From history we know that the Comet Shoemaker-Levy 9 collided with Jupiter in 1994. It was the first planetary comet collision observed by scientists. It raised many questions about what would happen if a comet collided with Earth again.
When Shoemaker-Levy 9 Collided with Jupiter
In July of 1994, astronomers caught a glimpse of the destructive capabilities and consequences of a comet colliding with the Earth. The comet Shoemaker-Levy 9, which had been captured by Jupiter's gravity, was torn apart by tidal forces and collided with the planet. The destructive power was immense as seismic shocks ripped through the planet. The impact sites could be seen for months afterwards and were more prominent than the Great Red Spot, Jupiter's most noticeable characteristic.
Considering that Jupiter is a gas giant and unpopulated, this event was more of a interesting example of the powerful and destructive capabilities of the universe in general. It was something to be observed and was valuable for the data gathered, but it would have been a much different story if an impact event of this magnitude had happened to Earth.
Dangers Comets Pose
Comets have particularly unusual orbits as opposed to asteroids which typically stay within the asteroid belt. Most have elongated elliptical orbits, meaning that their orbit takes them from the far reaches of the solar system to much closer runs near the sun. These orbits range in the number of years they take to complete. This is fortunate in that comets only have one chance on average every two hundred years to come anywhere near approaching Earth. Comets with shorter orbital periods have orbits that are well known, and there are no comets in danger of impacting the Earth any time soon.
Longer periods comets, with orbital periods greater than two hundred years, have orbits which are less predictable; there are some comets that might be approaching Earth within the next few hundred years that we are unaware of. Even though these comets have rather small chances of colliding with the Earth, the facts that their orbits are less known and less predictable make them something to keep in mind and worry about.
Possibilities of Earth Impact Events
Comets are not the only objects that could collide with Earth. The possibility of a collision with an asteroid is very real as well. Though most asteroids in the solar system can be found in the Asteroid belt that lies between Mars and Jupiter, some asteroids have irregular orbits which take them near the orbital path of Earth. Because Jupiter is so massive, its gravity is more likely to capture celestial objects than Earth, but the Earth's gravity will still pull objects towards it than come within close enough range.
The Chicxulub crater is the largest and best known impact crater on Earth. It is a crater that was discovered beneath the Yucatan Peninsula and was the result of a tremendous impact. It is associated with the extinction event that wiped out the dinosaurs in the Cretaceous Period. The Chicxulub crater, the Shoemaker-Levy collision, and the discovery of other sites that have been determined to be caused by impacts with celestial objects served as a wake-up call for astronomers.
The Spaceguard Survey Report, enacted by Congress in 1992, is an effort track near-earth objects with special attention paid to potentially dangerous objects. The purpose is to discover any objects on a collision course with the Earth and attempt to prevent the collision from happening. While there are many theories about how to accomplish this, no techniques for diverting the path of near-earth objects have been tested.
The Effects of Earth Impacts
Asteroids and meteoroids collide regularly with Earth's atmosphere, but few of these even make it through the atmosphere. Objects that do make it through the atmosphere usually are still not large enough to do any kind of damage. As previous extinction events in the fossil record and craters that have been caused by celestial objects show, impacts by objects large enough to do damage to the Earth have occurred and could possibly occur again.
The most pressing consequence of an Earth impact would be an extinction event. While the actual impact would be catastrophic and destructive beyond anything humans have experienced, the further consequences of the amount of debris thrown into the air would be responsible for even more deaths. The sort of climate change due to the blockage of the sun's light would be detrimental to all biological life, and it would only be life that could adapt quickly to these conditions that could survive.