Japan shoots for a piece of an asteroid

A celestial “smash-and-grab” space mission that could become the greatest triumph in the history of the Japanese space program is entering its most challenging stage in deep space.

Together with NASA's comet-crashing Deep Impact mission, the Stardust mission to bring cometary material back to Earth and the European Space Agency's comet-chasing Rosetta spacecraft, Japan's Hayabusa probe is helping inaugurate a new era of interplanetary exploration: the physical examination of the surfaces of small bodies such as comets and asteroids.

The Hayabusa probe is slowly closing in on a distant asteroid named Itokawa. Within a few months, after surveying the asteroid thoroughly from a safe distance, Hayabusa will swoop down to its surface and grab samples of the dirt for return to Earth, like a spacefaring bird of prey. In fact, the spacecraft's name comes from the Japanese word for "peregrine falcon."

Already closer to the asteroid than the distance between Earth and its moon, Hayabusa is approaching at a relative speed of 225 mph (100 meters per second) and is firing its gentle but exceedingly persistent engine to further slow its speed.

The probe uses an ion-drive system pioneered by NASA’s Deep Space 1 comet scout, but with a distinctive design innovation. It is the first probe to use microwaves to ionize the xenon fuel. Through electrostatic deflection — like charges repel each other — the four engines can produce a slow-but-steady gain or loss in total speed amounting to as much as 27 mph (12 meters per second) per day.

That’s about the same velocity gain that a rising rocket booster racks up in half a second. But traditional thrusters operate only sporadically for a few seconds or a few minutes at a time. In contrast, Hayabusa has been thrusting for two years, and now it has essentially reached its distant goal.

“The distance is within 400,000 kilometers [250,000 miles],” project manager Junichiro Kawaguchi of Japan’s Institute of Space and Astronautical Science reported in an e-mail exchange with msnbc.com.

Although Hayabusa took detailed pictures of Earth during a high-speed flyby last year, it has yet to turn its camera onto its target. “The asteroid is small,” Kawaguchi explained, “and also the attitude constraint [for firing its braking engines] restricts the camera to be oriented to the object.”

The probe and nearby asteroid will pass behind the sun next month, restricting radio contact. Images should start coming in once the radio interference clears up, Kawaguchi said. “In mid-August, it should be seen at the magnitude of 5 or so,” he explained, initially only as a dim speck of light that will grow gradually brighter. (As seen from the spacecraft's point of view, a magnitude-5 object would be slightly brighter than the dimmest light sources that can be seen from Earth with the naked eye.)

Delayed arrival
When Hayabusa was launched two years ago, mission planners expected the probe to be already at the asteroid by this time — but the schedule was delayed because of a crippling event that occurred during the voyage.

“It is due to the heavy solar cell damage caused by the historical largest solar flare that occurred in 2003,” Kawaguchi explained. The consequent reduction in electrical power from the craft’s solar cells caused Hayabusa's electrical propulsion system to lose thrust.

The probe is now expected to arrive at its "home station" stand-off position early in September.

The delay has been worrisome, however, because there’s only one safe route home with the samples, and that requires the probe’s departure from the asteroid in mid-November. For the kind of trajectory that the craft’s ion-powered engines can achieve, it’s a matter of leaving on time, or becoming a permanent asteroid resident.

Consequently, all the scouting and remote-sensing operations that had originally been planned to take three or four months must now be accomplished in half the time. This advance reconnaissance is essential if the actual landings — and several are planned — are to have any reasonable chance of success.

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