Asteroid impacts are unlikely to wipe out humanity, with larger asteroids hitting Earth less than once a century. However, asteroids over 250m in diameter hit Earth about once every 50,000 years, and scientists are looking for ways to redirect them. Proposed methods include nuclear bombs, kinetic collisions, and the “gravity tractor” approach, but they take years to implement. If an asteroid is spotted too close to impact, there may not be enough time to prevent damage.
In considering the challenge of asteroid impacts, it is first important to keep the probabilities in perspective. If humans were likely to be wiped out by an asteroid impact, it probably would have happened already in our 2+ million year history. Asteroids larger than 25 m (82 ft) in diameter hit us less often than once a century, and appreciable quantities of the asteroid never even make it to the surface. The effect of such an asteroid impact is an air explosion the size of a small atomic bomb.
Human cities cover only a tiny fraction of a percentage point of the planet. If an atomic bomb (or an atomic bomb-sized impact) happened randomly somewhere on the planet, the probability that it would kill someone would be less than one in a million. In more than 2,000 years of recorded history, no asteroid impact has killed a single person.
Slightly more serious than the small asteroids that continually hit Earth are asteroids larger than 250 m (820 ft) in diameter, which hit Earth about once every 50,000 years. Such an asteroid would release about a gigaton of energy, fifty times smaller than the largest nuclear weapon ever tested, but still significant. This would be enough to tear down buildings and set fire to everything within a 10-50 mile radius. The probability of such an asteroid hitting us in the next 50 years is only 1/1000, but the odds are high enough that scientists have started looking for ways to redirect nuisance asteroids if we notice them heading our way.
One of the earliest ideas for countering an asteroid impact was to simply bomb an asteroid, using a nuclear tipped space rocket. Part of the problem with this approach is that the asteroid fragments would still largely contain their former kinetic energy and orbit. However, being in smaller pieces, they are more likely to burn up in the atmosphere.
An even simpler strategy for avoiding an asteroid impact has been proposed: a kinetic collision. If the astronaut’s trajectory is known years in advance, in many cases it would be enough to send a spacecraft weighing less than a ton into her side to divert the asteroid’s course. This was judged to be simpler than the nuclear weapon approach.
Yet another proposed method is the “gravity tractor” approach: sending a small spacecraft to tether itself to the side of the asteroid and use its gravitational or thrust-based influence to slowly alter the asteroid’s course.
Of course, most of these proposed strategies take many years to implement. In some cases, especially with comets, it can be weeks, days, or even hours before impact when we see the object coming. If so, we should just cross our fingers that it doesn’t affect our city or our country.
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