A "gravity tractor" could deflect an Earth-threatening asteroid if it was deployed when the asteroid was more than one orbit away from the potential impact, according to a new study. If the space rock was found heading straight for Earth, a combination of techniques - including a gravity tractor - might save the day.

Dan Durda
©Dan Durda/FIAAA/B612 Foundation
A gravity tractor would fly near an asteroid and gravitationally nudge it off course (Illustration: Dan Durda/FIAAA/B612 Foundation)

The study, carried out by NASA's Jet Propulsion Laboratory in Pasadena, California, shows that the weak gravitational pull of a nearby spacecraft could deflect a hypothetical asteroid 140 metres across, big enough to cause regional devastation if it hit Earth.

"Prior to this study, the gravity tractor deflection technique had been proven in only a conceptual way," says Clark Chapman of the Southwest Research Institute in Boulder, Colorado, who was not involved in the study.

"Although there were few, if any, substantive criticisms of these concepts, some of us had the feeling that the ideas were viewed as quaint but not-ready-for-prime-time," he says. "The JPL study gives it the solid engineering underpinnings that we never really doubted, but now are there for anyone to see."

Exactly how much of a push is needed to deflect an asteroid depends on how long before a potential impact the intervention begins, and what kind of orbit the object is going to follow in the interval, says Rusty Schweickart, a former Apollo astronaut and chairman of the B612 Foundation, which funded the study.

Narrow keyhole

In some cases, the asteroid will pass through a narrow "keyhole" in space before returning on a future orbit to hit Earth. If it misses the keyhole, which may be only a few hundred metres across, it will go on to miss Earth.

That's where a gravity tractor alone could do the job. "The gravity tractor is a wimp, but it's a precise wimp," Schweickart told New Scientist. "It can make very small, precise changes in orbit, and that's what you need to avoid a keyhole."

The well-known asteroid Apophis could pass through such a keyhole in 2029, leading to an impact with Earth just seven years later.

In the JPL study, the imagined asteroid is initially discovered on a direct path to impact, so a gravity tractor would be too feeble to deflect it alone. Instead, the team envisages a one-two punch.

Gentle force

First a spacecraft would be crashed directly into it, similar to the Deep Impact mission that impacted a comet in 2005. That would provide a much greater change of direction, but in a less controllable fashion. There's a chance it could even push the path of the asteroid into one of those dangerous keyholes.

Then a second spacecraft, the gravity tractor, would come into play. Weighing around a tonne and hovering about 150 metres away from the asteroid, it would exert a gentle gravitational force, changing the asteroid's velocity by only 0.22 microns per second each day. But over a long enough time, that could steer it away from the keyhole.

In the simulation, a simple control system kept the spacecraft in position, and a transponder on the asteroid helped monitor its position and thus determine its trajectory more precisely than would be possible otherwise.

The simulated asteroid was 140 metres wide and its elongated shape was copied from an asteroid called Itokawa, which was visited by Japan's Hayabusa spacecraft in 2005.

Like real asteroids, it was spinning in the study - important since asymmetric gravitational effects could push the tractor out of position. "We didn't want some nice easy, smooth shape," says Schweickart. "We have a little bitty spacecraft with this monster swinging its butt at it."

A preliminary report on the simulation was presented by JPL's Don Yeomans at the recent Asteroids, Comets and Meteors meeting in Baltimore, Maryland.

Comets and Asteroids - Learn more about the threat to human civilisation in our special report.