We’ve all seen it in movies: scientists spot a giant comet/asteroid hurtling toward Earth, and Bruce Willis/Clint Eastwood must fly out to it and blow it up/nudge it aside before it devastates the planet/blows the special effects budget.
But in real life, we don’t really know exactly what it would take to deflect an oncoming asteroid. We’re not even very good at deciding whether or not one is a threat.
The European Space Agency is out to change that, with a proposed mission called Don Quijote (presumably because of Quijote’s well-known penchant for tilting at windmills).
The importance of studying ways to assess and possibly deflect asteroids from a collision path with Earth has been brought home not only by mediocre Hollywood movies, but by a very real scare astronomers had a few months ago.
On December 19, 2004, astronomers rediscovered a 400-metre-diameter asteroid (originally called MN4 and recently renamed Apophis), which had first been discovered six months earlier but hadn’t been seen since. The second sighting allowed its orbit to be computed. Those calculations revealed there was an unusually high chance that Apophis could collide with the Earth in 2029.
Usually such initial concerns are quickly alleviated by repeated observation, but in this case, repeated observation made such a collision seem even more likely. The asteroid reached record levels on two scales used to measure the risk of an asteroid impact (rather like the Richter scale tells us how strong an earthquake is).
Fortunately, observations from the first sighting of Apophis were found that made it clear the asteroid will not hit Earth in 2029 (an impact at some later date can’t be ruled out, although it’s considered unlikely). It will, however, pass within just 32,000 kilometres of Earth, the closest approach we’ve ever seen of an object this size. That’s inside the orbit of some satellites, which means Apophis will be visible to the naked eye.
But if Apophis had turned out to be a real threat, we would have been scrambling to find some way to deflect it before its orbit and ours intersect in 24 years. If Don Quijote works out, maybe we’ll have a system in place before the next threat shows up.
The current plan calls for two spacecraft to be launched on different trajectories. One, Sancho, will rendezvous with the asteroid and go into orbit around it. The second, Hidalgo, will arrive sometime later and slam into the asteroid. Sancho will observe the asteroid before and after Hidalgo’s impact, and will measure any changes to the asteroid’s orbit.
Two possible target asteroids, designated 2002 AT4 and 1989 ML, were selected earlier this year by the NEO (Near Earth Object) Mission Advisory Panel. One important criterion was that the test object not be one whose orbit crosses Earth: that ensures that whatever change the mission brings about in the asteroid’s orbit won’t have the unfortunate side-effect of causing it to slam into Earth at some time in the future.
The panel also sought to pick targets whose orbits could be easily matched without the use of too much propellant (which keeps the cost down), and whose shape, density and size are fairly well known. That’s important for properly positioning the orbiting spacecraft, and for targeting the impactor spacecraft at the asteroid’s centre of mass.
Finally, the experts had to choose between C-type asteroids, asteroids with dark surfaces (due to the presence of carbon compounds), rather than an S-type asteroids, which have brighter surfaces (due to the presence of silicates). It’s generally thought we’re at more risk from C-type asteroids, because they’re more numerous among the NEO population. On the other hand, it will be easier for the spacecraft to navigate toward a brighter S-type asteroid. On the other other hand, if the “rehearsal” is too easy, it won’t be realistic.
The final decision on which asteroid to aim for won’t be made until 2007. In the meantime, the ESA has issued an open invitation to European space companies to submit proposals on possible designs for the two spacecraft. Sometime toward the end of the year, the most promising ones will be selected. Early next year, two teams will start further development of those ideas. In early 2007, ESA will select a final design.
All of which means, of course, that the actual launch date for Don Quijote is still well in the future.
One hopes Earth’s next close encounter with an asteroid is even further away.
