Space A to Z

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

XYZ

Pick a Letter!

XYZ

Show All

Hide All

For your enjoyment and delight, we proudly present a list of space words and their definitions. And for extra convenience, they're even in alphabetical order! Pick a letter from our wonderful letter picker to begin.

Word	Meaning
A.S.I.	Agenzia Spaziele Italiana - The Italian Space Agency.
Abū Ishāq Ibrāhīm al-Zarqālī (Al-Zarqali / Arzachel)	A 11th-century Arab astronomer and mathematician who lived in what is now Spain. He made highly accurate calculations of the motions of the Sun, planets, and Moon. At the time, people believed Earth was the centre of the Solar System, but many of his measurements remained correct even after later astronomers confirmed that the Sun was at the centre. A lunar crater, Arzachel, is named in his honor.
Accretion	The build-up and accumulation of matter (dust and gas) which results in the formation of stars, planets, moons and other objects in solar systems.
Achernar	The brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Like Altair, it spins so fast that it is stretched out at the poles.
Acrux	The brightest star in the constellation Crux (the Southern Cross) and one of the brightest stars in the southern hemisphere. It is actually a system of multiple stars.
John Couch Adams	An English astronomer and mathematician (1819–1892) who was the first to predict the existence of Neptune using mathematics. In 1846, German astronomer Johann Galle discovered Neptune, but he used the predictions of French astronomer Urbain Le Verrier. Adams had made similar calculations earlier but did not publish them in time, so Le Verrier received most of the credit for the discovery.
Adrastea	Moon of Jupiter, discovered in 1979 by Voyager, 2nd closest moon to Jupiter, diameter 40 km (25 miles)
Aegir	A moon of Saturn, part of the Norse group of irregular moons. It has a retrograde orbit, meaning it moves in the opposite direction to Saturn’s rotation.
Aitne	Moon of Jupiter, discovered in 2003 by a group of astronomers at the University of Hawaii. It is irregularly shaped with a diameter of about 4 km (2 and a half miles). It orbits the planet at a distance of 23,981,000 km (14,901,100 miles).
Albedo	The measure of how much light a surface reflects. A high albedo means an object is very reflective (e.g. ice), while a low albedo means it absorbs more light (e.g. asphalt).
Albedo Feature	A bright or dark area on the surface of a planet, moon, or asteroid that reflects different amounts of sunlight. These features help scientists study the composition and history of celestial objects. For example, the dark patches on Mars show volcanic plains, while bright areas on the Moon are covered in reflective dust.
Albiorix	A moon of Saturn, the largest of the Gallic group of irregular moons. It has a reddish surface and may have originated from a larger body that broke apart.
Aldebaran	The brightest star in the constellation Taurus, often called the “Eye of the Bull.” It is an orange giant star and one of the brightest in the night sky.
Edwin "Buzz" Aldrin	Apollo 11 astronaut. Second man to walk on the Moon's surface.
Algenib	A bright star in the constellation Pegasus, part of the Great Square of Pegasus, a famous star pattern.
Alien	A general term for any lifeform that originates beyond Earth. Scientists search for microbial or intelligent alien life using methods like studying exoplanets, detecting biosignatures, and listening for signals from space. While no confirmed alien life has been discovered, potential candidates include Mars, Europa, and Enceladus due to their possible subsurface oceans.
Alioth	The brightest star in the Big Dipper (part of the constellation Ursa Major) and the third-brightest star in the whole constellation.
Alkaid	The last and brightest star in the handle of the Big Dipper (Ursa Major).
Almach	A beautiful multiple star system in the constellation Andromeda. Through a telescope, it appears as a striking blue and orange pair.
Alpha Centauri	The closest star system to Earth after the Sun. It consists of three stars: Alpha Centauri A, Alpha Centauri B, and Proxima Centauri (which is actually the closest individual star). Alpha Centauri A and B form a binary system, while Proxima Centauri orbits them at a distance.
Altair	The brightest star in the constellation Aquila and one of the closest bright stars to Earth, about 16.7 light-years away. It spins very fast, making it more stretched at the poles than at the equator. Altair is part of the Summer Triangle, a group of three bright stars visible in the summer sky.
Amalthea	Moon of Jupiter, discovered in 1892 by Edward Emerson Barnard. 200 km (150 miles) in diameter.
Amino Acids in Space	Basic building blocks of life, found in comets and meteorites. These molecules are important because they suggest that the ingredients for life might exist beyond Earth and could have arrived on the planet from space.
Ananke	A retrograde moon of Jupiter, thought to be the largest remnant of a captured asteroid that broke apart due to Jupiter’s gravitational forces.
Andromeda	A large constellation best known for the Andromeda Galaxy (M31), the nearest major galaxy to the Milky Way.
Angular Momentum	The tendency of a spinning or orbiting object to keep moving in the same way unless something changes it. This is why planets stay in stable orbits around the Sun and why a spinning ice skater speeds up when they pull their arms in.
Anthe	A small moon of Saturn, discovered in 2007. It is embedded in the Anthe Ring Arc, a faint ring created by particles ejected from the moon itself.
Antimatter	The opposite of normal matter. When matter and antimatter collide, they annihilate each other, releasing energy.
Antlia	A faint constellation whose name means "the Air Pump."
Aoede	A small, irregular moon of Jupiter, part of the Pasiphae group. It has a highly inclined, retrograde orbit.
Apastron	The point at which a star in a binary system is farthest from its companion.
Aphelion	The point at which an object in orbit around the Sun is farthest from it. Opposite to perihelion.
Aphelion Precession	The slow movement of a planet's farthest point from the Sun due to gravitational forces.
Apoapsis	The point at which an object in orbit around any celestial body is farthest from that body. For example, a moon around Jupiter.
Apogee	The greatest distance of a body (the Moon or an artificial satellite) in its orbit around Earth.
Apogee Kick Motor (AKM)	A rocket motor used to adjust the orbit of satellites after launch.
Apollo (Space Program)	A series of U.S. space missions in the 1960s and 1970s designed to land astronauts on the Moon and return them safely to Earth. The program, run by NASA, included both unmanned and manned missions. Apollo 11 was the first successful Moon landing in 1969. Apollo 13's landing was aborted due to an explosion onboard, but the crew returned safely. The program ended with Apollo 17 in 1972.
Apollo Asteroids	A group of Earth-crossing asteroids named after 1862 Apollo, the first discovered of its kind.
Apollo Telescope Mount (ATM)	A solar observation module attached to Skylab, America’s first space station.
Apus	A small, faint constellation in the southern sky, named after a bird-of-paradise.
Aquarius	One of the zodiac constellations, representing the Water Bearer. It contains the famous Helix Nebula.
Aquila	A constellation in the northern sky, home to the bright star Altair.
Ara	A southern constellation whose name means "the Altar."
Dominique François JeanArago	French astronomer and physicist. He discovered the production of magnetism by rotation. He was also director of the Paris Observatory and lived from 1786 to 1853.
Arche	A tiny irregular moon of Jupiter, part of the Carme group, orbiting the planet in a distant, retrograde motion.
Arcturus	Arcturus is the brightest star in the constellation Boötes and the fourth brightest star in the night sky.
Arecibo Message	A powerful radio signal sent from Earth in 1974 to a star cluster called Messier 13, about 25,000 light-years away. It contained information about humans, our number system, DNA, and our solar system, designed as a message for potential extraterrestrial civilizations.
Ariel	One of the five major moons of Uranus, discovered in 1851. It has a bright, icy surface with canyons and fault lines, suggesting past geological activity.
Neil Armstrong	Apollo 11 astronaut. First man to walk on the Moon's surface.
Artemis Program	NASA’s current lunar exploration program, aiming to return humans to the Moon for the first time since Apollo and establish a sustainable presence. Artemis will test new technologies, including the Orion spacecraft and the Space Launch System (SLS), and pave the way for future Mars missions. The first crewed lunar landing is planned for mid-2027 (Artemis III), with the goal of landing the first woman and the first person of colour on the Moon. The first mission, Artemis I, launched in 2022 as an uncrewed test flight. The program is named after Artemis, the Greek goddess of the Moon and twin sister of Apollo.
Asteroid	Medium-sized rock fragments left over from the formation of the Solar System left to float around space. Smaller than planets but larger than meteoroids. Also known as "Planetoids"
Asteroid Belt	Region in between the orbits of Mars and Jupiter where a large number of asteroids orbit the Sun.
Asteroid Family	A group of asteroids that share similar orbits, likely originating from a single larger body.
Asteroid Number	Each asteroid discovered is given a number based on the order that it was discovered. 1 Ceres means that Ceres was the first asteroid to be discovered. 4 Vesta means that Vesta was the fourth asteroid to be discovered.
Astrobiology	The study of life in the universe. It looks at how life started on Earth, where life might exist on other planets or moons, and what kinds of living things could survive in space. Scientists search for clues in extreme places on Earth, like deep in the ocean or in deserts, to understand where life might exist beyond the planet.
Astrobiome	All the places beyond Earth where life could exist. This includes planets, moons, and even deep space environments where tiny life forms might survive in extreme conditions.
Astrochemistry	The study of chemical elements and molecules in space, including in stars and interstellar clouds.
Astrology	A belief system, not a science, that suggests the positions of celestial bodies influence people's lives.
Astronaut	A person who travels into space. Russia/Soviet equivalent is Cosmonaut.
Astronomical Unit	A unit of measurement used for measuring distances in space. 1 A.U. is equal to the approximate distance from the Earth to the Sun (150 million kilometres).
Astronomy	The study of the objects in the sky, eg: stars, planets, moons, galaxies, comets, asteroids, etc.
Astrophysics	The part of science that explains how space works. It uses ideas from physics to study things like stars, planets, black holes, and galaxies. Astrophysicists try to understand how these objects are made, how they move, and what forces (like gravity) affect them.
Atlas	A small moon of Saturn, shaped like a flying saucer due to an equatorial ridge formed by ring material. It orbits near the A ring.
Atmosphere	The layer of gas surrounding the surface of a planet, moon or star.
Atmospheric Drag	The resistance a satellite or space object experiences when moving through the outer layers of a planet’s atmosphere. In low Earth orbit (LEO), drag gradually slows satellites down, causing them to lose altitude and eventually re-enter the atmosphere. Space agencies must adjust satellite orbits to counteract this effect, or the objects will fall back to Earth and burn up.
Atom	A tiny particle of matter, itself made up of many particles of matter.
Aurora	Glowing lights in the sky caused by the interaction between the Solar Wind (charged particles from the Sun) and a planet's magnetic field.
Aurora Australis	An aurora which can be seen in the Southern Hemisphere. The glowing lights are caused by the interaction of charged particles from the Sun and Earth's magnetic field in the upper atmosphere (or ionosphere). Also known as the "Southern Lights"
Aurora Borealis	An aurora which can be seen in the Northern Hemisphere. The glowing lights are caused by the interaction of charged particles from the Sun and Earth's magnetic field in the upper atmosphere. Also known as the "Northern Lights"
Aurora Oval	The ring-shaped region around a planet’s magnetic poles where auroras are most commonly seen.
Autonoe	A tiny, distant moon of Jupiter, part of the Pasiphae group, with an irregular, retrograde orbit.
Axis	The imaginary line around which a planet, moon, star spins.
Azimuth	The direction of an object in the sky, measured in degrees from north (0°). For example, an object at 90° azimuth is directly east, while 180° is south. Azimuth is used in astronomy and navigation to help locate stars, planets, and satellites.
Heinrich Louis d'Arrest	Danish astronomer involved in the first observation of the planet Neptune. He was assisting German astronomer Galle who was using information provided by French mathematician Le Verrier about the position of an object beyond Uranus. While Galle observed the sky, d'Arrest read from a star chart. Galle spotted an object which wasn't listed on d'Arrest's chart, and the first observation of Neptune was made! D'Arrest lived from 1822 to 1875.

Word	Meaning
Edward Emerson Barnard	An American astronomer (1857–1923) who discovered Barnard’s Star, the fourth closest known star to Earth. He also discovered Amalthea, a moon of Jupiter, in 1892.
Barnard's Star	Fourth nearest star to Earth. 5.9 light years away.
Barnard’s Loop	A large red nebula in Orion, linked to past supernova explosions.
Barycentre (Barycenter)	The common centre of mass around which two or more celestial bodies orbit. For example, the Earth and Moon orbit a barycentre located just inside the Earth’s surface.
Bebhionn	A small moon of Saturn, discovered in 2004. It is part of the Gallic group of irregular moons, which have prograde orbits (moving in the same direction as Saturn's rotation). It is named after a figure from Irish mythology.
Belinda	A small moon of Uranus, approximately 80 km in diameter. It was discovered in 1986 by Voyager 2 and is the 10th closest moon to Uranus.
Bellatrix	A bright blue giant star in the Orion constellation. It is sometimes called the "Amazon Star" and is one of the hotter, more massive stars visible to the naked eye.
BepiColombo	A joint ESA (European Space Agency) and JAXA (Japan Aerospace Exploration Agency) mission launched in 2018 to study Mercury, the least explored planet in the inner Solar System. It consists of two orbiters that will map Mercury’s surface, study its magnetic field, and analyze its thin atmosphere once it arrives in 2025.
Bereshit (Beresheet)	An Israeli private lunar lander launched in 2019 by the company SpaceIL. It was part of an attempt to become the first privately funded mission to land on the Moon. However, the lander crashed due to a technical failure during descent.
Bergelmir	A small, irregular moon of Saturn, discovered in 2004. Part of the Norse group of moons.
Bestla	A small, irregular moon of Saturn, part of the Norse group of moons, discovered in 2004. It has a retrograde orbit, meaning it moves in the opposite direction to Saturn's rotation.
Beta Centauri	The second-brightest star in the Centaurus constellation, located near Alpha Centauri. It is actually a system of at least three stars and is much farther from Earth than Alpha Centauri.
Beta Lyrae	A famous binary star system where one star is being pulled apart by the other.
Betelgeuse	A bright red supergiant star in the constellation Orion, known for being one of the largest stars visible to the naked eye. It is expected to explode as a supernova sometime in the next 100,000 years.
Bianca	A small inner moon of Uranus, discovered by Voyager 2 in 1986. It is part of the Portia group of moons.
Big Bang	The event that marked the beginning of the universe around 13.8 billion years ago. Scientists believe the universe started as a tiny, incredibly hot and dense point, then expanded rapidly. This expansion continues today, causing galaxies to move farther apart.
Big Dipper	A well-known star pattern that is part of the larger constellation Ursa Major. It is often used to locate the North Star, Polaris.
Billion	In the U.S., a billion is one thousand million (1,000,000,000). In older British usage, a billion was one million million (1,000,000,000,000), but most scientists today use the American definition or scientific notation to avoid confusion.
Binary Star	A system of two stars that orbit a common center of mass. Many stars in the universe are part of binary or multiple star systems.
Bion Program	A series of Soviet and later Russian biological research missions launched between 1973 and 1996. The Bion satellites carried animals, plants, and microorganisms into space to study the effects of weightlessness on living organisms. These missions helped scientists understand how space travel affects life on Earth.
Biosignature	A chemical or physical feature that could indicate the presence of past or present life on another planet or moon.
Black Hole	A region of space where gravity is so strong that nothing, not even light, can escape. Black holes form when massive stars collapse under their own gravity at the end of their life cycle. They can be detected by their effect on nearby objects and radiation emitted from material falling into them.
Blazar	A type of extremely bright and active galactic nucleus, powered by a supermassive black hole.
Blueshift	When light from an object moving toward an observer appears to shift toward the blue end of the spectrum. This is the opposite of redshift, which happens when an object moves away.
Boötes Void	A vast, nearly empty region of space in the Boötes constellation. It is one of the largest known voids in the universe, with very few galaxies inside it. Sometimes called the "Great Nothing," it stretches about 330 million light-years across.
Johann Elert Bode	German astronomer, mostly known for "Bode's Law" (see below). Lived from 1747 to 1826.
Bode's Law	A mathematical formula designed by Johann Bode to predict the distances of planets from the Sun in astronomical units (A.U.). It accurately estimated the positions of planets up to Uranus and even hinted at the existence of the asteroid belt. However, it failed to predict Neptune’s orbit, suggesting that the pattern is more of a coincidence than a fundamental law of planetary formation.
Bok Globule	A small, dense cloud of gas and dust where stars may be forming. These dark patches are often seen in nebulae.
Bolide	An extra-bright meteor that becomes a fireball as it travels through the atmosphere, shining as bright or brighter than the light of a full moon. Geologists also refer to meteorites that have impacted a surface and form a crater as bolides.
Booster Rocket	A rocket used to provide extra thrust during the launch of a spacecraft. Many boosters, such as those on the Space Shuttle and Falcon Heavy, are jettisoned after use.
Bow Shock	The boundary where the solar wind (charged particles from the Sun) collides with a planet's magnetic field or an interstellar medium. For example, Earth's bow shock protects the planet by diverting harmful space radiation around it.
Brown Dwarf	A celestial object that is larger than a planet but too small to sustain nuclear fusion like a star. Often considered a "failed star."
Bubble Nebula	A nebula shaped like a bubble, located in the constellation Cassiopeia.
Bus Classification	A system used to classify asteroids based on the type of light they reflect (their spectral properties). It was developed by astronomer David Tholen in 1984 and later expanded by Richard P. Binzel in the 2000s. Asteroids are grouped into categories like C-type (carbon-rich), S-type (silicate or rocky), and X-type (metallic), helping scientists understand their composition and origins.

Word	Meaning
Caldera	A volcano that has sunk into the collapsed land below it (usually because all the magma below has erupted and there is nothing to support the newly formed mountain). Fairly common on Earth. An example elsewhere in the Solar System is Olympus Mons on Mars.
Caliban	An irregular moon of Uranus, named after a character in William Shakespeare’s play The Tempest.
Callirrhoe	A small moon of Jupiter, discovered in 1999. About 7 km in diameter, it follows a retrograde orbit, meaning it moves in the opposite direction to Jupiter's rotation.
Callisto	The second-largest moon of Jupiter and the third-largest moon in the Solar System. Callisto is about 99% the size of Mercury and is the most cratered body in the Solar System. It was discovered by Galileo in 1610.
Calypso	A small moon of Saturn, discovered in 1980. It shares its orbit with the larger moon Tethys and is part of Saturn’s Trojan moons.
Capella	The brightest star in the constellation Auriga and one of the closest giant stars to Earth.
Carme	Discovered in 1938, Carme became the eleventh moon known to orbit Jupiter. Irregularly shaped, about 48 km in diameter. The moon didn't receive its present name until 1975. Before then it was known as Jupiter XI, and between 1955 and 1975, some people knew it as Pan. Pan is now the name of a different moon that orbits Saturn, discovered in 1990.
Carpo	A small moon of Jupiter, discovered in 2003. It is only about 2 km in diameter and has a highly inclined orbit.
Catena	A chain of craters of similar size. They are believed to be created when an object like a comet is broken up into fragments and these fragments impact the surface of a planet or moon.
Celestial object	Any object which appears in the sky - the Moon, Sun, planets, stars, galaxies, etc. are all celestial objects.
Centaur (Astronomical Object)	A type of celestial object that shares characteristics of both asteroids and comets. Centaurs orbit between Jupiter and Neptune, with some showing comet-like activity.
Cepheid Variable	A type of star that changes in brightness over time. These stars are used as 'cosmic yardsticks' to measure distances in space.
Cetus	A large constellation named after a sea monster from Greek mythology. It contains several bright stars and galaxies.
Chaldene	Small moon of Jupiter, discovered in 2000, about 4 km in diameter.
Challenger (Space Shuttle)	A NASA space shuttle that tragically exploded 73 seconds after launch on January 28, 1986. It was the first space shuttle disaster and led to major changes in space safety protocols.
Chandrasekhar Limit	The maximum mass (about 1.4 times the Sun’s mass) that a white dwarf star can have before it collapses into a neutron star or black hole.
Chang’e Program	China’s lunar exploration program, named after the Chinese Moon goddess. It includes several successful robotic landers and sample return missions.
Chariklo	The largest known Centaur object, orbiting between Saturn and Uranus. It is the first minor planet discovered to have rings.
Charon	Moon of Pluto, discovered in 1978 and about one third the size of Pluto. Because Pluto and Charon orbit an invisible axis between the two objects, they are sometimes referred to as a Binary Planet.
James W. Christie	American astronomer who discovered Pluto's moon Charon in 1978. He was born in 1938.
Chromosphere	Part of the atmosphere of the Sun or any other star. It lies above a star's visible surface, its photosphere and below its outermost layer, its corona.
Churyumov–Gerasimenko	A comet visited by the ESA’s Rosetta spacecraft in 2014, famous for being the first comet where a lander (Philae) touched down.
Circumpolar Stars	Stars that never set below the horizon as seen from a particular location on Earth. These stars appear to move in circles around the celestial pole. In the Northern Hemisphere, an example is Polaris, the North Star.
Circumstellar Habitable Zone	Also called the Goldilocks Zone or Habitable Zone, this is a region around a star where temperatures and other conditions may be just right for any planets situated in them to be able to hold liquid water, greatly increasing the possibility of life existing on them. Earth is situated in the solar system's Habitable Zone.
Cluster	A group of stars or galaxies held together by gravity. There are two main types of star clusters: open clusters, which contain younger stars, and globular clusters, which are tightly packed groups of older stars.
Coma	The cloud of gas and dust surrounding the nucleus of a comet. It forms when the Sun’s heat causes the comet’s ice to vaporize, creating a glowing halo.
Comet	A small ball of ice, dust, and gas that orbits the Sun, often in an elliptical (oval) path. Comets become visible when they get close to the Sun, causing their ice to vaporize and form a glowing coma and tail. Some comets take decades to complete an orbit, while others take thousands of years.
Conjunction	A term used to describe the position of a planet relative to Earth and the Sun. An 'inferior conjunction' occurs when an inner planet (Mercury or Venus) is between Earth and the Sun. A 'superior conjunction' happens when the inner planet is on the opposite side of the Sun. A superior planet (Mars, Jupiter, etc.) is in conjunction when it is behind the Sun from Earth's perspective, and in 'opposition' when it is on the same side as Earth.
Constellation	A group of stars, named by ancient astronomers because of their appearance (kind of like dot-to-dot). The twelve signs of the zodiac are named after twelve constellations.
Copernican System	A model of the Solar System proposed by Nicolaus Copernicus in the 16th century. It placed the Sun at the center, with planets—including Earth—orbiting around it. This was a major shift from the earlier geocentric model, which claimed Earth was at the center. The Copernican System laid the foundation for modern astronomy.
NicolausCopernicus	A Polish astronomer and mathematician (1473–1543) best known for proposing the heliocentric model of the Solar System, where the Sun is at the center and planets orbit around it. His book De revolutionibus orbium coelestium ('On the Revolutions of the Celestial Spheres') published in 1543 challenged the long-held belief that Earth was the center of the universe
Cordelia	A small inner moon of Uranus, discovered by the Voyager 2 spacecraft in 1986. It is the innermost of Uranus’s moons and orbits just inside the planet’s bright epsilon ring, helping to keep the ring in place through its gravitational influence. Cordelia is named after the daughter of King Lear in Shakespeare’s play King Lear.
Corona	The outermost part of the atmosphere surrounding a celestial object like the Sun. Can often be seen during a solar eclipse. The corona extends many millions of kilometres into space. Lower in density than the rest of the star, not as bright (the Sun's disc has to be covered for it to be visible), but much hotter.
Cosmonaut	A person trained by Russia (or the former Soviet Union) to travel into space. The term is similar to "astronaut" but is specifically used for Russian space missions. The first person in space, Yuri Gagarin, was a cosmonaut, launching aboard Vostok 1 in 1961. Cosmonauts have been part of major space programs, including the Salyut, Mir, and International Space Station (ISS) missions.
Crater	A bowl-shaped depression formed when a meteorite hits the surface of a planet or moon.
Cressida	A small inner moon of Uranus, discovered by Voyager 2 in 1986. It is part of a group of moons that orbit close to Uranus and may eventually collide with another moon or break apart due to gravitational forces. It is about 80 km in diameter and named after a character in Shakespeare’s play Troilus and Cressida.
Crust	The outermost solid layer of a planet, moon, or other rocky body. It is made of rock and minerals and is much thinner than the layers beneath it, such as the mantle and core. On Earth, the crust is where land, oceans, and all known life exist. Other planets and moons also have crusts, though they may be thicker, thinner, or made of different materials, depending on their composition and history.
Cupid	Moon of Uranus with a diameter of 18km (11 miles). Discovered in 2003 by Mark Robert Showalter and Jonathan Lissauer. Named after a character in William Shakespeare's play Timon of Athens

Word	Meaning
Dactyl	A tiny moon of the asteroid Ida, discovered by the Galileo spacecraft in 1993. It was the first moon ever discovered orbiting an asteroid.
Daphnis	A small moon of Saturn, discovered on May 1, 2005, from images taken by the Cassini spacecraft. It has a diameter of about 8 km (5 miles) and orbits within Saturn’s A Ring, where its gravity creates waves in the ring’s structure. Named after Daphnis, a Sicilian shepherd and poet in Greek mythology.
Dark Dust Cloud	A dense region of dust and gas in space that blocks visible light. These clouds can obscure stars and other celestial objects behind them, making them difficult to observe with optical telescopes.
Dark Matter	An invisible type of matter that makes up most of the universe. Scientists can't see it directly because it doesn't give off light, but they know it's there because its gravity affects how galaxies move.
Dawn (Spacecraft)	A NASA mission launched in 2007 to study the dwarf planet Ceres and the asteroid Vesta in the asteroid belt.
Day	The time it takes for a planet to spin once on its axis. On Earth, a day is 24 hours, but other planets have different day lengths. A day on Mars is called a 'sol' and lasts about 24.6 Earth hours.
Deep Ecliptic Survey (DES)	A project from 1998 to 2005 that searched for objects beyond Neptune in the Kuiper Belt. It discovered 663 objects using telescopes in Arizona and Chile. Further info is available here
Deep Space	Any region of space beyond Earth’s Moon, including interstellar and intergalactic space.
Deep Space Network (DSN)	A system of giant radio antennas used by NASA to communicate with spacecraft exploring the Solar System and beyond.
Deimos	One of Mars’ two small moons (the other is Phobos). Deimos is the smaller and more distant moon, orbiting Mars at 23,460 km (15,577 miles). It is only about 16 km (10 miles) across. It was discovered on August 12, 1877, by American astronomer Asaph Hall, who also discovered Phobos on the same night.
Density	A measure of how closely packed matter is. Gas is less dense (less compact) than water. Water is less dense than a solid.
Desdemona	A small moon of Uranus, discovered by Voyager 2 in 1986. It is one of Uranus’s inner moons and orbits close to the planet. Named after the heroine in William Shakespeare’s play Othello.
Despina	Moon of Neptune, third closest to the planet. Its diameter is 152 kilometres (94 miles) and it orbits at 52,526 km (32,638 miles) from Neptune. Despina was discovered in 1989 by the Voyager 2 space craft.
Diameter	The distance from one side of a round object to the opposite side going through its centre. In astronomy, this usually refers to the width of a planet, moon or star. Some of these objects are not completely spherical (like Jupiter) so the diameter is wider than the distance from top to bottom.
Dione	A large moon of Saturn, discovered in 1684 by Giovanni Cassini. It has a diameter of 1,123 km (698 miles) and is mostly made of ice. Dione has bright, wispy streaks on its surface, thought to be caused by icy cliffs or fractures in its crust.
Doppler Effect	The change in sound or light when something moves closer or farther away. In space, it helps scientists know if a star or galaxy is moving toward or away from Earth.
Double Planet	A pair of planets that are gravitationally bound and orbit around a common center of mass. Pluto and Charon are often considered a double planet because their center of mass lies outside Pluto. Some scientists suggest that Earth and the Moon may also qualify as a double planet system.
Double Star	A pair of stars that orbit around a common center of mass due to their mutual gravitational attraction. Also called a Binary Star system. Some double stars are 'true binaries,' while others just appear close together in the sky but are actually unrelated.
Dragon (Spacecraft)	A reusable spacecraft developed by SpaceX to transport cargo and crew to the International Space Station (ISS). It was the first commercial spacecraft to dock with the ISS.
Dust Tail (of a Comet)	A stream of dust that trails behind a comet, pushed away by the Sun’s radiation. It often forms a long, curved tail that can be visible from Earth.
Dwarf Planet	A celestial body that orbits the Sun and is large enough to be rounded by its own gravity but has not cleared its orbital path of other debris. Examples include Pluto, Eris, and Haumea.
Dysnomia	A small moon of Eris, the second-largest dwarf planet. It was discovered in 2005.

Word	Meaning
Earth	The third planet from the Sun and the fifth largest in the Solar System. Earth is the only planet known to support life and has large amounts of liquid water on its surface. It has one natural satellite, the Moon.
Ecliptic	The apparent path the Sun follows across the sky over the year, caused by Earth’s orbit around the Sun.
EDT (Eastern Daylight Time)	Eastern Daylight Time is used during spring and summer in the eastern United States, parts of South America, the Bahamas, and some Caribbean islands. It is four hours behind GMT (Greenwich Mean Time) or UTC (Universal Time Coordinated). Because Florida uses EDT, it is often referenced for launch times from Kennedy Space Center.
Elara	A small moon of Jupiter, discovered in 1905 by Charles Dillon Perrine. It is part of the Himalia group, a set of moons that share a similar orbit around Jupiter. Elara is about 86 km (53 miles) in diameter.
Electromagnetic Spectrum	The full range of wavelengths of light, from radio waves to gamma rays. Different types of telescopes detect different parts of this spectrum.
Element	A chemical substance that cannot be broken down into simpler substances. Elements, such as hydrogen, helium, oxygen, and carbon, make up all matter. There are 118 known elements. The Solar System was formed from a cloud of gas and dust containing various elements, which later combined to form planets and stars.
Elliptical/Ellipse	An elongated circle or oval shape. Most planets in the Solar System follow elliptical orbits rather than perfect circles. Venus has the most circular orbit, while dwarf planets like Pluto and Eris have the most elliptical orbits.
Enceladus	A medium-sized moon of Saturn, known for its icy surface and geysers that shoot water into space. It was discovered in 1789 by William Herschel and is considered one of the best places to search for life beyond Earth.
Endeavour (Space Shuttle)	A NASA Space Shuttle that completed 25 missions, including servicing the Hubble Space Telescope.
Epimetheus	A small moon of Saturn, discovered in 1966 but confirmed in 1977. It shares an unusual orbit with another moon, Janus, meaning they occasionally swap places. Epimetheus is about 116 km (72 miles) in diameter.
Equinox	The time of year when day and night are nearly equal in length, occurring in March and September when Earth's axis is not tilted toward or away from the Sun.
Erebus Montes	A group of mountains on Mars, named after the HMS Erebus, an Antarctic exploration ship.
Erinome	A small moon of Jupiter, discovered in 2000. It is part of the Carme group, a set of irregular moons that orbit Jupiter in the opposite direction of its rotation (retrograde motion).
Erriapus	A small moon of Saturn, discovered in 2000. It is part of the Gallic group of irregular moons and has a diameter of about 10 km (6 miles).
ESA (European Space Agency)	A space agency formed by 22 European countries to design, develop, and launch satellites, probes, and human space missions. Notable missions include Mars Express, Venus Express, and the Rosetta mission, which landed on a comet.
Escape Velocity	The speed an object must reach to escape the gravitational pull of a larger body, such as a planet or moon. For example, a rocket must travel at about 11.2 kilometres a second (km/s) / 7 miles a second (mi/s) to escape Earth’s gravity, while a much lower speed is needed on the Moon due to its weaker gravity.
EST (Eastern Standard Time)	Eastern Standard Time is used during autumn and winter in the eastern United States, parts of South America, the Bahamas, and some Caribbean islands. It is five hours behind GMT (Greenwich Mean Time) or UTC (Universal Time Coordinated). EST is often used when referring to space launches from Kennedy Space Center.
Euanthe	A small moon of Jupiter, discovered in 2001. It is part of the Ananke group, a collection of irregular moons that orbit Jupiter in a retrograde direction (opposite to Jupiter's rotation). Euanthe has a diameter of about 3 km (2 miles).
Eukelade	A small irregular moon of Jupiter, discovered in 2003. It is part of the Carme group, a family of moons that orbit Jupiter in a distant, retrograde path. Eukelade is about 4 km (2.5 miles) in diameter.
Europa	A large moon of Jupiter, discovered in 1610 by Galileo Galilei. Europa has an icy surface and a subsurface ocean beneath its ice crust, making it one of the best places to search for alien life in the Solar System.
Eurydome	A small retrograde moon of Jupiter, discovered in 2001. It is also part of the Pasiphae group, a set of irregular moons that follow distant, inclined orbits around Jupiter. Eurydome is about 3 km (2 miles) in diameter.
EVA (Extravehicular Activity)	An activity performed by an astronaut outside a spacecraft, also known as a spacewalk. EVAs can occur during orbital missions, space station maintenance, or lunar surface exploration. The first spacewalk was by Alexei Leonov (1965), and the first American EVA was by Ed White (1965).
Event Horizon	The boundary around a black hole where nothing, not even light, can escape. Anything that crosses the event horizon is pulled into the black hole and cannot return.
ExoMars	A European Space Agency (ESA) mission designed to search for signs of life on Mars. Originally planned as a joint European-Russian project, cooperation with Russia ended in 2022, and ESA is now developing the mission independently. It consists of an orbiter, which launched in 2016, and a planned rover, which will drill beneath the Martian surface to look for evidence of past or present life.
Exoplanet/Extrasolar Planet	A planet that orbits a star outside our Solar System. Thousands of exoplanets have been discovered, many of them gas giants like Jupiter, but some are Earth-sized and may have conditions suitable for life. Most exoplanets are found by detecting how they affect their stars rather than being directly imaged.
Extra Terrestrial	Literally means 'beyond Earth.' It refers to anything not from planet Earth, including meteors, space dust, and potential alien life

Word	Meaning
Falcon 9	A reusable rocket developed by SpaceX, used to launch satellites and astronauts into space.
Farbauti	A small irregular moon of Saturn, discovered in 2004. It is part of the Norse group of moons, which have retrograde orbits (moving in the opposite direction to Saturn's rotation). Farbauti is about 5 km (3 miles) in diameter.
Fast Radio Burst (FRB)	A brief but powerful burst of radio waves from space. The cause of most FRBs is still unknown, but they may come from neutron stars or other extreme objects.
Fenrir	A tiny moon of Saturn, discovered in 2004. It belongs to the Norse group of irregular moons and follows a retrograde orbit. Fenrir is one of the smallest known moons of Saturn, with a diameter of about 4 km (2.5 miles).
Ferdinand	A small moon of Uranus, discovered in 2001. It is an irregular moon with a retrograde orbit, meaning it moves in the opposite direction of Uranus's rotation. Ferdinand is the outermost known moon of Uranus and has a diameter of about 20 km (12 miles).
Field of View	The area of the sky visible through a telescope, camera, or other optical instrument at one time. A larger field of view allows more objects to be seen at once, while a smaller field of view provides a more detailed view of a smaller region.
Field Star (or Field Galaxy)	A star (or galaxy) which is in the area being viewed by an astronomer but is not the object that they are actually looking at.
Filter	A device used in telescopes and cameras to absorb certain colors or reduce light levels, improving the visibility of details. For example, filtering out certain wavelengths while observing a planet can reveal surface or atmospheric features more clearly. Special solar filters allow safe viewing of sunspots by reducing the intensity of sunlight.
Fireball	An especially bright meteor that burns up in Earth's atmosphere, often leaving a glowing trail.
First Quarter Moon	The phase of the Moon when half of its surface is illuminated as seen from Earth, occurring about a week after a new moon.
Flare	A sudden burst of energy from a star, such as the Sun. These usually occur near sunspots and release radiation and charged particles into space. When solar flares reach Earth, they can disrupt radio signals and affect satellites. They are best observed using special solar telescopes or during eclipses.
Flat Earth Society	A group that promotes the belief that Earth is flat, despite overwhelming scientific evidence proving it is a sphere. The society was originally based in England but later moved to California. Modern space missions and satellite images have repeatedly confirmed the spherical shape of Earth.
Flyby	A space mission in which a spacecraft passes close to an object (such as a planet, moon, comet, or asteroid) to take pictures and collect data without entering orbit or landing. Flybys were common in early space exploration but are still used for missions traveling to multiple destinations, such as Voyager 2, which flew past all four gas giants.
Fomalhaut	A bright star in the constellation Piscis Austrinus, known for having one of the first directly imaged exoplanetary systems.
Fornjot	A distant moon of Saturn, discovered in 2004. It is the largest of the Norse group of irregular moons and has a diameter of about 6 km (3.7 miles). It follows a retrograde orbit far from Saturn.
Francisco	A small moon of Uranus, discovered in 2001. It has an irregular, retrograde orbit and is part of the group of outer moons of Uranus. Francisco is about 22 km (14 miles) in diameter.
Fraunhofer Lines	Dark lines in the Sun's spectrum caused by elements absorbing specific wavelengths of light.

Word	Meaning
Galactic Centre	The central region of a galaxy, usually densely packed with stars, gas, and a supermassive black hole. The Galactic Centre of the Milky Way is about 28,000 light-years from Earth and contains the supermassive black hole Sagittarius A.
Galatea	Moon of Neptune, fourth closest to the planet. Galatea is 175 kilometres (109 miles) in diameter and orbits Neptune at an average distance of 61,953 km (38,496 miles). It was discovered by Voyager 2 in 1989.
Galaxy	A vast collection of stars, gas, dust, and dark matter held together by gravity. Galaxies come in different shapes, including spiral (like the Milky Way), elliptical, and irregular. Some galaxies contain billions or even trillions of stars.
Galilean Satellites / Galilean Moons	Jupiter's four largest moons, discovered by Galileo Galilei in 1610. In order of increasing distance from Jupiter, these are Io, Europa, Ganymede, and Callisto. Ganymede is the largest moon in the Solar System, bigger than Mercury.
Galileo (Spacecraft)	A NASA spacecraft launched in 1989 to study Jupiter and its moons. It provided detailed images of Io’s volcanoes, Europa’s ice, and Ganymede’s surface before crashing into Jupiter in 2003.
Galileo Galilei	An Italian astronomer, physicist, and engineer (1564–1642) known as the 'father of modern astronomy.' He improved the telescope, discovered Jupiter’s four largest moons (Io, Europa, Ganymede, and Callisto) in 1610, and provided strong evidence that Earth orbits the Sun, supporting the heliocentric model. His discoveries challenged the Church’s view of the universe, leading to his house arrest for the rest of his life.
Galle, Johann Gottfried	A German astronomer who made the first confirmed observation of Neptune in 1846, assisted by Heinrich Louis d’Arrest. This discovery was based on predictions by French mathematician Urbain Le Verrier.
Gamma Rays	A type of electromagnetic radiation with the shortest wavelength and highest energy. Gamma rays are produced by powerful cosmic events, such as supernova explosions, neutron stars, and black holes. They are also used in medical treatments and imaging.
Gamma-Ray Burst (GRB)	A sudden, intense burst of gamma rays from space. These bursts can come from collapsing stars or neutron star collisions and are some of the most energetic events in the universe.
Ganymede	The largest moon of Jupiter and the largest moon in the Solar System. It has a diameter of 5,262 km (3,273 miles), making it larger than Mercury and Pluto. Ganymede has a mix of old, cratered regions and younger, grooved terrain. It orbits Jupiter every 7 days and 3 hours.
Gas Giant	A large planet with a deep atmosphere, mostly made of hydrogen and helium, and a small (possibly rocky) core. The four gas giants in the Solar System are Jupiter, Saturn, Uranus, and Neptune. Most exoplanets discovered so far are gas giants because their size makes them easier to detect.
Gemini Program	A NASA program from the 1960s that tested spacewalking, docking, and long-duration spaceflight, preparing for the Apollo Moon landings.
Geostationary Orbit	An orbit around Earth where a satellite remains fixed over the same spot on the planet’s surface. A satellite in geostationary orbit takes 24 hours to complete one orbit, matching Earth's rotation. These orbits are used for weather and communication satellites.
Geosynchronous Orbit	An orbit where a satellite takes exactly 24 hours to go around Earth, but it is not necessarily fixed over one spot. Unlike a geostationary orbit, a geosynchronous satellite may move north and south or in an elliptical path. This means it appears to 'drift' from a fixed point over time.
Gliese 581	A red dwarf star about 20 light-years from Earth that has several exoplanets, some of which may be in its habitable zone.
Goldilocks Zone	A region in the orbit of a star where bears live in cottages and eat porridge for breakfast. Ignore the stuff about bears, although it does get its name from the children's story! It's actually a region around a star where temperatures are 'just right' for liquid water to exist, making it a possible location for life. Earth is situated in the Goldilocks Zone in the solar system. It can also be called the habitable zone or circumstellar habitable zone.
Gravitational Lens	When a massive object (like a galaxy) bends and magnifies light from another object behind it, allowing astronomers to see distant galaxies that would otherwise be too faint.
Gravity	A force that pulls objects toward each other. The more massive an object, the stronger its gravity. Gravity keeps planets in orbit around stars and holds galaxies together. On Earth, gravity keeps us on the ground, while the Sun’s gravity keeps planets orbiting around it.
Gravity Assist	A technique used to speed up or change the path of a spacecraft by using a planet’s gravity. The spacecraft flies close to a planet, gaining momentum and altering its trajectory. Mariner 10 was the first mission to use gravity assist (using Venus to reach Mercury), and the Voyager probes used it to visit multiple planets before heading beyond the Solar System.
Greenhouse Effect	The process by which gases like carbon dioxide trap heat in a planet's atmosphere, keeping it warm. It is responsible for Earth’s climate and Venus’s extremely high temperatures.
Greenwich Mean Time (GMT)	The time standard based on the prime meridian (0° longitude) in Greenwich, England. Historically used as the world’s time reference, GMT is now mostly replaced by Coordinated Universal Time (UTC) in modern timekeeping. It is effectively the same as Universal Time (UT) in practical use.
Greip	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group of moons, which follow retrograde orbits (moving opposite Saturn’s rotation). Greip is about 6 km (3.7 miles) in diameter.

Word	Meaning
Habitable Zone	Also called the Goldilocks Zone or Circumstellar Habitable Zone, this is the region around a star where temperatures and conditions may be just right for liquid water to exist on a planet’s surface. Earth is in the Sun’s habitable zone, making it suitable for life.
Halimede	A small moon of Neptune, discovered in 2002 by a team led by Matthew J. Holman. It is the fifth most distant known moon of Neptune, with a diameter of about 60 km (37 miles) and an orbit 15.7 million km (9.8 million miles) from the planet.
Harpalyke	A small irregular moon of Jupiter, discovered in 2000. It is part of the Ananke group, a family of moons that orbit Jupiter in retrograde motion (opposite to the planet’s rotation). Harpalyke has a diameter of about 4 km (2.5 miles).
Hati	A small irregular moon of Saturn, discovered in 2004. It is part of the Norse group, a collection of moons that orbit Saturn in retrograde motion. Hati is about 6 km (3.7 miles) in diameter and follows a distant, inclined orbit around Saturn.
Hayabusa (Spacecraft)	A Japanese spacecraft that collected and returned samples from asteroids Itokawa (2003-2010) and Ryugu (2014-2020).
Heat Shield	A protective covering that shields a spacecraft from intense heat during re-entry into Earth’s atmosphere. The Space Shuttle’s heat shield was made of thousands of heat-resistant tiles, while modern spacecraft like Orion and Dragon use a single ablative heat shield that burns away to absorb heat.
Hegemone	A small irregular moon of Jupiter, discovered in 2003. It is part of the Pasiphae group, a set of moons that orbit Jupiter in retrograde motion. Hegemone is about 3 km (2 miles) in diameter.
Helike	A small irregular moon of Jupiter, discovered in 2003. It is part of the Ananke group, which consists of moons that follow retrograde orbits. Helike has a diameter of about 4 km (2.5 miles).
Heliocentric	A system where the Sun is at the center. The Solar System is heliocentric, meaning all planets orbit the Sun. This idea, proposed by Nicolaus Copernicus, replaced the earlier belief that Earth was at the center of the universe (geocentric model).
Heliopause	The boundary where the solar wind from the Sun meets interstellar space. The Voyager probes crossed the heliopause, entering interstellar space.
Helium	The second-most common element in the universe after hydrogen. Helium is a key component of gas giants like Jupiter and Saturn. In stars, hydrogen is converted into helium through a process called nuclear fusion, which produces heat and light.
Hermippe	A small irregular moon of Jupiter, discovered in 2001. It is part of the Ananke group, a family of retrograde moons orbiting Jupiter. Hermippe has a diameter of about 4 km (2.5 miles).
Himalia	The largest irregular moon of Jupiter, discovered in 1904 by Charles Dillon Perrine. It is part of the Himalia group, a set of moons with prograde orbits (moving in the same direction as Jupiter’s rotation). Himalia is about 170 km (106 miles) in diameter.
Hippocamp	The smallest known moon of Neptune, discovered in 2013 from Hubble Space Telescope images taken between 2004 and 2009. It is an inner moon, about 34 km (21 miles) in diameter, and orbits 105,300 km (65,433 miles) from Neptune, completing an orbit in 23 hours.
Horsehead Nebula	A famous dark nebula in the constellation Orion, shaped like a horse's head.
Hubble Space Telescope	A space telescope launched in 1990, famous for capturing detailed images of galaxies, nebulae, and exoplanets.
Hydrogen	The most abundant element in the universe and the simplest. Stars in their early life are made up almost entirely of hydrogen, which is converted into helium through nuclear fusion, producing heat and light.
Hyperion	A large, irregularly shaped moon of Saturn, discovered in 1848. It has a spongy appearance due to its many craters.
Hypernova	An extremely powerful stellar explosion, producing even more energy than a supernova. Hypernovae are linked to black hole formation and gamma-ray bursts.
Hyrrokkin	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group of moons, which follow retrograde orbits. Hyrrokkin has a diameter of about 8 km (5 miles).

Word	Meaning
Iapetus	A large moon of Saturn, discovered in 1671 by Giovanni Cassini. It is famous for its two-tone coloration, with one side much darker than the other. Iapetus is about 1,470 km (913 miles) in diameter and orbits far from Saturn.
Ijiraq	A small irregular moon of Saturn, discovered in 2000. It is part of the Inuit group, a set of moons that have prograde orbits and are named after figures from Inuit mythology. Ijiraq is about 12 km (7.5 miles) in diameter.
Inclination	The angle of a planet or moon’s orbit compared to a reference plane, such as the Earth's equator or the Solar System’s plane.
Indian Space Research Organisation (ISRO)	India's space agency, responsible for launching satellites and missions like Chandrayaan (Moon missions) and Mangalyaan (Mars mission).
Infrared	A type of invisible light with longer wavelengths than visible red light. Infrared radiation can be felt as heat and is used in thermal imaging, astronomy, and remote controls. Infrared telescopes can see through dust clouds in space to reveal hidden stars and galaxies.
Inner Planets	The four planets closest to the Sun: Mercury, Venus, Earth, and Mars. They are the smallest planets in the Solar System and have rocky surfaces, so they are also called terrestrial planets. The Asteroid Belt separates them from the outer planets.
Interferometry	A technique where multiple telescopes work together to act as a much larger telescope, improving resolution.
International Space Station (ISS)	A space station orbiting Earth where astronauts live and conduct research in microgravity. Construction of the ISS began in 1998, and it became operational in 2000. It is a joint project involving NASA (United States), Roscosmos (Russia), ESA (Europe), JAXA (Japan), and CSA (Canada). The station orbits Earth about every 90 minutes and is expected to remain in operation until at least 2030.
Interstellar Medium	The gas, dust, and cosmic rays that fill the space between stars in a galaxy.
Io	The most volcanically active moon in the Solar System, orbiting Jupiter. Io was discovered by Galileo Galilei in 1610 and has hundreds of active volcanoes that constantly reshape its surface. It is about 3,643 km (2,264 miles) in diameter.
Iocaste	A small irregular moon of Jupiter, discovered in 2000. It is part of the Ananke group, a collection of moons with retrograde orbits (moving in the opposite direction to Jupiter’s rotation). Iocaste is about 5 km (3.1 miles) in diameter.
Ishtar Terra	A large highland region on Venus, similar in size to Australia.
Isonoe	A small irregular moon of Jupiter, discovered in 2000. It belongs to the Carme group, a family of moons that orbit Jupiter in a retrograde direction. Isonoe is about 4 km (2.5 miles) in diameter.

Word	Meaning
J002E3	An object that was temporarily captured by Earth's gravity in 2002. It was originally thought to be an asteroid, but later discovered to be a Saturn V rocket stage from the Apollo 12 mission.
James Webb Space Telescope (JWST)	A space telescope launched in 2021 as a successor to the Hubble Space Telescope. JWST observes space in infrared light, allowing it to study distant galaxies, exoplanets, and the early universe. It orbits the Lagrange Point 2 (L2), about 1.5 million km (930,000 miles) from Earth.
Janus	A small moon of Saturn, discovered in 1966. It shares an unusual orbit with another moon, Epimetheus, and the two swap places every few years. Janus is about 179 km (111 miles) across.
Jarnsaxa	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group of moons, which orbit Saturn in a retrograde motion (opposite to the planet’s rotation).
JAXA (Japan Aerospace Exploration Agency)	Japan's space agency, responsible for missions like Hayabusa (asteroid sample return) and Kaguya (Moon exploration).
Jet Propulsion Laboratory (JPL)	A research center in California, USA, managed by NASA and the California Institute of Technology (Caltech). JPL is responsible for many robotic space missions, including the Voyager probes, the Mars rovers, and the Cassini mission to Saturn.
Jovian Planets	The gas giant planets of the Solar System—Jupiter, Saturn, Uranus, and Neptune. They are called 'Jovian' because they are similar in composition to Jupiter. These planets have thick atmospheres, strong winds, and many moons.
Jovian System	The system of Jupiter and its moons, rings, and magnetic field.
Jules Verne ATV	A European robotic spacecraft launched in 2008 to deliver supplies to the ISS.
Juliet	A small inner moon of Uranus, discovered in 1986 by Voyager 2. It is part of the Portia group, a set of moons that share similar orbits and may be gradually breaking apart due to Uranus’s gravitational pull. Juliet has a diameter of about 94 km (58 miles) and orbits Uranus at 64,400 km (40,000 miles).
Juno (Spacecraft)	A NASA spacecraft that has been studying Jupiter since 2016. Juno's mission is to learn about Jupiter’s atmosphere, magnetic field, and deep interior. It takes detailed images of Jupiter’s storms, including the Great Red Spot.
Jupiter	The largest planet in the Solar System, the fifth from the Sun. Jupiter is a gas giant mostly made of hydrogen and helium. It has a strong magnetic field, the largest storm in the Solar System (the Great Red Spot), and at least 95 known moons, including the four large Galilean moons (Io, Europa, Ganymede, and Callisto).

Word	Meaning
Kale	A small irregular moon of Jupiter, discovered in 2001. It is part of the Carme group, a set of moons that orbit Jupiter in retrograde motion (opposite to Jupiter’s rotation). Kale has a diameter of about 2 km (1.2 miles).
Kallichore	A small irregular moon of Jupiter, discovered in 2003. It is part of the Carme group and follows a retrograde orbit. Kallichore has a diameter of about 2 km (1.2 miles).
Kalyke	A small irregular moon of Jupiter, discovered in 2000. It belongs to the Carme group and orbits in a retrograde motion. Kalyke is about 5 km (3.1 miles) in diameter and has a reddish surface, suggesting a possible link to captured asteroids.
Kari	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group, a set of moons that orbit Saturn in retrograde motion. Kari has a diameter of about 7 km (4.3 miles).
Kármán Line	The internationally recognised boundary between Earth's atmosphere and outer space, located at 100 km (62 miles) above sea level. This definition is used by the Fédération Aéronautique Internationale (FAI). However, NASA and the U.S. Air Force sometimes use a lower boundary of 80 km (50 miles) to define space.
Kelvin Scale	A temperature scale used in astronomy and physics, where 0 K (absolute zero) is the coldest possible temperature.
Kepler Space Telescope	A NASA space telescope launched in 2009 to search for exoplanets. It discovered thousands of planets before being retired in 2018.
Kepler-22b	An exoplanet in the habitable zone of its star, discovered by NASA’s Kepler Space Telescope in 2011. It is one of the first known Earth-like exoplanets.
Kepler’s Laws	Three laws of planetary motion, formulated by Johannes Kepler, describing how planets orbit the Sun in elliptical paths.
Kiviuq	An irregular moon of Saturn, discovered in 2000. It is part of the Inuit group, a set of prograde moons with distant, elliptical orbits. Kiviuq is about 16 km (10 miles) in diameter and may have a heavily cratered surface.
Kore	A small irregular moon of Jupiter, discovered in 2003. It is part of the Pasiphae group, a collection of moons that orbit Jupiter in retrograde motion. Kore has a diameter of about 2 km (1.2 miles).
Kosmos (Space Program)	A Soviet Union program that launched satellites and military spacecraft under the Kosmos designation starting in the 1960s.
Kuiper Belt	A region of icy objects beyond Neptune, including dwarf planets like Pluto, Haumea, and Makemake. It is the source of many comets.

Word	Meaning
Lagrange Points	Five regions in space where the gravitational pull of two large objects (such as the Sun and Earth) balances, allowing satellites to remain in a stable position.
Lander	A type of spacecraft designed to descend and land on a planet, moon, or asteroid. Landers often carry scientific instruments to study the surface. Some well-known landers include the Viking landers (Mars), Chang’e landers (Moon), and Philae (Comet 67P).
Laomedeia	A small irregular moon of Neptune, discovered in 2002. It has a diameter of 42 km (26 miles) and orbits Neptune at an average distance of 23.5 million km (14.6 million miles), taking 3,168 days (8.7 years) to complete one orbit. It is named after Laomedeia, one of the 50 Nereids in Greek mythology.
Larissa	The fourth-largest moon of Neptune, with a diameter of 194 km (121 miles). It orbits Neptune at 73,548 km (45,703 miles) and takes 13 hours to complete an orbit. It was first detected in 1981 and later imaged by Voyager 2 in 1989.
Leda	A small irregular moon of Jupiter, discovered in 1974 by Charles Kowal. It is part of the Himalia group, a set of moons with prograde orbits (moving in the same direction as Jupiter’s rotation). Leda is about 20 km (12 miles) in diameter.
Lenticular Galaxy	A type of galaxy that has characteristics of both spiral and elliptical galaxies, often lacking spiral arms.
Light	A form of electromagnetic radiation that is visible to the human eye. Light travels at 299,792 km per second (186,282 miles per second) in a vacuum. Different types of light exist beyond the visible spectrum, such as infrared, ultraviolet, X-rays, and gamma rays.
Light-Year	A unit of distance used in astronomy, equal to the distance light travels in one year - about 9.46 trillion km (5.88 trillion miles). Light-years are used to measure distances between stars and galaxies.
Loge	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group, a set of moons that orbit Saturn in retrograde motion (opposite to the planet’s rotation). Loge is about 6 km (3.7 miles) in diameter.
Luminosity	A measure of how much energy a star emits per second, determining its brightness.
Luna	The Latin name for Earth’s Moon, often used in scientific contexts.
Luna Program	A series of Soviet and Russian lunar missions initiated in the 1950s, achieving milestones like the first human-made object to reach the Moon (Luna 2, 1959) and the first successful lunar landing (Luna 9, 1966). After a hiatus post-1976, the program was revived with Luna 25 in 2023, aiming to explore the Moon's south pole. Despite Luna 25's crash, future missions (Luna 26, Luna 27) are planned to continue lunar exploration.
Lunar Module (LM)	The part of the Apollo spacecraft that landed on the Moon, carrying astronauts to and from the surface.
Lunar Reconnaissance Orbiter (LRO)	A NASA spacecraft launched in 2009 to map the Moon and study potential landing sites for future missions.
Lysithea	A small irregular moon of Jupiter, discovered in 1938 by Seth Barnes Nicholson. It is part of the Himalia group, a set of moons with prograde orbits. Lysithea has a diameter of about 36 km (22 miles) and orbits Jupiter at a distance of 11.7 million km (7.3 million miles).

Word	Meaning
"M" numbers	A catalogue system used to classify deep-sky objects, including galaxies, star clusters, and nebulae. The most famous is the Messier Catalog, compiled by French astronomer Charles Messier in the 18th century. It includes objects such as M31 (Andromeda Galaxy), M42 (Orion Nebula), and M45 (Pleiades).
Mab	A small inner moon of Uranus, discovered in 2003 using the Hubble Space Telescope. It orbits Uranus at a distance of about 97,700 km (60,700 miles) and is one of the faintest moons of the planet. Mab is thought to contribute dust to Uranus’s μ-ring.
Magnetic Field	A region around a planet, star, or other celestial body where magnetic forces influence charged particles. Earth's magnetic field protects us from the solar wind and creates the auroras (Northern and Southern Lights). Gas giants, stars, and some moons also have magnetic fields.
Magnetosphere	The region around a planet dominated by its magnetic field, protecting it from solar wind.
Magnitude	A measure of an object's brightness in the sky. The lower the magnitude number, the brighter the object. A star with a magnitude of 0 is brighter than one with a magnitude of +5. The Sun has a magnitude of -26.7, making it the brightest object in our sky.
Makemake	A dwarf planet in the Kuiper Belt, discovered in 2005.
Mantle	A layer of rock between a planet’s crust and core. The Earth's mantle is mostly solid, but convection within it drives plate tectonics. Other planets and moons, such as Mars, Venus, and Io, also have mantles.
Margaret	A small irregular moon of Uranus, discovered in 2003. It is part of the retrograde moons of Uranus, meaning it orbits in the opposite direction of Uranus’s rotation. Margaret is about 20 km (12 miles) in diameter.
Mars	The fourth planet from the Sun and the second smallest in the Solar System. Mars is known as the Red Planet due to iron-rich dust covering its surface. It has polar ice caps, massive canyons, and extinct volcanoes. Scientists believe Mars once had liquid water, and current missions are searching for signs of past or present life.
Mars Rover	A robotic vehicle that explores the surface of Mars, including Curiosity, Perseverance, and Opportunity.
Nevil Maskelyne	An 18th-century British astronomer (1732–1811) and the fifth Astronomer Royal of Britain. He contributed to accurate navigation at sea by improving methods for determining longitude. He was involved in testing John Harrison’s marine chronometers and helped establish astronomical observations for timekeeping. The Moon’s Maskelyne crater is named in his honor.
Mass	A measure of how much matter is in an object. Unlike weight, mass does not change based on location. An astronaut’s mass is the same on Earth and the Moon, but their weight is different due to gravity.
Matter	Anything that has mass and takes up space. Everything in the universe, from planets and stars to gases and dust, is made of matter. The three main states of matter are solid, liquid, and gas, with plasma being the fourth state found in stars.
Megaclite	A small irregular moon of Jupiter, discovered in 2000. It is part of the Pasiphae group, a set of moons that orbit Jupiter in retrograde motion. Megaclite is about 5 km (3.1 miles) in diameter.
Mercury	The closest planet to the Sun and the smallest in the Solar System. Mercury has no atmosphere, causing extreme temperature changes between day and night. It has a heavily cratered surface, similar to the Moon, and completes an orbit around the Sun in just 88 Earth days.
Mercury Program	NASA’s first human spaceflight program, sending the first Americans into space (1961–1963).
Messier Objects	A catalogue of galaxies, nebulae, and star clusters compiled by Charles Messier.
Meteor	A streak of light caused when a space rock burns up in Earth’s atmosphere. Often called a shooting star, meteors occur when meteoroids enter the atmosphere and heat up due to friction.
Meteorite	A meteoroid that survives its journey through Earth’s atmosphere and lands on the ground. Meteorites can provide clues about the early Solar System and are studied by scientists worldwide.
Meteoroid	A small rock or particle traveling through space, smaller than an asteroid but larger than a dust grain. If a meteoroid enters Earth’s atmosphere and burns up, it becomes a meteor (a 'shooting star'). If it survives the atmosphere and reaches the ground, it is called a meteorite.
Methone	A small, smooth moon of Saturn, discovered in 2004. It is one of Saturn’s 'egg-shaped' moons, with a diameter of about 3 km (1.9 miles). Unlike most moons, Methone has a surprisingly smooth surface without visible craters.
Metis	The innermost moon of Jupiter, discovered in 1979 by the Voyager 1 spacecraft. It orbits 128,000 km (79,500 miles) from Jupiter and is about 60 km (37 miles) in diameter. Metis helps maintain Jupiter's main ring by supplying dust from its surface.
Micrometeorite	A tiny dust-sized meteorite that falls to Earth without burning up.
Mimas	A moon of Saturn, discovered in 1789 by William Herschel. Mimas is about 396 km (246 miles) in diameter and is best known for its Herschel Crater, which makes it resemble the Death Star from Star Wars.
Miranda	A moon of Uranus, discovered in 1948 by Gerard Kuiper. It is one of the most geologically active moons in the Solar System, with giant canyons, cliffs, and strange surface features. Miranda is about 470 km (292 miles) in diameter.
Mneme	A small irregular moon of Jupiter, discovered in 2003. It is part of the Ananke group, a family of retrograde moons. Mneme has a diameter of about 2 km (1.2 miles).
Moon	Earth’s only natural satellite, formed about 4.5 billion years ago. The Moon has no atmosphere, meaning its surface remains unchanged by wind or weather. It influences Earth's tides and is the only celestial body humans have visited, with the Apollo 11 mission landing astronauts on the Moon in 1969.
Sir Patrick Moore	A British astronomer and broadcaster (1923–2012) known for popularising astronomy through television and books. He hosted The Sky at Night, the world’s longest-running space-focused TV program, for over 50 years. Moore contributed to lunar mapping, and his work was used by NASA during the Apollo program. In addition to astronomy, he was a talented xylophonist and composer, frequently performing on television. He also played the role of the GamesMaster on the 1990s British TV show GamesMaster, where he provided video game tips and challenges.
Mundilfari	A small irregular moon of Saturn, discovered in 2000. It is part of the Norse group, a set of retrograde moons. Mundilfari has a diameter of about 7 km (4.3 miles).

Word	Meaning
Naiad	The innermost moon of Neptune, discovered in 1989 by Voyager 2. Naiad orbits Neptune at a distance of 48,227 km (29,967 miles) and is just 67 km (42 miles) in diameter. It moves in an irregular, wavy orbit due to gravitational interactions with another moon, Thalassa.
Narvi	A small irregular moon of Saturn, discovered in 2003. It is part of the Norse group, a collection of moons that orbit Saturn in retrograde motion. Narvi is about 7 km (4.3 miles) in diameter.
NASA (National Aeronautics and Space Administration)	The United States space agency, responsible for missions like Apollo, the Space Shuttle, and Mars rovers.
Nebula (plural: nebulae)	A giant cloud of gas and dust in space. Nebulae can be the birthplaces of new stars (stellar nurseries) or the remnants of dead stars. Famous nebulae include the Orion Nebula (M42), the Crab Nebula, and the Helix Nebula.
Neptune	The eighth and farthest planet from the Sun in the Solar System. Neptune is a gas giant with a deep blue atmosphere, mostly made of hydrogen, helium, and methane, which gives it its color. It has strongest winds in the Solar System, reaching over 2,000 km/h (1,200 mph). Neptune was discovered in 1846, making it the only planet found through mathematical predictions rather than direct observation. It has 14 known moons, including Triton, which orbits in the opposite direction of Neptune’s rotation.
Nereid	Neptune's third-largest moon, discovered in 1949 by Gerard P. Kuiper. It is the sixth most distant moon of Neptune, orbiting at 5.5 million km (3.4 million miles) from the planet. Nereid has a highly eccentric (oval-shaped) orbit and is about 340 km (211 miles) in diameter.
Neso	The most distant known moon of Neptune, discovered in 2003 by Matthew J. Holman and Brett J. Gladman. It orbits Neptune at a record-breaking distance of 48.4 million km (30 million miles), making it the most distant moon of any planet in the Solar System. Neso is about 60 km (37 miles) in diameter.
Neutral Hydrogen	A type of cold, uncharged hydrogen gas found in interstellar space, used to map galaxies with radio telescopes.
Neutron Star	The collapsed core of a massive star that exploded as a supernova. Neutron stars are incredibly dense—a sugar-cube-sized piece would weigh billions of tons. Some neutron stars spin rapidly, emitting beams of radiation, and are known as pulsars.
New Horizons (Spacecraft)	A NASA spacecraft that flew past Pluto in 2015, providing the first close-up images of the dwarf planet.
Isaac Newton (1643–1727)	An English physicist, mathematician, and astronomer who formulated the three laws of motion and the law of universal gravitation, which explain how objects move and how gravity works. His book Principia Mathematica (1687) laid the foundation for classical mechanics and planetary motion. Newton also developed calculus and built the first reflecting telescope.
Newton’s Laws of Motion	Three laws, formulated by Isaac Newton, that describe how objects move and interact with forces.
Norma (Constellation)	A small southern hemisphere constellation near Scorpius and Ara.
Nova	A sudden brightening of a star when it temporarily increases in brightness due to a nuclear explosion on its surface.
Nuclear Energy	Energy released from the nucleus (center) of atoms, either by splitting atoms (nuclear fission) or combining them (nuclear fusion). Nuclear energy powers stars, nuclear reactors, and some spacecraft.
Nuclear Fusion	The process that powers stars, where lighter atoms (like hydrogen) fuse together to form heavier atoms (like helium), releasing enormous amounts of energy. The Sun converts 600 million tons of hydrogen into helium every second through nuclear fusion.
Nucleus (plural: Nuclei)	In astronomy, the solid core of a comet, made of ice, dust, and rock. When a comet approaches the Sun, its nucleus heats up, forming a glowing coma and tail. In physics, a nucleus is the central part of an atom, containing protons and neutrons.

Word	Meaning
Oberon	The second-largest moon of Uranus, discovered in 1787 by William Herschel.
Observatory	A facility used for observing celestial objects and space phenomena. Observatories can be ground-based (such as the Mauna Kea Observatory) or space-based (such as the Hubble Space Telescope). Some observatories use radio telescopes, while others focus on visible light, infrared, or X-ray astronomy.
Occultation	When one celestial object blocks another from view, such as the Moon passing in front of a star.
Oort Cloud	A hypothetical cloud of icy objects surrounding the Solar System, thought to be the source of long-period comets.
Open Cluster	A group of young stars formed from the same gas cloud, such as the Pleiades (M45).
Ophelia	A small inner moon of Uranus, discovered in 1986 by Voyager 2. It orbits close to Uranus at 53,800 km (33,400 miles) and has a diameter of about 43 km (27 miles). Ophelia is a shepherd moon, helping to maintain Uranus’s Epsilon ring.
Orbit	The curved path that an object follows around a larger body due to gravity. Planets orbit stars, moons orbit planets, and artificial satellites orbit Earth. Orbits can be circular or elliptical, depending on the object's speed and gravitational influences.
Orbiter	A spacecraft designed to enter and remain in orbit around a planet or moon, rather than landing. Orbiters can take high-resolution images, study atmospheres, and map planetary surfaces. Examples include the Mars Reconnaissance Orbiter (MRO) and the Cassini spacecraft around Saturn.
Orion (Spacecraft)	A NASA spacecraft designed for deep-space exploration, including future crewed missions to the Moon and Mars.
Orthosie	A small irregular moon of Jupiter, discovered in 2001. It is part of the Ananke group, a collection of moons that orbit Jupiter in retrograde motion (opposite to Jupiter’s rotation). Orthosie has a diameter of about 2 km (1.2 miles).
OSIRIS-REx	A NASA mission launched in 2016 to collect samples from the asteroid Bennu, returning to Earth in 2023.
Outer Planets	The four planets farthest from the Sun - Jupiter, Saturn, Uranus, and Neptune. These gas giants are the largest planets in the Solar System, made mostly of hydrogen and helium, with possible small solid or metallic cores. They are also referred to as Jovian planets.

Word	Meaning
Paaliaq	A small irregular moon of Saturn, discovered in 2000. It is part of the Inuit group, a set of moons that have prograde orbits. Paaliaq has a diameter of about 22 km (14 miles) and is thought to be composed of ice and rock.
Pallas	The second-largest asteroid in the asteroid belt, discovered in 1802.
Pallene	A small inner moon of Saturn, discovered in 2004 by the Cassini spacecraft. It is only 4 km (2.5 miles) in diameter and orbits between the larger moons Mimas and Enceladus. Pallene is thought to contribute dust to Saturn’s faint Pallene ring.
Pan	The innermost moon of Saturn, discovered in 1990 using images from Voyager 2. It orbits within the Encke Gap of Saturn’s rings and helps keep the gap clear. Pan has a distinctive 'flying saucer' shape due to material accumulating along its equator.
Pandora	A moon of Saturn, discovered in 1980 by the Voyager 1 spacecraft. It is a shepherd moon of Saturn’s F ring, helping to keep the ring in place through its gravitational influence. Pandora is about 84 km (52 miles) in diameter.
Parallax	The apparent shift in position of a nearby star when viewed from different points in Earth's orbit. Used to measure distances in space.
Parker Solar Probe	A NASA spacecraft launched in 2018 to study the Sun’s corona by flying closer to the Sun than any previous probe.
Particle	A tiny fragment of matter, such as dust, ice, or gas. Space is filled with particles, from the fine dust in planetary rings to charged particles in the solar wind.
Pasiphae	An irregular moon of Jupiter, discovered in 1908. It is the largest member of the Pasiphae group, a collection of moons that orbit Jupiter in retrograde motion. Pasiphae has a diameter of about 58 km (36 miles) and may have originated from a captured asteroid.
Pasithee	A small irregular moon of Jupiter, discovered in 2001. It is part of the Carme group, a set of moons that orbit Jupiter in retrograde motion. Pasithee has a diameter of about 2 km (1.2 miles).
Penumbra	The lighter outer part of a shadow in an eclipse, where only part of the Sun or Moon is blocked.
Perdita	A small inner moon of Uranus, first photographed by Voyager 2 in 1986 but not confirmed until 1999. It orbits Uranus at 76,400 km (47,500 miles) and is about 30 km (19 miles) in diameter.
Perihelion	The closest distance that an object like a planet, asteroid or comet gets to the Sun while orbiting it. Most objects in the solar system orbit in eccentic orbits, so there is a difference between their closest point to the Sun and their furthest point. Earth for example gets as close as 147 million kilometres to the Sun. This is its perihelion. An object's most distant distance from the Sun is called its aphelion. Earth's aphelion is around 152 million kilometres.
Periodic Comet	A comet that follows a regular, repeating orbit around the Sun, returning at predictable intervals. Well-known periodic comets include Halley’s Comet (returns every 76 years) and Encke’s Comet (returns every 3.3 years).
Philae Lander	A lander from the Rosetta mission that landed on a comet (67P/Churyumov–Gerasimenko) in 2014.
Phobos	The larger and innermost of Mars' two moons, discovered in 1877 by Asaph Hall. Phobos is highly irregular in shape and orbits extremely close to Mars, completing an orbit in just 7 hours and 39 minutes. It is slowly spiraling toward Mars and may eventually crash into the planet or break apart.
Phoebe	An irregular moon of Saturn, discovered in 1899. Phoebe orbits farther from Saturn than any major moon and has a retrograde orbit, suggesting it may be a captured object from the Kuiper Belt. It is about 213 km (132 miles) in diameter.
Phoenicis Lacus	A large dark region on Mars, thought to be composed of basaltic rock or ancient lava flows.
Photosphere	The visible surface of the Sun (or another star). The photosphere is where sunspots and solar flares originate. It has a temperature of about 5,500°C (9,932°F).
Pioneer (Spacecraft)	A series of NASA probes that explored the outer Solar System in the 1970s, including Pioneer 10 & 11, which were the first spacecraft to travel beyond Pluto.
Planet	A large, spherical object that orbits a star. Planets can be rocky (like Earth and Mars) or gaseous (like Jupiter and Saturn). In 2006, the International Astronomical Union (IAU) defined a planet as an object that orbits the Sun, is round due to gravity, and has cleared its orbit of debris.
Planetary Nebula	A glowing shell of gas ejected by a dying star. Despite their name, planetary nebulae have nothing to do with planets—they were named by early astronomers because of their round shape. Famous planetary nebulae include the Ring Nebula (M57) and the Helix Nebula.
Planetesimal	A small, solid object that formed in the early Solar System and contributed to the growth of planets. Planetesimals are thought to have clumped together over time, forming protoplanets, and later, full-sized planets.
Plasma	A state of matter found in stars and the Sun, made of hot, ionized gas where electrons are free-moving.
Pleiades	A famous open star cluster also known as the Seven Sisters, located in the constellation Taurus.
Pluto	A dwarf planet in the Kuiper Belt, discovered in 1930. Pluto was classified as the ninth planet until 2006, when it was redefined as a dwarf planet due to its inability to clear its orbit. Pluto has five known moons, including its largest, Charon. It takes 248 years to complete one orbit around the Sun.
Poles	The north and south points on a planet or moon where the rotation axis meets the surface. Many planets have polar ice caps, including Earth, Mars, and Pluto.
Polydeuces	A small moon of Saturn, discovered in 2004. It is a Trojan moon of Dione, meaning it shares Dione’s orbit but stays 60 degrees ahead or behind due to gravitational balance. Polydeuces is about 3 km (1.9 miles) in diameter.
Portia	A moon of Uranus, discovered in 1986 by Voyager 2. It is part of Uranus’s inner moons and orbits at 66,100 km (41,000 miles). Portia is about 140 km (87 miles) in diameter.
Praxidike	A small irregular moon of Jupiter, discovered in 2000. It is part of the Ananke group, which consists of retrograde moons. Praxidike is about 7 km (4.3 miles) in diameter.
Project Mercury	NASA’s first human spaceflight program (1958-1963), which sent the first Americans into space before Apollo.
Prometheus	A shepherd moon of Saturn, discovered in 1980 by Voyager 1. It orbits just inside Saturn’s F ring and helps shape and maintain the ring's structure by pulling on ring particles with its gravity. Prometheus is elongated, about 119 km (74 miles) long and 87 km (54 miles) wide.
Prospero	A small irregular moon of Uranus, discovered in 1999. It follows a retrograde orbit and is about 50 km (31 miles) in diameter.
Proteus	Neptune's second largest moon. In was discovered in July 1989 by Voyager 2. Its diameter is about 420 km (261 miles) and it takes 27 hours to orbit Neptune. It is situated about 117,647 km (73,106 miles) from Neptune
Protoplanet	A large body of rock and gas that forms in the early stages of a solar system. Protoplanets grow by colliding and merging with other objects, eventually becoming full-sized planets. Earth, Mars, and other planets likely started as protoplanets billions of years ago.
Protostar	A young star in the early stages of formation, before nuclear fusion begins.
Proxima Centauri	The closest known star to the Sun, about 4.24 light-years away.
Psamathe	Moon of Neptune, discovered in 2003. It has a diameter of 38 km (24 miles) and orbits at an average distance of 46.7 million km (29 million miles). One orbit takes 9,117 days. It is named after a Nereid, a sea nymph. Psamathe in mythology was the goddess of sand.
Puck	A moon of Uranus, discovered in 1985 by Voyager 2. It is the largest of Uranus’s inner moons, with a diameter of 162 km (101 miles). Puck’s surface is heavily cratered and appears to be covered in dark material.

Word	Meaning
Quadrantid Meteor Shower	An annual meteor shower that peaks in early January, producing bright meteors. It originates from debris left behind by an asteroid or comet and appears to radiate from the constellation Boötes.
Quantum Mechanics	A branch of physics that explains how particles behave at the smallest scales. Quantum mechanics helps scientists understand the behavior of light, atoms, and even the energy inside stars and black holes.
Quaoar	A large Kuiper Belt Object (KBO) and potential dwarf planet, discovered in 2002. It is about 1,100 km (680 miles) in diameter, making it one of the largest objects in the Kuiper Belt after Pluto, Eris, and Makemake. Quaoar has a small moon named Weywot.
Quasar	A massive, extremely bright object powered by a supermassive black hole at the center of a distant galaxy. Quasars emit huge amounts of energy, often outshining entire galaxies. They are among the most distant objects we can observe, providing clues about the early universe.
Queen’s Wreath Nebula	A faint nebula in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. It is a star-forming region where new stars are being born.

Word	Meaning
Radiation	Energy that travels through space as waves or particles. Radiation can come from stars, radioactive materials, or cosmic sources. Types of radiation include radio waves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Radiation Belt	A zone of high-energy charged particles trapped by a planet’s magnetic field. Earth has two major radiation belts, called the Van Allen belts.
Radio Waves	A type of electromagnetic radiation with the longest wavelengths and lowest energy. Radio waves are used in communication (radio, TV, cell phones) and for astronomical observations through radio telescopes.
Ranger Program	A series of NASA probes in the 1960s that took the first close-up images of the Moon before crashing into its surface.
Re-entry	The process of a spacecraft returning to Earth's atmosphere (or another planet’s atmosphere). During re-entry, a spacecraft experiences extreme heat and friction, requiring heat shields to protect it.
Red Giant	A large, aging star that has expanded and cooled after burning most of its hydrogen. The Sun will become a red giant in about 5 billion years.
Redshift	The stretching of light waves as an object moves away from Earth, making them appear redder. Used to measure the expansion of the universe.
Rhea	The second-largest moon of Saturn, discovered in 1672 by Giovanni Cassini. Rhea is about 1,527 km (949 miles) in diameter and has a heavily cratered surface with thin rings or dust clouds surrounding it.
Ring System	A collection of ice, dust, and rock particles that orbit a planet, such as Saturn’s spectacular rings or the faint rings around Jupiter, Uranus, and Neptune.
Roche Limit	The minimum distance a moon or satellite can orbit a planet before its gravity pulls it apart due to tidal forces.
Rocket	A vehicle that uses propulsion from controlled explosions to travel into space. Rockets are used for launching satellites, astronauts, and probes
Rosalind	A small inner moon of Uranus, discovered in 1986 by Voyager 2. Rosalind orbits at 69,900 km (43,400 miles) from Uranus and is about 72 km (45 miles) in diameter. It is named after a character from Shakespeare’s play As You Like It.
Rosetta (Spacecraft)	A European Space Agency (ESA) spacecraft that orbited Comet 67P/Churyumov-Gerasimenko and deployed the Philae lander in 2014.
Rupes (Lunar/Martian)	A long cliff or escarpment found on the Moon, Mars, and Mercury. Examples include Rupes Recta on the Moon.

Word	Meaning
Sao	Moon of Neptune, discovered in 2002. It has a diameter of 44 km (27 miles) and orbits at a distance of 22.4 million km (13.9 million miles) from Neptune, taking 2,914 days to complete an orbit. It is named after a Nereid, a sea nymph in Greek mythology. Sao helped sailors to navigate stormy seas safely, or rescue those that weren't so lucky.
Satellite	An object that orbits a planet, star, or other celestial body. Satellites can be natural (like the Moon) or artificial (like the Hubble Space Telescope). Artificial satellites are used for communication, weather monitoring, and space exploration.
Saturn	The sixth planet from the Sun and the second-largest in the Solar System. Saturn is a gas giant best known for its spectacular ring system, made of ice and rock. It has over 140 known moons, including Titan, the largest moon with a thick atmosphere. Saturn takes 29.5 Earth years to orbit the Sun.
Walter Schirra	An American astronaut (1923–2007) and the only person to fly in Mercury, Gemini, and Apollo missions. He commanded Apollo 7 (1968), the first crewed Apollo mission, which tested spacecraft systems in Earth orbit.
David Scott	An American astronaut who flew on Gemini 8 (1966), Apollo 9 (1969), and Apollo 15 (1971). He was the seventh person to walk on the Moon and the first to drive the Lunar Roving Vehicle during Apollo 15.
Sedna	A distant, icy object in the outer Solar System, discovered in 2003. It is a possible dwarf planet, about 1,000 km (620 miles) in diameter. Sedna has one of the longest orbits, taking 11,400 years to circle the Sun.
Setebos	A small irregular moon of Uranus, discovered in 1999. It orbits Uranus at a far distance, following a retrograde motion. Setebos is about 48 km (30 miles) in diameter.
Shooting Star	A bright streak of light in the sky caused when a meteoroid burns up as it enters Earth's atmosphere. Shooting stars are also called meteors and are commonly seen during meteor showers.
Siarnaq	A large irregular moon of Saturn, discovered in 2000. It is the largest member of the Inuit group, a set of moons that orbit in prograde motion. Siarnaq has a reddish color and is about 40 km (25 miles) in diameter.
Singularity	A point of infinite density, such as the center of a black hole, where the laws of physics break down.
Sinope	A small irregular moon of Jupiter, discovered in 1914. It is part of the Pasiphae group, a set of moons that orbit Jupiter in retrograde motion. Sinope has a diameter of about 38 km (24 miles).
Skathi	A small irregular moon of Saturn, discovered in 2000. It is part of the Norse group, a collection of moons that orbit Saturn in retrograde motion. Skathi has a diameter of about 8 km (5 miles).
Skoll	A small irregular moon of Saturn, discovered in 2006. Like Skathi, it is part of the Norse group and follows a retrograde orbit. Skoll is about 6 km (3.7 miles) in diameter.
Skylab	America’s first space station, launched in 1973 and occupied by astronauts for science experiments.
Solar Constant	The amount of solar energy received per second on a square meter of Earth’s upper atmosphere.
Solar Eclipse	An event that occurs when the Moon passes between the Sun and Earth, blocking some or all of the Sun’s light. A total solar eclipse happens when the Sun is completely obscured, while a partial or annular eclipse occurs when only part of the Sun is blocked.
Solar Flare	A sudden explosion of energy from the Sun’s surface, releasing radiation and charged particles into space. Solar flares can disrupt satellites, power grids, and communications on Earth.
Solar System	The name given to the Sun and its family of planets and dwarf planets (plus their moons), comets and asteroids. The Sun is at the centre of the Solar System with all of the other objects held in orbit of it by the Sun's gravitational pull. Earth is one of the planets in the Solar System, and the only one known to have life. The Solar System is considered to end when the Sun's gravity no longer has an effect. Other stars also have objects like planets and comets orbiting them, so can also be considered to be solar systems.
Solar Wind	A stream of charged particles released by the Sun’s upper atmosphere. The solar wind interacts with Earth's magnetic field, creating the auroras (Northern and Southern Lights).
Space	The vast, seemingly infinite expanse beyond Earth's atmosphere. Space contains planets, stars, galaxies, and cosmic dust. It begins at the Kármán line (100 km or 62 miles above Earth) and is mostly a vacuum, with some particles and radiation.
Space Probe	A type of uncrewed spacecraft designed for exploring planets, moons, asteroids, and comets. Space probes send back data about temperature, atmosphere, surface conditions, and magnetic fields. Examples include Voyager 1 & 2, New Horizons, and the Parker Solar Probe.
Spacecraft	A vehicle designed for travel and operations in space. Spacecraft can be crewed (like the International Space Station) or uncrewed (like the Hubble Space Telescope). They can orbit planets, land on surfaces, or explore deep space.
Spacewalk	An extravehicular activity (EVA) performed by an astronaut outside a spacecraft. The first spacewalk was done by Alexei Leonov in 1965, and NASA’s first was by Ed White in 1965.
Spectroscopy	The study of light and radiation to determine the composition and movement of celestial objects.
Spicule	A thin jet of hot gas that rises from the Sun’s surface into its chromosphere. Spicules are short-lived and can reach speeds of 20 km per second (12 miles per second).
Spiral Galaxy	A galaxy with a rotating disk of stars, gas, and dust, with spiral arms. The Milky Way is a spiral galaxy.
Sponde	A small irregular moon of Jupiter, discovered in 2001. It belongs to the Ananke group, a set of moons that orbit Jupiter in retrograde motion. Sponde has a diameter of about 2 km (1.2 miles).
Sputnik Program	A series of Soviet space missions, beginning with Sputnik 1 in 1957, which became the first artificial satellite to orbit Earth. The Sputnik 2 mission carried Laika, the first living being in space. The program paved the way for human spaceflight.
Star	A massive ball of hot gas, primarily hydrogen and helium, held together by gravity. Stars produce energy through nuclear fusion. The closest star to Earth is the Sun, while distant stars form galaxies.
Stephano	A small irregular moon of Uranus, discovered in 1999. It follows a retrograde orbit and is about 32 km (20 miles) in diameter. Stephano may have originated from a captured asteroid.
Summer Solstice	The longest day of the year, occurring in June in the Northern Hemisphere and December in the Southern Hemisphere. It happens when Earth's axial tilt is most directly facing the Sun, leading to the most daylight hours in a single day.
Sun	The star at the centre of the Solar System, made mostly of hydrogen and helium. The Sun is about 4.6 billion years old and provides heat, light, and energy to Earth. It will eventually become a red giant before shrinking into a white dwarf.
Sunspot	A dark, cooler region on the Sun’s surface, caused by intense magnetic activity. Sunspots appear darker because they are thousands of degrees cooler than the surrounding areas.
Supernova	The explosion of a massive star at the end of its life. A supernova can outshine an entire galaxy and spread elements like iron and gold into space, helping to form new stars and planets.
Surtur	A small irregular moon of Saturn, discovered in 2006. It is part of the Norse group and follows a retrograde orbit. Surtur has a diameter of about 6 km (3.7 miles).
Suttungr	A small irregular moon of Saturn, discovered in 2000. It orbits in a retrograde direction and is part of the Norse group. Suttungr is about 7 km (4.3 miles) in diameter.
Sycorax	One of the largest irregular moons of Uranus, discovered in 1997. It follows a retrograde orbit and is about 165 km (103 miles) in diameter. Sycorax is named after a character from Shakespeare’s The Tempest.
Synchronous Rotation	When a moon’s rotation period matches its orbit, causing the same side to always face its planet (like Earth’s Moon).

Word	Meaning
T Coronae Borealis (T CrB)	A recurrent nova in the constellation Corona Borealis, expected to explode again between 2024 and 2026.
Tarqeq	A small irregular moon of Saturn, discovered in 2007. It is part of the Inuit group, a set of prograde moons that have similar orbits. Tarqeq has a diameter of about 7 km (4.3 miles) and takes 894 Earth days to complete one orbit of Saturn.
Tarvos	A small irregular moon of Saturn, discovered in 2000. It is part of the Gallic group, a set of moons with similar orbits. Tarvos has a diameter of about 15 km (9.3 miles) and follows a highly elliptical orbit around Saturn.
Taygete	A small irregular moon of Jupiter, discovered in 2000. It is part of the Carme group, a set of moons that orbit in retrograde motion. Taygete has a diameter of about 5 km (3.1 miles).
Telescope	An instrument used to magnify distant objects in space. The first telescopes were developed in the Netherlands in 1608, and Galileo Galilei used one in 1610 to discover the moons of Jupiter and Saturn’s rings. Telescopes use mirrors and lenses to collect and focus light. Space telescopes like Hubble and James Webb provide clearer views because they avoid Earth's atmosphere.
Telesto	A small moon of Saturn, discovered in 1980. It is one of Saturn’s Trojan moons, meaning it shares its orbit with Tethys. Telesto has a diameter of about 25 km (16 miles) and orbits Saturn at 295,000 km (183,000 miles).
Terrestrial Planets	The four rocky planets closest to the Sun - Mercury, Venus, Earth, and Mars. These planets have solid surfaces, unlike the gas giants. They are also called the inner planets and are separated from the outer planets by the Asteroid Belt.
TESS (Transiting Exoplanet Survey Satellite)	A NASA mission launched in 2018 to find exoplanets around nearby stars.
Tethys	A large moon of Saturn, discovered in 1684 by Giovanni Cassini. Tethys has a diameter of 1,062 km (660 miles) and is mostly water ice. Its surface features include the Odysseus Crater and the Ithaca Chasma, a massive canyon system.
Thalassa	The second-closest moon of Neptune, discovered in 1989 by Voyager 2. It has a highly irregular shape and is about 83 km (52 miles) in diameter. Thalassa orbits Neptune at 50,075 km (31,115 miles) and may be a fragment of a larger moon that broke apart.
Thebe	The fourth-largest moon of Jupiter’s inner moons, discovered in 1979 by Voyager 1. Thebe is about 98 km (61 miles) across and orbits Jupiter at 222,000 km (138,000 miles). Its surface is heavily cratered due to Jupiter’s strong gravitational pull.
Thelxinoe	A small irregular moon of Jupiter, discovered in 2003. It is part of the Ananke group, which consists of moons in retrograde orbits. Thelxinoe is about 2 km (1.2 miles) in diameter.
Themisto	A small moon of Jupiter, discovered in 1975 and later rediscovered in 2000. Unlike most of Jupiter’s irregular moons, Themisto does not belong to a specific moon group. It has a diameter of 9 km (5.6 miles) and orbits Jupiter between the Galilean moons and the smaller irregular moons.
Thrust	The force that moves a spacecraft or rocket forward. Thrust is created when a rocket expels exhaust gases in the opposite direction. More thrust is needed to escape a planet’s gravity.
Thruster	A small engine used to control a spacecraft’s movement. Thrusters adjust a spacecraft’s position, speed, and orientation in space. They are often powered by chemical fuels, ion propulsion, or cold gas.
Thrymr	A small irregular moon of Saturn, discovered in 2000. It is part of the Norse group and follows a retrograde orbit. Thrymr has a diameter of about 7 km (4.3 miles).
Thyone	A small irregular moon of Jupiter, discovered in 2001. It is part of the Ananke group, a set of retrograde moons. Thyone has a diameter of about 4 km (2.5 miles).
Titan	The largest moon of Saturn, discovered in 1655 by Christiaan Huygens. Titan has a thick atmosphere rich in nitrogen and methane, and it is the only moon in the Solar System with liquid lakes and rivers (made of methane and ethane). Titan is larger than Mercury, with a diameter of 5,150 km (3,200 miles).
Titania	The largest moon of Uranus, discovered in 1787 by William Herschel. It has a diameter of 1,578 km (980 miles) and features deep canyons and fault lines, suggesting past geological activity.
Trans-Neptunian Object (TNO)	Any object that orbits the Sun beyond Neptune, including Pluto, Eris, and Sedna.
Trinculo	A small irregular moon of Uranus, discovered in 2001. It follows a retrograde orbit and is about 18 km (11 miles) in diameter.
Triton	The largest moon of Neptune, discovered in 1846 by William Lassell. Triton is unique because it orbits Neptune in a retrograde motion, meaning it moves in the opposite direction of Neptune's rotation. It has geysers of liquid nitrogen, suggesting active geology. Triton has a diameter of 2,707 km (1,681 miles) and is likely a captured Kuiper Belt Object.
Konstantin Tsiolkovsky	A Russian scientist and one of the founders of modern rocketry.

Word	Meaning
UFO (Unidentified Flying Object)	A term for any unexplained aerial phenomenon, often linked to theories about alien life but usually explained by natural or human-made objects.
UK Space Agency	The national space agency of the United Kingdom, established in 2010 to coordinate space exploration, satellite technology, and commercial spaceflight. The agency supports scientific missions, UK involvement in ESA programs, and projects like Mars exploration, Earth observation, and spaceports in the UK.
Ultraviolet (UV)	A type of electromagnetic radiation with shorter wavelengths than visible light but longer than X-rays. The Sun emits ultraviolet light, which can cause sunburns and is absorbed by Earth’s ozone layer. UV telescopes, like those on the Hubble Space Telescope, help study hot stars and galaxies.
Ulysses (Spacecraft)	A NASA & ESA mission (1990–2009) that studied the Sun’s poles, providing valuable data about the solar wind and magnetic field.
Umbriel	A large moon of Uranus, discovered in 1851 by William Lassell. Umbriel has a dark, heavily cratered surface, with a bright ring-shaped feature called Wunda. It is about 1,169 km (726 miles) in diameter and takes 4.1 days to orbit Uranus.
Universal Time (UT)	A timekeeping system based on Earth’s rotation, used as the standard in astronomy and global timekeeping. Universal Time is effectively the same as Greenwich Mean Time (GMT) but is more precise and used in scientific contexts.
Universe	Everything that exists, including all matter, energy, space, and time. The observable universe extends about 93 billion light-years across.
Upper Stage (Rocket)	The final stage of a rocket, responsible for placing satellites or probes into their final orbit or sending them into deep space.
Uranian Moons	The moons of Uranus, which are named after Shakespearean and Alexander Pope characters instead of mythological deities (e.g., Titania, Oberon, and Miranda).
Uranus	The seventh planet from the Sun, classified as an ice giant. Uranus has a pale blue-green color due to methane gas in its atmosphere and is the only planet that rotates on its side. It has a system of thin rings and at least 27 known moons, including Titania, Oberon, and Miranda. Uranus was discovered in 1781 by William Herschel.

Word	Meaning
Van Allen Radiation Belt	A region around Earth filled with charged particles, trapped by Earth's magnetic field. It was discovered by the Explorer 1 spacecraft in 1958.
Vanishing Point (Astronomical Perspective)	The point at which parallel lines appear to converge in an image of the sky, commonly used in astrophotography.
Variable Star	A star whose brightness changes over time, due to pulsations, eclipses, or other processes. Examples include Cepheid variables and Mira variables.
Venera Program	A series of Soviet space missions (1961–1984) designed to explore Venus. Several Venera spacecraft successfully landed on Venus, making them the first probes to return images from the surface of another planet. Due to Venus’ extreme heat and pressure, landers survived for only minutes to hours before being destroyed.
Venus	The second planet from the Sun, similar in size to Earth but with a thick, toxic atmosphere made mostly of carbon dioxide. Venus experiences an extreme greenhouse effect, making it the hottest planet in the Solar System, with surface temperatures reaching 465°C (869°F)—hot enough to melt lead. Its surface is covered with volcanoes and vast plains. Venus has no moons and rotates in the opposite direction to most planets.
Vesta (Asteroid)	One of the largest objects in the asteroid belt, about 525 km (326 miles) across. It was visited by NASA's Dawn spacecraft in 2011.
Viking Program	A NASA Mars exploration program (1970s) that sent the first successful landers to Mars, conducting experiments for signs of life.
Virgin Galactic	A commercial spaceflight company aiming to make space tourism possible using the VSS Unity spaceplane.
Volcano	A mountain or vent where magma (molten rock), gas, and ash erupt from beneath a planet’s surface. Many planets and moons have volcanoes, including Venus, Mars, and Io (a moon of Jupiter). Some, like Olympus Mons on Mars, are much larger than those on Earth.
Voyager Program	A NASA mission that launched Voyager 1 & 2 in 1977, exploring Jupiter, Saturn, Uranus, and Neptune before heading into interstellar space.
Vulcanoid Asteroids	A hypothetical group of small asteroids that might exist between Mercury and the Sun, but none have been confirmed.

Word	Meaning
Waning	A term used to describe the Moon’s phases when it appears to shrink in size after a full moon. The waning phases include Waning Gibbous (after the full moon), Last Quarter (half-lit) and Waning Crescent (thin sliver before new moon). Once the Moon reaches the new moon phase, it starts waxing again.
Waxing	A term used to describe the Moon’s phases when it appears to shrink in grow in size after a new moon. The waxing phases include Waxing Crescent thin sliver after new moon), First Quarter (half-lit) and Waxing Gibbous (almost full). Once the Moon reaches the full moon phase, it starts waning again.
Weak Force	One of the four fundamental forces of nature, responsible for radioactive decay and nuclear reactions inside stars.
James E. Webb	An American government official (1906–1992) who served as NASA Administrator from 1961 to 1968 during the Apollo program. Under his leadership, NASA expanded space exploration, launching crewless Apollo test missions and laying the groundwork for the first Moon landing. The James Webb Space Telescope (JWST) was named in his honour.
WFIRST (Now Roman Space Telescope)	A NASA space telescope planned to launch in the mid-2020s, designed to study dark energy and exoplanets.
White Dwarf	A small, hot, and dense star that is the remnant of a sun-like star after it has exhausted its nuclear fuel. White dwarfs are the final stage of a star’s life cycle, following the red giant phase. The Sun will eventually become a white dwarf in about 5 billion years.
Wilkins Ice Shelf	An ice shelf in Antarctica that is being studied for its links to climate change and its potential similarities to icy moons like Europa.
Winter Solstice	The shortest day of the year, occurring in December in the Northern Hemisphere and June in the Southern Hemisphere.
Wolf-Rayet Star	A massive, extremely hot star nearing the end of its life, shedding large amounts of material into space.
Wright Flyer (Moon Crater)	A crater on the Moon named after the Wright brothers, who invented the first successful aeroplane.

Word	Meaning
X-ray	A type of electromagnetic radiation with shorter wavelengths than ultraviolet light but longer than gamma rays. The Sun, black holes, and neutron stars emit X-rays, which can be detected by space telescopes like Chandra X-ray Observatory.
X-ray Binary	A system where a neutron star or black hole pulls material from a companion star, emitting powerful X-rays.
Xena	The nickname for the dwarf planet Eris before it was officially named in 2006. Discovered in 2005, Eris was temporarily called Xena, inspired by the TV show Xena: Warrior Princess. The discovery of Eris contributed to the reclassification of Pluto as a dwarf planet.
XMM-Newton	An ESA X-ray telescope launched in 1999 to study black holes, neutron stars, and galaxies.
Year	The length of time it takes for a planet to complete one orbit around its star. For Earth, this is 365.25 days. Other planets have different year lengths�Mercury's year is just 88 days, while Neptune�s year lasts 165 Earth years.
Yellow Dwarf star	A G-type main sequence star, like the Sun. These stars convert hydrogen into helium through nuclear fusion, producing heat and light. Despite the name, yellow dwarfs appear white in space; Earth's atmosphere makes the Sun look yellow. They typically last 10 billion years before expanding into red giants and later shrinking into white dwarfs.
Ymir	A small irregular moon of Saturn, discovered in 2000. Ymir orbits Saturn at a far distance and takes more than 3.5 Earth years to complete one orbit. It belongs to the Norse group of moons, following a retrograde orbit (moving opposite Saturn�s rotation). Named after Ymir, a giant from Norse mythology.
Zarya	The first module of the International Space Station (ISS), launched in 1998 as part of the Russian segment.
Zenith	Look up, straight up. The point in the sky directly above you is the zenith.
Zero Gravity (Weightlessness)	The sensation of floating in space due to freefall, experienced by astronauts in orbit.
Zodiac	The part of the sky containing the 12 zodiac constellations, through which the Sun appears to move during the year. This apparent motion is caused by Earth�s orbit around the Sun. The 12 zodiac constellations are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. While astrology claims these constellations influence personality and destiny, astronomy treats them as part of the celestial sphere with no supernatural influence.
Z�llner, Johann Karl Friedrich	A German astrophysicist (1834�1882) who made the first measurement of the Sun�s apparent magnitude (missing the true value by just 0.1 magnitude). He also studied optical illusions, the paranormal, and photometry (the measurement of light). A crater on the Moon is named in his honor.
Zwicky Transient Facility (ZTF)	A sky survey that detects supernovae, asteroids, and other transient events in space.
Zwicky, Fritz	A Swiss astronomer (1898�1974) known for his work at Caltech. He and Walter Baade coined the term supernova to describe the explosion of massive stars. Zwicky also predicted dark matter�s existence and cataloged thousands of galaxies. He has both a Moon crater and an asteroid (1803 Zwicky) named in his honor.

Pick a Letter!

XYZ

Show All

Hide All