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The Menace of the Asteroids. Part 2: Defending Earth

Artwork showing a large asteroid impact with the Earth. Image Credit: Don Davis / NASA

A 500-meter-wide asteroid has just been discovered that will almost certainly collide with the Earth in 2030. No one knows yet where it will strike—on land or at sea—but its potential for regional devastation is high. The question is: what, if anything, can be done to stop it? 

Fortunately, this shock announcement isn’t true: nothing is heading our way as far as we’re aware. But it’s very possible, given that only about a quarter of all the potentially hazardous asteroids out there have yet been detected, that some big dumb object has our name on it. So, having some form of planetary defense system in place makes a lot of sense.

Approaches to dealing with the threat of asteroid (or comet) impacts fall broadly into three categories that we can call the three D’s: duck, destroy, or deflect. “Duck” means we’d do our best to get out of the way of the impact zone by evacuating the area or, in the most extreme case of a planet-wide disaster, retreating to purpose-built underground shelters. Needless to say, this would be a last-ditch measure if other strategies had failed or not even been attempted.

A gravity tractor being used to alter the trajectory of an asteroid. Image Credit: Dan Durda

Another option would be to try to blow up a threatening asteroid into smaller pieces using, for example, powerful nuclear weapons. The problem with this is that many of the fragments might continue on the same course and strike the Earth anyway, subjecting us to the equivalent of a blast of lead shot no less damaging than a single bullet.

By far the best solution would be to gradually alter the motion of the problem asteroid so that it missed. This could involve deflecting the object sideways so that it no longer intersected the Earth’s orbit, or speeding up or slowing down the intruder so that it arrived at the wrong time to collide. Direct methods, involving short, sharp shoves from explosions or kinetic impactors, might be necessary in the case of a short-notice threat, but would probably not work well against many near-Earth asteroids which are of the “rubble-pile” variety, made of loose aggregations of boulders.

The preferred strategy of most experts is to use an indirect, slow-acting method. For example, the asteroid could be given its own propulsion system in the form of chemical rockets, ion engines, or a mass driver attached to its surface. These would fire over an extended period of time and allow fine control over the asteroid’s trajectory.

A more subtle method would be to park a large, massive spacecraft over the asteroid to serve as a gravity tractor or space tug. Because the spacecraft and asteroid would mutually attract one another, the spacecraft would only need to counter the force toward its much more massive companion, using some kind of long-duration thruster, such as an ion engine, to cause a net movement of the asteroid toward the spacecraft and thus slightly alter its heading. Other approaches have also been suggested, including wrapping an asteroid in a sheet or reflective material to use the pressure of solar radiation to nudge the asteroid out of harm’s way.

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