Deflecting Earth-Bound Asteroids

Image: Don Davis/NASA

Last April, I wrote a fairly detailed post about asteroids and near-Earth objects. In it, I mention the gravity deflection (a.k.a. “gravity tractor”) method which consists of stationing a spacecraft a short distance from the asteroid and using gravitational attraction to pull the asteroid off course.

New Scientist reports on a related study led by Massimiliano Vasile of the University of Glasgow in Scotland:

[researchers] compared nine of the many methods proposed to ward off such objects, including blasting them with nuclear explosions.

The team assessed the methods according to three performance criteria: the amount of change each method would make to the asteroid’s orbit, the amount of warning time needed and the mass of the spacecraft needed for the mission.

The method that came out on top was a swarm of mirror-carrying spacecraft. The spacecraft would be launched from Earth to hover near the asteroid and concentrate sunlight onto a point on the asteroid’s surface.

Image: M Vasile et al, University of Glasgow

The concentrated light would be enough to heat up the surface of the asteroid to more than 2,100° Celcius, vaporising it into gases, creating enough thrust to change the course of the asteroid.

The scientists found that 10 of these spacecraft, each bearing a 20-metre-wide inflatable mirror, could deflect a 150-metre asteroid in about six months. With 100 spacecraft, it would take just a few days, once the spacecraft are in position.

To deflect a 20-kilometre asteroid, about the size of the one that wiped out the dinosaurs, it would take the combined work of 5000 mirror spacecraft focusing sunlight on the asteroid for three or more years.

5,000 ships is a lot, but 20-kilometer asteroids are also very rare. Chances are we’ll have to deal with much smaller ones a lot sooner.

What are the problems with some of the other methods according to the study? For the same mass launched into space, the gravity tractor takes more time. The nuclear explosion is about as effective as the mirrors, but there’s a danger that the asteroid could break up in many smaller pieces, making it harder to deal and unpredictable, even potentially more dangerous than it was.

Another option, especially for smaller rocks, is to simply ram a spacecraft into them (and then, as needed, use a gravity tractor or mirror(s) to fine tune the trajectory).

The most important thing is to start building up both our deflection tools and our detection tools. We don’t want to be caught with our pants down.

Sources:

• Are mirrors the best way to deflect asteroids?
• Mirrors ‘could deflect’ asteroids
• Inflatable Mirrors on spacecraft would move asteroids fastest

See also:

• Near Earth Objects and Asteroids: Are We Whistling in the Dark?
• Orbit@Home Funded by NASA