But it remains unclear how a sighted asteroid can really be stopped from striking the Earth. Would it be a good idea to pre-empt an impact by hitting the approaching celestial body with a nuclear or hydrogen bomb? And at what distance would such a pre-emptive strike be safe enough for the planet?

Mathematical modeling has enabled scientists to calculate the lower distance limit for detonating a nuclear warhead in outer space. An asteroid moves with an average speed of 25 kilometers per second, so it should be no closer to the Earth than 464,000 km when a bomb targeting it goes off.

Detractors argue that using such a method would be extremely unwise and fraught with grave implications for the Earth.

Boris Shustov, president of the Astronomy Institute at the Russian Academy of Sciences, is one of the opponents. He heads an academic task force set up in Russia to study threats posed by asteroids and comets to the Earth.

In Shustov's view, the best solution would be to make celestial bodies veer off a potentially dangerous course rather than destroying them. He says modifying an asteroid's velocity by just a few centimeters per second would be enough to divert it.

One way of doing so would be to attach a spacecraft with a solar sail shaped like a concave mirror to an asteroid. The sail will focus solar radiation on a small area of the asteroid's surface. Owing to the asteroid's weak gravity, warmed-up substance will quickly evaporate off the surface and the resultant jet thrust will make the rock shift its trajectory.

Another technique involves attaching rocket engines onto the surface of an asteroid itself to activate them when the need arises.

If an attempt to move a large asteroid off course fails, we could then try to provoke its collision with another, smaller asteroid. Such processes happen naturally all the time, resulting in asteroids shifting their orbits.

Unfortunately, such methods are workable only with well-studied asteroids whose potential impact on the Earth can be predicted at least several years in advance.

A much bigger danger is posed by undiscovered asteroids and so-called long-period celestial bodies, which approach the Earth once in a thousand years or even less often.

The orbits of asteroids may change gradually over time. Sharp shifts are also possible, for example, in the event of gravitational perturbation by large-sized planets.

Creating a special asteroid interception service and keeping that service on permanent alert will require major investment. Some of its elements should be tried out while implementing current space programs. This way, we could ensure an immediate mobilization of all available means to counter a specific asteroid impact threat as soon as it arises.

-- (Yury Zaitsev is an expert with the Russian Institute for Space Research. This article is reprinted by permission of RIA Novosti. The opinions expressed in this article are the author's and do not necessarily represent those of RIA Novosti.)

(United Press International's "Outside View" commentaries are written by outside contributors who specialize in a variety of important issues. The views expressed do not necessarily reflect those of United Press International. In the interests of creating an open forum, original submissions are invited.)

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