14 DAY TRIAL // If an asteroid were to be discovered tomorrow and proven that it would impact the Earth in less than a week, we wouldn't be able to do much to prevent this from happening. All we can do so far is hope that such events do not occur any time soon. Deflecting asteroids from a possible Earth-threatening trajectory may sound like an easy task, but in fact it is anything but that.
Fortunately, there are some ideas that could actually work and one of them is the 'gravity tractor' proposed by NASA's Jet Propulsion Laboratory in Pasadena, California, which in theory should be able to deflect asteroids as large as 140 meters across. "Prior to this study, the gravity tractor deflection technique had been proven in only a conceptual way. 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. The JPL study gives it the solid engineering underpinnings that we never really doubted, but now are there for anyone to see," says Clark Chapman of the Southwest Research Institute in Boulder.
'Gravity tractors' are double edged interventions. The pulling or pushing force exerted on the asteroid is greatly related to the time left before the asteroid falls into a trajectory from which it can no longer be influenced and to other variables such as the future orbit of the celestial body. If executed improperly, the procedure could just as well put the asteroid into a trajectory away from Earth, only to later pass through a gravitational keyhole that will bring it in the future in an equally dangerous position.
"The gravity tractor is a wimp, but it's a precise wimp. It can make very small, precise changes in orbit, and that's what you need to avoid a keyhole," said Rusty Schweickart, chairman of the B612 Foundation and former Apollo astronaut.
The best example would be the asteroid Apophis which, towards the end of 2004, caused a lot of concern amongst astronomers who predicted that during its 2029 fly-by around Earth it could pass through a gravitational keyhole that would send its crashing into the surface of our planet on April 13, 2036. Later calculations of its orbit indicated that it had a chance of 1 in 45,000 of impacting the Earth in 2036.
The 'gravity tractor' imagined by the JPL researchers works in two stages, and should work effectively in the case of previously discovered asteroids whose trajectory was calculated. First, the deflected body will be impacted with the help of an object similar to the one used in the Deep Impact mission. Then, a massive spacecraft with a mass of roughly a ton would approach the asteroid at a distance of about 150 meters. The gravitational force exerted by the spacecraft should be enough to move the asteroid by about 0.22 microns every day, not very much but still enough to steer it away from future encounters with gravitational keyholes.
A transponder placed on board the spacecraft would then send precise data regarding the position of the asteroid, so that its orbit is calculated. During the simulations conducted at JPL, researchers used available data collected from an asteroid known as Itokawa, a 140 meter wide body with an elongated shape that was studies in more detail by the Hayabusa spacecraft in 2005.
"We didn't want some nice easy, smooth shape. We have a little bitty spacecraft with this monster swinging its [back] at it," said Schweickart.