Over the long term, asteroids pose perhaps the most serious threat to life on Earth. Large asteroid impacts are believed to have been responsible for at least some of the nearly catastrophic extinction events in Earth's long history, such as that between the Cretaceous and Tertiary periods (which resulted in the extinction of the dinosaurs). Since an asteroid-caused extinction-level event today would devastate and perhaps entirely eliminate the human race, asteroid defense strategies are a vital, if largely speculative, under-funded, and under-appreciated, component of the human space program.
Since rising awareness of the potential risk posed by asteroids in the 1960s and 1970s, quite a number of asteroid defense strategies have been put forward. Since asteroids are large enough that they probably cannot be safely destroyed, most asteroid defense strategies involve ways of moving them out of the way instead.
- Nuclear Explosives -
The standard default response to an impending collision is simply to blow it up with nuclear explosives. That we could use nuclear weapons against incoming asteroids has entered the popular imagination through movies such as Armageddon. This would not involve manned landers as featured in the movie; however, it would involve some sort of spacecraft delivering current nuclear weapons or, more likely, much larger variants.
However, it is now generally accepted that such an approach would be futile. An asteroid large enough to devastate life on Earth could not be vaporized in a nuclear explosion. Instead, the best-case scenario would be to break it up into chunks. However, unlike in Armageddon, there is no easy way to prevent most or even all of those chunks from then going on to impact the Earth anyways - and if they do, then the total amount of kinetic energy involved might be more or less the same as it would have been anyways. Thus, nuclear explosives are a somewhat dubious asteroid defense strategy.
On the other hand, it is conceivable that a series of nuclear explosions at a safe distance from the asteroid's surface could be used to make course corrections, nudging the asteroid progressively in a slightly new direction. Such minor course corrections are the holy grail of asteroid defense strategies: because both velocity and distance are so enormous in space, if we could nudge an asteroid even slightly off course years in advance, it could still miss the Earth by a very safe margin.
- Kinetic Impactor -
A variant on the above is the so-called kinetic impactor: a large but non-explosive spacecraft, preferably as heavy as possible but potentially surprisingly lightweight (perhaps a ton or so), could simply be rammed into the side of the asteroid. The kinetic impactor would not detonate; instead, its purpose would be to knock the asteroid off-course. Again, even a minor change in velocity, delivered years in advance, would be enough to make the asteroid in question miss the Earth by a wide margin.
The kinetic impactor is currently the generally favoured asteroid defense strategy. The European Space Agency is designing a concept kinetic impactor spacecraft, and it is conceivable that a real kinetic impactor would have been built if the asteroid Apophis, discovered in 2004, lived up to original estimates which suggested it could strike the Earth in the 2030s. (Fortunately, subsequent analysis by NASA indicated that the chances of this happening are only about one in a quarter-million.)
- Other Asteroid Defense Strategies -
A number of more novel asteroid defense strategies have been proposed, all of which rely on the same principle as the kinetic impactor - a small push, delivered far in advance - but do so with more speculative technology. One approach favoured by physicists and former astronauts Edward Lu and Stanley Love, who have emerged as activists demanding better funding for asteroid defense strategies, is the so-called gravitational tractor. Instead of colliding the heavy spacecraft (the kinetic impactor) into the asteroid, Love and Lu suggest simply parking it on a very close, parallel trajectory. Over time, the very small but non-zero gravity of the "tractor" spacecraft would tug the asteroid off course. Using a "tractor" approach rather then an impact, Love and Lu point out, means there is zero risk of accidentally breaking up the asteroid. This is important because, as noted above, simply breaking up an asteroid does not necessarily reduce the chance of an impact.
A number of other ways of producing the same slow, steady push as the gravity tractor have also been proposed. For example, the same objective might be achieved by delivering onto the asteroid either a specially designed rocket motor, or a mass driver, which would shoot off debris from the surface into space. Either approach would have a negligible impact on the asteroid, but over time, even such a tiny force could move the asteroid far enough that it misses the Earth. Given enough warning time, even solar radiation - either directed at the object from lenses in Earth orbit, or received via large solar sails attached to the surface - might be enough to slightly alter the orbit of an asteroid.
- Putting It Into Practice -
Unfortunately, while developing a feasible asteroid defense strategy is vital, in focusing on it we risk missing out on an even more important step, which historically has been equally under-funded by the major space programs: space surveillance. Put simply, we cannot defend against an asteroid we do not see coming - which is even more important given that the most feasible asteroid defence strategies assume we will have years of advance preparation time to work with.
However, an impact by a comet or by a previously unseen smaller asteroid could be catastrophic to human life, and although the risk of this occurring in any given year is negligible, we could have only weeks or months of warning. American and European programs currently hope to make comprehensive catalogues of all near-Earth objects, but every year a few such small objects are detected passing closer to the Earth than our own Moon, only days prior to making a close approach. In 1989, the asteroid Asclepius was discovered shortly after it passed by Earth. Although it never actually came close to the surface, it did pass straight through the position Earth had occupied six hours before; had the 300-metre-wide rock actually struck the Earth, it would have done so with a force equivalent to hundreds of megatons of TNT (far greater than the most powerful nuclear weapon ever built).
An even more serious problem is that testing an asteroid defense system would be extremely dangerous and politically controversial. Some countries will predictably feel extremely insecure if rival nations develop the ability to redirect asteroids - since, by definition, anything that can deflect an object away from Earth could also deflect an object towards Earth, and perhaps even a specific target on the Earth's surface. Moreover, since it would alter the orbit of at least one asteroid and (in doing so) possibly others, even a seemingly harmless test deflection might have the unforeseen consequence of setting up a real impact in the distant future.