Do you know about Hayabusa?
Thought not. It's a Japanese robotic space probe, designed to rendezvous with the asteroid Itokawa, study it, grab a sample from its surface, and return that sample to Earth. Furthermore, it is semi-autonomous (meaning that it needs very little help from Earth) and powered by an exotic ion-drive engine.
And it has just arrived at its target. Hayabusa is now hovering about 20 kilometers (12 miles) from Itokawa, with nearly zero relative velocity. It's ready to start its two-month exploration, after which the ion drive will light up again to boost the little spacecraft back to Earth, where it will re-enter the atmosphere over the outback of Australia and its payload of asteriod samples will be recovered.
The ion engine has only been used a few times before, mainly on experimental flights. Instead of generating thrust by chemical heat-generating reactions (as conventional chemical rockets work), the ion engine uses electricity generated by solar panels to accelerate electrically charged particles (ions) at extremely high speeds. The "kickback" from this acceleration provides the thrust, and it does so with great efficiency. The high efficiency comes from two things: the high velocity of the accelerated particles (far higher than the exhaust velocity of a chemical rocket), and the fact that the source of energy (the sun) has almost no mass on the spacecraft (just the solar panels) — so the spacecraft doesn't have to push it's own fuel, as with a chemical rocket.
Congratulations to the team who put Hayabusa together! And may this initial success be a preview of the entire mission...
