| MadSci Network: Astronomy |
Hi Piotr,
The possibility of a comet crashing into the Earth is pretty small. For example, comet Hyakutake passed within about 5,000,000 km of the Earth and this was a fairly close approach. One can estimate the probability of a comet like Hyakutake hitting the earth in the following way. If we assume that a comet passes within 5,000,000 km of the Earth once every year, then taking the radius of the Earth to be about 5,000 km, the frequency with which comets will hit the earth turns out to be about once every million years.
However, historical records of large impacts on the Earth tend to indicate that the actual interval between large impacts is much longer. There was a mysterious explosion near Tunguska, in Siberia, in 1908. This has been explained as the vaporisation of a fragment of a comet in the atmosphere and was sufficiently powerful to be felt 50 or 60 km away (and was, I believe, heard in London). There a numerous craters, for example the Barrington Crater in Arizona, which point to impacts by objects a few meters across in the past few million years. Finally, there is evidence that a body large enough to cause a change in the Earth's climate with the debris from its impact (this could have been an object a few km across, which is about how big a comet is) hit the Earth 65 million years ago and was responsible for the extinction of the dinosaurs.
What can we do about this? Well, first we can try and detect objects before they are too close for us to do anything. This is a problem as even meteors a few hundred metres across could create a crater a few km in size and it is difficult to see rocks unless they pass in front of a star and eclipse its light. Comets are easier to detect but have larger velocities than meteors and therefore need to be detected several months before they hit in order for anything to be done. In either case, we need a team of people who watch the sky constantly looking for objects which might be dangerous. Needless to say this would be quite an intensive project, however an effort is now under way to catalogue all the near earth asteroids, for example, so that we can predict if any of those will hit the earth. For more information about this, see the link at the bottom of this reply.
If an object were detected on a collision course with the Earth, most scientist seem to think that the best thing to do would be to try and detonate a nuclear weapon on the its surface in order to give it a slight push and deflect its orbit away from the Earth.
I think that if the object were only a few meters across then we could also try to predict its point of impact and evacuate that area. On the other hand, if it was an object a few km in size, we would have to try and deflect it because an impact that large could lead to effects similar to a 'nuclear winter' and the possibility of mass extinctions of species here (including us.) Breaking up a large object with a nuclear weapon is, unfortunately, not a good idea because the resulting pieces could still be devastating and would still have orbits that collided with the Earth.
For more information on the detection of comets and asteroids on collision courses with the Earth and what steps we can take to them, I recommend taking a look at the Asteroid and Comet Impact Hazard site at the NASA Ames Research Centre. This page has a great deal of information about the hazard from collisions with comets and asteroids and links to many intersting pages dealing with this problem.
Jim O'Donnell