Expert: Philip Stahl - 4/9/2005

Question
If there was a large comet or asteroid on a collision coarse with earth, would we know before it hit? Coild astronomers tell were it will hit so they can evacuate the area?

Answer
Hello.

If enough resources, telescopes were trained on the sky - then yes, astronomers would be able to tell if a comet or asteroid were on a collision course with Earth.

Right now, one major Observatory - at The University of Arizona's Lunar & Planetary Laboratory, is carrying on a project called 'Spacewatch' with its 1.8 meter (~ 6' dia.) telescope - which is more or less dedicated to the detection of small objects in the solar system, with a view to studying its dynamical evolution.

Spacewatch has specific focus on the Trojan, Main-Belt, Trans-Neptunian, Centaur and Earth-approaching asteroid populations, as well as any hitherto unknown comets that enter its investigation domains.

It also provides follow-up for the above - including astrometry (exact position determinations) for all the named classes given, and assessment of which constitute hazards.

Other amateur astronomers also keep an eye on the sky - using whatever telescopes they have at their disposal - and reporting anything suspicious to the International Astronomical Union. The IAU then does more exhaustive analyses to determine if there is any genuine threat.

A government-backed project, "Space Guard", was also to have come online, but it remains short of critical funding.

Unfortunately, in regard to your second question, there is no way at present to be able to predict very exactly - with enough time in advance- where an Earth-intercepting asteroid or comet may hit. That may well improve in the future, but for the time being, pretty well the whole planet has to be on alert - unless the object is within 2-3 days of striking perhaps.

The difficult thing about Earth-intercepting asteroids *and* comets, is that while the probability of collision with a massive planet-killer is vanishingly small (less 1 every 10 million years or so) it is not zero. A true killer asteroid - such as wiped out the dinosaurs 65 million years ago - could certainly wreak havoc with humans. (Similarly with any comet nucleus of the same size)

Hopefully, the nations of the planet will one day see that this risk - however low in probability  - is worth the investment of observing resources. Which will certainly have vastly more import (to all humans) than their wasteful arms' spending!