Asteroid deflection strategies

Since 1994 when Comet Shoemaker-Levy 9 collided with Jupiter, scientists have been researching if it may be possible to deflect incoming asteroids. With the discovery in 1999 that (29075) 1950 DA has a 1 in 300 chance of colliding with Earth in 2880, and the more recent discovery of 2004 MN4, whose chance of impacting Earth in 2029 had initially been estimated at 1 in 37 before it was revised to approximately 1 in 50,000, this problem has been taken more seriously by politicians and scientists.

An impact by a 10 km asteroid on the Earth is widely viewed as an extinction-level event, likely to cause catastrophic damage to its existing biosphere. There is also a threat of impacts by comets falling into the inner Solar System after having been disturbed from their orbits in the "Oort Cloud," a huge, tenuous sphere of comets surrounding the Solar System. Such "long period" comets are only infrequent visitors into the inner Solar System and they do not generally fall in orbits in the same plane as that of Earth, but there is nothing to rule out the possibility that one might collide with the Earth. The impact velocity of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making it much more destructive.

One might propose firing nuclear missiles at incoming asteroids would solve the problem, but it can be easily demonstrated that the energy required to destroy a 1km wide asteroid would be much larger than the energies available from nuclear explosives. Furthermore, simply breaking up an asteroid does not necessarily prevent the fragments from impacting, and potentially even causing greater destruction than the intact asteroid would have. On the other hand, if the asteroid is destroyed years in advance, the fragments would disperse over a very wide volume before, so only a few if any fragments would impact the Earth.

A more promising idea is to explode smaller nuclear devices alongside the asteroid. Providing this was done well in advance, a small deflection from a nuclear blast would be enough to alter the orbit and avoid an Earth impact. This is a form of nuclear pulse propulsion. In 1968, students at the Massachusetts Institute of Technology designed a system using nuclear explosions to prevent a hypothetical impact on the earth by the asteroid Icarus. This design study was later published as the Icarus Project.

However, NASA scientist Dan Durda has argued that if an asteroid was of low density and high porosity, it would be more likely to absorb the energy from a stand-off explosion, rather than be deflected. Although (29075) 1950 DA is dense, many other asteroids in the Solar System were found to be of low density and high porosity, and therefore could not be deflected in this way.

In addition, deflecting an asteroid impact requires that the asteroid be identified years before impact.

Cynics suggest that nuclear scientists are simply trying to find new excuses to keep nuclear missiles around, and to keep 'improving' them, as their careers have been endangered since the end of the Cold War.

Researcher Jay Melosh has proposed that it may be possible to deflect the orbit of an asteroid by focusing solar energy on its surface. This would create a small thrust from the resulting vaporisation of material, which over time would be enough to deflect any incoming asteroids, regardless of their density and porosity. Such a 'solar collector' is not as far fetched as it sounds, as the US military have already launched satellites designed to collect and focus radio waves.

Other proposals include:

• Setting up mass drivers on the object to scoop up dusty material and shoot it away, giving the object a slow, steady nudge.
• Flying a big sheet of reflective Mylar to wrap itself around the asteroid, acting as a solar sail to use the pressure of sunlight to shift the object's orbit.
• Dusting the object with powdered chalk or soot to perform a similar adjustment, using the Yarkovsky effect.

See also

• Torino scale
• Palermo scale
• Deep Impact (film).
• Armageddon (film).

References

• Donald W. Cox, and James H. Chestek. 1996. Doomsday Asteroid: Can We Survive? New York: Prometheus Books. ISBN 1573920665. (Note that despite its sensationalist title, this is a good treatment of the subject and includes a nice dicussion of the collateral space development possibilities.)
• Russell L. Schweickart, Edward T. Lu, Piet Hut and Clark R. Chapman; "The Asteroid Tugboat"; Scientific American (November 2003).

External links

• BBC Horizon - Averting Armageddon (summary) (http://www.bbc.co.uk/science/horizon/2003/armageddon.shtml)