Asteroid collision: How to defend Earth, II

warning times for 30-meter objects to more than a month. Even so, every ground-based lookout suffers from interference from the sun and moon.

A dedicated space telescope would fix this problem, but such a mission could cost more than a billion dollars. “We’re talking about investing in an insurance policy,” says Irwin Shapiro of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Shapiro is leading a U.S. National Research Council panel that by year’s end will recommend a strategy to better address the threat from near-Earth objects (see 22 December 2008 HSNW). That study, along with the air force’s report on its asteroid impact exercise, is intended to help the White House develop an official policy on the near-Earth object hazard by October 2010, which Congress has requested.

Asteroid impacts are much rarer than hurricanes and earthquakes, but they have the potential to do much greater damage, Johnson warns: “It’s not something I think there needs to be billions of dollars per year spent on, but it does warrant some priority in the list of things that we ought to be worried about.” The cash would at least give us a better idea of when the next asteroid might strike. “From what we know today,” he says, “it could be next week.”

What is to be done?
An asteroid blast like the one that flattened Tunguska in Siberia in 1908 is expected only once every 500 years or so, on average. It is likely to be a lot longer than that before one hits a populated area, given how small a fraction of Earth’s surface is taken up by cities and towns. A NASA study in 2003 concluded that only one in four Tunguska-like impacts would kill anyone, and only one in 17 such impacts would have a death toll of 10,000 or more, comparable to severe earthquakes and tsunamis.

The fastest way to deflect an asteroid away from Earth would be to send a nuclear bomb aboard a spacecraft, à la the film Deep Impact, though we would still need several years’ warning.

The spacecraft would have to be able to home in on the asteroid and to trigger the explosion at just the right distance. Precision is needed to avoid breaking up the hurtling rock while still giving it enough of a nudge to prevent the Earth impact years down the line.

Shiga writes that this assumes we are already prepared. Designing and building new spacecraft typically takes a few years. With current rocket technology, it would probably take several additional years to reach a threatening asteroid. And since the explosion would need to occur years ahead of the predicted impact in order to make the asteroid miss Earth, we’d need decades of lead time if we hoped to deflect Armageddon. A confounding factor is that nukes in space are forbidden by the Outer Space Treaty of 1967, signed by the US, Russia, and other nuclear powers, though they might agree to turn a blind eye on this one.

With several decades of warning time, other deflection technologies could come into play (see discussion in “New Ideas for Deflecting Earth-threatening Asteroids,” 26 march 2009 HSNW). The gravity tractor, for example, would see a spacecraft hover near the asteroid for several years, gradually pulling the asteroid off its collision course using the tiny gravitational pull of the spacecraft’s mass.

Another option would be to focus sunlight on a spot on the asteroid using a fleet of mirror-bearing spacecraft, heating it enough to vaporise rock. The escaping gases would act like the exhaust from a rocket engine, giving the asteroid a slight push in the opposite direction that could produce a substantial course change over many years.