Hayabusa on track for landing in two weeks
BY STEPHEN CLARK
Posted: May 30, 2010
Two weeks before its scheduled return to Earth, Japan's Hayabusa asteroid explorer is halfway through a series of unprecedented ion engine burns to aim the probe for a narrow re-entry corridor toward Australia.
The Hayabusa mothership will release the 16-inch-wide entry capsule about three hours before landing as the probe travels around 25,000 miles from Earth. During re-entry, temperatures around the capsule will reach about 4,900 degrees Fahrenheit, but the tiny craft will be protected by a carbon-fiber heat shield.
Parachutes will deploy to slow the capsule's speed for touchdown in the Australian outback.
Because Hayabusa's chemical fuel tanks are empty, Japanese engineers had to devise ways to keep the spacecraft on course using ion thrusters, highly-efficient engines typically used for long-duration burns lasting thousands of hours.
Hayabusa's sole operational ion thruster, afflicted by its own technical trouble, has fired three times since early April to guide the spacecraft toward Earth. Each trajectory correction maneuver, which would normally be executed using chemical engines, takes up to several days to complete because of the ion engine's low thrust.
The craft completed its third correction firing early Thursday, Japanese time. The nearly 100-hour burn changed Hayabusa's velocity by 11 mph and put the probe on course for the Earth rim point, an imaginary target 200 kilometers above the planet's surface.
The fine-tuning burns beginning in April came after the ion thruster completed its long-term propulsion phase in late March.
Hayabusa will fire its ion engine two more times in the coming weeks to target the spacecraft's trajectory for the Woomera landing site.
The next burn is scheduled to begin around June 6 to bend Hayabusa's trajectory from the Earth rim point to its destination in Australia. A final maneuver, tentatively timelined for three days before re-entry, will correct any errors in the course toward the landing site.
Hayabusa, which is about the size of a compact car, launched from Japan in 2003 and spent three months exploring asteroid Itokawa in late 2005. Hayabusa means falcon in Japanese.
Although the craft likely did not achieve its objective of collecting samples from Itokawa, scientists are hopeful Hayabusa's landing capsule carries some asteroid residue. Even if the container is empty, the $200 million mission would still complete the first round-trip voyage to and from an asteroid, assuming the crippled spacecraft can complete the final two weeks of its journey.