Even with a relatively primitive telescope (read binoculars) and a clear night, you can make out the cavernous depressions that define our moon. This is direct evidence of the Late Heavy Bombardment — a shower of asteroids that collided with our earth and moon from 4.1 billion to 2 billion years ago.
But does this now depleted asteroid belt actually pose a real risk to life on earth now?
Across the globe there are several programs focused on our skies, monitoring potential threats from asteroids and comets. Mostly they are looking for the larger asteroids (more than1km in diameter), because they are the ones that could vaporize oceans, destroy mankind etc.
In 2010, NASA's Spaceguard Survey, for example, reported over 500,000 asteroids in our solar system, of which 6,758 are classified as Near Earth Objects (NEOs), and 1086 as Potentially Hazardous Objects (PHOs). Within the next hundred years, they estimate that around 200 of the PHOs have a small but finite chance of impacting earth.
However, the chances of Armageddon-level collisions are very low. We know that every few years, an object from space will impact earth with energy near that of the Hiroshima bomb. Fortunately, that energy usually dissipates high in the atmosphere and causes little damage. The really scary asteroids would need to be greater than 1 km in diameter and it has been predicted that those monsters only hit earth every few hundred thousand years. A 300-meter body is still formidable, however, but again they are rare, only striking earth every 60,000 years or so.
Despite these encouragingly low probabilities, there is a threat of some manner of collision and we need to be ready to deflect danger. If there was a sinister object on a crash course for earth, scientists suggest an international collaboration could blow it out of the way with a nuclear explosion (see Bruce Willis in Armageddon), attach a rocket engine to the beast to nudge it off course, focus sunlight onto it using a giant solar mirror creating a jet of vaporizing rock to disrupt its route, or possibly, bash it up into smaller pieces to minimize the damage once it breaches earth’s atmosphere.
However, all of these ideas would depend on the compositional make up of the object; which is actually a potential problem. In order to address the individual and specific risk of a potential humanity killer, we would send out a scoping mission to probe its structure. And just like in Bruckheimer’s blockbuster, if you hit un-mineable geology, for example, you could just bounce off the surface failing to push the object out of our way. Some scientists believe that NASA’s budget should, therefore, include a provision (it would need to be a large one) to address the problem of deflection. Because this is a huge unknown, the previous list of solutions is theoretical at this point.
On balance, if an asteroid is making its way towards us we would see it and most likely we would have time, possibly decades, to plan our move. Also the chances of a truly devastating impact are really very small so please rest easy tonight: an impending dinosaur-style catastrophe is doubtful.