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Can a spacecraft save Earth from an asteroid?

Winning mission proposes to track threat.

To hunt prey, one must first track it.

That’s the logic behind a competition on how best to hypothetically track Apophis, the 300-metre-wide asteroid that has a tiny chance of striking Earth in the year 2036. The winning spacecraft design was unveiled on Tuesday.

The mission would orbit an Earth-threatening asteroid. Credit: Planetary Society

Advocates say that not only would a mission to the asteroid better assess its trajectory than ground-based telescopes, but it would also gather information about the asteroid’s composition, shape and spin — crucial if engineers wanted to knock it off a path bound for Earth. “We wanted to raise awareness of the near-Earth object threat and encourage deep thinking about a not-well-studied niche,” says Bruce Betts, director of projects for the Planetary Society in Pasadena, California, which sponsored the competition.

The winner of the $25,000 first prize was SpaceWorks Engineering of Atlanta, Georgia, which proposed a simple spacecraft built largely from off-the-shelf components. Dubbed Foresight, it would orbit Apophis and send back radio signals to Earth even as it mapped the asteroid with a camera and laser rangefinder. Total price tag: just under $140 million.

Few expect NASA — or anyone else — to actually launch the mission given the remote chance of a Apophis impact in 2036, currently pegged at 1 in 45,000. But Steve Ostro, a radar astronomer at NASA's Jet Propulsion Laboratory in Pasadena, says it’s just a matter of time before an asteroid comes along that does warrant a bounty hunter. “These are not utterly rare situations,” he says.

Ground-based telescopes — particularly those equipped with radar — are very good at tracking threatening asteroids, but only when they’re close enough to spot, usually within about 15 million kilometres of Earth. In 2013, Apophis will swing that close, and astronomers will have a better idea of the probability of a 2036 impact. That would be confirmed in 2029, during another near-Earth pass that brings the asteroid closer than some satellites. But by then it could be too late to mount a serious Earth defence. “If the object is on a collision course, we need a lot more lead time," says Ostro. "A 2029 knowledge of a hit is way too late."

A tracking mission would allow scientists to confirm an Earth-bound trajectory early on, and give aerospace engineers more time to plan to divert it. For example, scientists have proposed ‘gravity tractors’ — heavy orbiters that could deflect an asteroid merely by adding their tiny gravitational tug. They suggest that a 20-tonne tractor could deflect an Apophis-sized asteroid in about a year, but, given a longer lead time, a one-tonne tractor could do the same job.

The Planetary Society, which received 37 mission proposals from 20 countries, also awarded $5,000 to a student team from the Georgia Institute of Technology in Atlanta.

Betts says that not all of the proposals were of the same quality. Two proposals sought to improve the asteroid’s visibility from telescopes on Earth: one sought to cover Apophis in aluminium foil, and another would spatter it with paintballs.

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Planetary Society Apophis competition

Jet Propulsion Laboratory Apophis Website

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Hand, E. Can a spacecraft save Earth from an asteroid?. Nature (2008). https://doi.org/10.1038/news.2008.626

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