The European Space Agency is putting together a mission to see how well current technology could handle the threat of an asteroid impact. The agency has now selected two targets for its rehearsal deflection mission, dubbed Don Quijote.

The agency has selected asteroids 2002 AT4 and (10302) 1989 ML as possible mission targets. It says they represent the best compromise of all the selection criteria. The final decision on which asteroid to target will be made in 2007.

ESA is calling for spacecraft design proposals from the space industry, and early in 2006 it will select two to be developed further. The final demonstration mission design will be chosen in early 2007, but the launch date has yet to be confirmed.

The mission will see two spacecraft travel to the chosen asteroid. The first, called Sancho, will arrive at the asteroid several months in advance of the second, Hidalgo. When Hidalgo arrives to smash into the asteroid, Sancho will be there to observe any changes to the asteroid's orbit.

There is no danger that the mission will knock a currently harmless asteroid into a path that would threaten Earth, ESA says. It argues that even a very heavy impact would only deflect an asteroid by a very small amount.

An impact of the size planned in the Don Quijote mission will alter the asteroid's path by such a small amount that it would not be detectable from Earth. This is why two space craft are being dispatched: the second is needed to monitor the object for subtle variations in its orbit, following the impact of the first.

ESA describes the impact of a Near-Earth Object as being one of the few natural disasters we have the technology to prevent. It argues that this kind of work is needed now, because although there is no imminent threat that we are aware of, our ability to identify and track NEOs is not yet very sophisticated.

It points to the 400m asteroid 2004 MN4, which astronomers managed to catch up with again at Christmas time, having lost it since its discovery in mid-2004.

Initial measurements indicated that the rock stood an unusually good chance of impacting Earth in 2029. However, once earlier measurements were found, and a better trajectory was calculated, scientists were able to rule out a 2029 collision. The threat of later impacts has not been ruled out, however. ®