Asteroids: Deflect or Destroy?

"Let's go burn down the observatory so this will never happen again."
Moe the bartender, after Springfield narrowly avoids being destroyed by a comet. From Bart's Comet.

How do we defend ourselves against an asteroid or comet impact? Burning the observatories and hoping for the best is one option, although not a particularly effective one. Although interestingly, previous mass extinctions on the Earth have always been followed by a period of rapid activity in which new, more advanced species have flourished. So if a large asteroid does threaten us, perhaps we should let it do its worse - in the interests of evolution of course!

However, assuming we decide that mankind is worth saving, what options do we have? Short of evacuating the Earth or moving it out of the way, it seems the only plausible course of action is to tackle the asteroid itself.

Destroy

The "gung-ho" approach of blowing the offending asteroid to smithereens is sometimes used as a secondary justification for the controversial and costly Space Defence Initiative (SDI, aka "Star Wars"), first proposed by the Reagan Administration and recently resurrected by George W. Bush. The use of nuclear warheads against an asteroid is very risky, especially if its composition and structure has not been accurately determined (i.e. is it a solid body or a "rubble-pile"?). Even if we could muster enough weaponry to destroy the approaching body, it would most likely be fragmented by the explosion and the Earth would suffer multiple impacts as opposed to a single giant one. It's a choice between being hit by a shotgun blast or a rifle bullet - which isn't much of a choice since they're both going to kill you.

Deflect

The preferred solution would be to "nudge" the asteroid when it is at its furthest point from the Earth, thus turning an initially small deflection into a significantly large one by the time it crosses the Earth's orbit. This could be done in a number of ways, for example by setting off explosions close to the asteroid or by hitting it with a series of projectiles, gradually imparting enough of a velocity change to alter its trajectory by the desired amount. In his book Entering Space, Robert Zubrin suggests the use of Nuclear Thermal Rocket engines (NTRs) positioned on the asteroid's surface to change its trajectory, using the asteroid's own water content as propellant. This is a less heavy-handed approach than the use of nuclear bombs, but if the asteroid's rotation is chaotic, i.e. tumbling through space, then the direction of thrust could be difficult to control.

Of course, we won't have the luxury of deciding how and when to move it if we only become aware of the asteroid a few hours or days before impact, which is why the ongoing asteroid detection vigils are so important. As mentioned on the previous page, it's unlikely that a significantly large impact (i.e. bigger than the Tunguska event) will occur for at least several decades or even centuries. However, the more warning we have, the better our chances of being able to do something about it.

Eye on the sky

So far the Americans have led the way with programs such as Spacewatch, LINEAR and NEAT, which - by using dedicated telescopes and automated search software - have tracked down most of the known NEOs.

Recently, Britain was presented with an opportunity to make its own contribution to the NEO initiative with the publication of the Report of the Task Force on Potentially Hazardous Near Earth Objects, which contained 14 key recommendations. These included the construction of a dedicated 3 metre telescope in the southern hemisphere (a region which is not well covered at present) and the establishment of a British Centre for Near Earth Objects. However, the Government response, while positive, failed to commit to any firm course of action, preferring instead to continue a policy of discussion and consultation for several more months (in other words, "don't spend any money on it, at least until the General Election is out of the way").

The full text of both the report and the Government's response to it can be found at the official UK Taskforce website. Further information can also be found at Spaceguard UK.

Of course, even if we manage to pinpoint the location and orbit of every PHA, there is still the wildcard threat posed by comets. They come hurtling into the inner solar system at great speed and with little warning. Hale-Bopp was an exception. This comet was so huge it was discovered while it was still beyond the orbit of Saturn. However, just two years later it crossed the Earth's orbit; still not enough time for us to stop it if it had been on a collision course. Fortunately, we have the massive planet Jupiter to thank for "mopping up" a lot of these cometary interlopers, often by simply ejecting them from the solar system, or occasionally - as in the spectacular demise of comet Shoemaker-Levy 9 (below) - by swallowing them whole.

On the plus side, the study of near-Earth asteroids may have another benefit besides warning us of potential impacts. The next page looks at how they could prove to be a valuable resource in years to come.

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