Mercury, named after the Roman messenger to the gods, is the closest planet to the Sun and the smallest traditional planet in the Solar System. It orbits the Sun in 88 days, meaning that for every year on Earth, four years have passed on Mercury. However, because it takes 59 days for Mercury spin once on its axis (one Mercurian day), it takes 176 days for the Sun to completely rise and set over the planet's horizon, compared with the 24 hours it takes for the same thing to happen on Earth. It is because of Mercury's speed across the face of the Sun that it is known as the "Messenger" planet. It's Greek equivalent is Hermes.
Mercury is so small and so close to the Sun that it is almost impossible to see. With telescopes, just after dawn or before sunset, the planet can be seen as a small spot in front of the Sun surface. Hundreds of years ago, astronomers thought it was two separate objects as it could be seen twice a day. Its "morning" name was Apollo.
Mercury's surface looks very similar to the Moon's. It is covered in craters, although does not have the large dark 'oceans' that we see on the Moon's surface. This is because the planet has no atmosphere or air. If small rocks (asteroids and meteors) head for Earth, most burn up in the planet's atmosphere and do not hit the surface. But, because Mercury and the Moon don't have atmospheres (explaining why their skies are always black), objects heading for them hit the surface and leave craters.
Life on Mercury is impossible. Temperatures on the planet can be extremely hot or extremely cold depending on which side of it is facing the Sun. On its sunlit side, temperatures reach 427 °C (800 °F), and on its night side, temperatures plummet to -183 °C (-297 °F). Mercury spins on its axis very slowly, so one side of the planet can be in complete darkness for weeks. The planet is very heavy for its size. Astronomers believe that it has a large iron core which may explain why the planet weighs so much. Earth is a geologically living planet. Its continents still move, and hot lava is churned from below its surface through volcanic eruptions. Mercury is geologically dead. The surface shows no sign of current volcanic activity or any other form of geological activity. It is for this reason that astronomers believe Mercury's core to be extremely cold.
Mercury is one of only two planets in the Solar System not to have a moon. The other moonless planet is Venus. Mercury has been visited a couple of times by spacecraft. Its first visit was Mariner 10 which launched in 1973. Mariner 10 took in the sights and sounds of Venus first before going on to visit Mercury in early 1974. Its only other visitor is Messenger which launched in 2004 and made its first flyby of Mercury in January 2008 (it too spent some time around Venus beforehand). It entered an orbit of Mercury in March 2011 - making it the first spacecraft to orbit Mercury. It mapped the entire surface of the planet, and discovered that water ice exists on Mercury in craters that never receive sunlight. Its mission ended in 2015 when it was sent to crash to the surface of Mercury. NASA's Messenger website contains more information about this mission.