Over 65 million years ago, dinosaurs ruled the world. These giant creatures dominated the planet, living on the land, in the air and in the sea. At that time, people simply didn't exist. All of a sudden, these huge creatures died out. There are different ideas about why the dinosaurs suddenly became extinct, but one of the most popular is that something from space crashed into Earth. This object could have been an asteroid, meteor or a comet. Its impact would have been so great, whether it crashed into the sea or on land, that it would have changed the climate of the planet completely, making it uninhabitable for many of the species living at the time. That was 65 million years ago, and we have been fortunate that since then, no other visitor from outer space has had such a devastating effect on life on Earth.
But, could it happen again in the future? Comets are strange objects in the Solar System because of their unusual orbits. The eight planets travel in a mostly regular circle around the Sun and on the same plane as each other. Dwarf planets, like Pluto, behave differently. Pluto can go as close as 4,443,000,000 kilometres (2,756,902,000 miles) from the Sun and as far as 7,682,900,000 kilometres (4,583,190,000 miles) away from it. This orbit is known as an elliptical orbit. Pluto also orbits on a different plane, going "higher" and "lower" than the other planets. The differences between these orbits can be seen in the picture below.
The orbits of comets are similar to Pluto's - for this reason it was sometimes argued that Pluto may actually be a comet and not a planet. Comets orbit the Sun in elliptical orbits and on different planes, some going up and under like Pluto (only more extreme) and some on the same plane as the other eight planets. Sometimes comets travel through the Solar System, passing the planets, spinning around the Sun and then going back outwards. Usually they complete this journey successfully and avoid hitting any of the planets on the way. But, sometimes it goes wrong and 65 million years ago may have been one of those occasions! It is possible that on its journey through the Solar System, one unfortunate comet may have bumped into Earth on its way. The collision would have destroyed the comet and caused devastation on Earth. And, since other comets still orbit the Sun, there is a chance that one day another one could collide with Earth! Unfortunately, because comets are usually only discovered a few months before they get close enough to the Sun and Earth to be visible, it is difficult to predict if one is heading your way! Fortunately, there is Jupiter. This gigantic planet grew so large during the formation of the Solar System by attracting most of the matter that made up the planets. its strong pull of gravity means that, if anything strays too close to it, it pulls it in and gobbles it up! Because of this, any objects which could potentially head for Earth may get stopped on their journey by Jupiter. And in 1994, we could see this happen. Introducing the Shoemaker-Levy Comet........
The Shoemaker-Levy Comet, also known as Shoemaker-Levy 9, was discovered by Carolyn and Eugene M. Shoemaker and David Levy on 24th March 1993. Unlike any other comet discovered previously, the Shoemaker-Levy Comet was orbiting Jupiter and not the Sun. The comet itself was actually in fragments, up to about 2 kilometres in diameter. It is believed that the comet had previously been orbiting the Sun, but in about 1970, it was attracted by Jupiter's gravity. In July 1992, the force of Jupiter's gravity on the comet, which was orbiting the planet every two years, actually pulled it apart. Comets have been known to have broken into fragments before, and it is also believed that many of the smaller moons (and maybe even the rings) of the larger planets are actually fragments of larger moons which have broken up. However, what got scientists most excited was that it looked like the fragments of the Shoemaker-Levy Comet would actually collide with Jupiter.
Hubble Space Telescope image of Shoemaker-Levy Comet in fragments, 17th May 1994 - Image courtesy of NASA
In July 1994, scientists and space enthusiasts all over the world pointed their telescopes, satellites and big binoculars towards Jupiter. The significance of the collision was that it would be the first time that a collision of major Solar System objects could be witnessed. Would the fragments have any effect on Jupiter's atmosphere or would they simply sink into the planet with nothing more than a flash? On 16th July 1994, the first fragment crashed into Jupiter at a speed of 60 kilometres a second. This collision caused a fireball which reached a temperature of almost 24,000 °C. This temperature was measured by the Galileo probe which was due to arrive at Jupiter in 1996. The impact itself however left a huge dark spot on Jupiter, about half the diameter of Earth. The largest dark spot was caused by an impact on 18th July 1994 which measured about 12,000 kilometres. The amount of energy released from this impact was 750 times more powerful than if all of the nuclear weapons on Earth were used at the same time! Remember, that energy was released by only a small 2 kilometre fragment of a comet, showing how dangerous a comet or meteor impact with Earth would be. The final fragment of the Shoemaker-Levy comet impacted with Jupiter on 22nd July 1994. In total there were 21 impacts. Each of the fragments were named Fragment A (the first to hit Jupiter) to Fragment W (the last impact). The impacts themselves allowed scientists to discover more about the chemical make up within Jupiter as the fragments kind of opened up the upper atmosphere of the cloud tops to reveal what was below it. They discovered sulphur, carbon disulfide, ammonia and hydrogen sulphide within the planet's atmosphere. Some scientists get very excited by chemicals! Scientists believe that Jupiter has a layer of water within its atmosphere. Although water was discovered to be present in Jupiter's atmosphere from the impacts, there wasn't as much as scientists thought there was. This could either be because the fragments didn't go deep enough to the water layer (each fragment would have been destroyed shortly after entering Jupiter's atmosphere) or simply because there isn't as much water there as the scientists thought there was!
The dark spots could be seen on Jupiter for several months after the impacts and were said to be easier to spot than the Great Red Spot on the planet. Since the collisions, other comets have been discovered to be orbiting Jupiter suggesting that these kind of events are more common than previously thought. As well as that, some of Jupiter's moons (Ganymede and Callisto) appear to have impact trails which are likely to have come from comets. This demonstrates how Jupiter plays an important role in protecting the Inner Planets. The gravitational pull of Jupiter attracts passing comets. Sometimes the comet is captured, meaning that instead of continuing it journey around the Sun, it now orbits Jupiter instead. Sometimes the comet is simply thrown off course, heading in a different direction to the one it was originally going in. And sometimes, the comet continues to orbit the Sun, but in a different orbit. Comet Hale-Bopp, which was discovered in 1995 and orbited the Sun in 1997 had its orbital period affected by Jupiter's gravity. Originally, Hale-Bopp took 4300 years to orbit the Sun. After its 1997 visit, Jupiter's gravitational influence on Hale-Bopp shortened its orbit to 2300 years. Comets can still head for Earth, and an impact from a comet 65 million years ago may have caused the extinction of the dinosaurs, but thanks to Jupiter basically getting in the way or "eating" up the comets, we may be saved from potential disaster.
But for how long.....?