More challenges await Japan's asteroid mission
BY STEPHEN CLARK
Posted: April 26, 2010
Japan's battered Hayabusa spacecraft must steer through several unprecedented course corrections using an improvised ion engine before reaching its long-awaited return to Earth in June.
The homecoming in Australia will occur around 1400 GMT, JAXA said in a statement. The probe is now traveling about 12 million miles from Earth.
The landing would complete an audacious seven-year round trip voyage to an asteroid. Hayabusa was supposed to gather samples at asteroid Itokawa, but its collection mechanism did not function as planned, leaving that objective in doubt.
Officials are not sure whether any rocks or dust from the asteroid were funneled into the craft's collection chamber.
Junichiro Kawaguchi, Hayabusa's project manager, said a lot can go wrong during the probe's final two months in space.
"The spacecraft is not in good shape," Kawaguchi said in an interview earlier this month. "If the spacecraft was in good condition, we would assume it was an easy thing from this point."
Since its launch in 2003, Hayabusa has lost three of its four ion engines, leaked out all of its chemical propellant and is down to a single reaction wheel.
The trouble delayed Hayabusa's departure from Itokawa, which forced JAXA to postpone the craft's return to Earth from 2007 until 2010.
Managers were close to declaring the $200 million mission a failure in November, when Hayabusa's sole operational ion engine stopped working. But controllers were unexpectedly able to link functioning components in two different thrusters to continue propelling the spacecraft toward Earth.
Kawaguchi chalks up the daring mission's multiple comebacks to the project's hardworking team and a lot of luck. He says he still is not sure Hayabusa can finish the trip home.
"I'm not optimistic, but I would love to be surprised," Kawaguchi said.
Despite firing for thousands of hours since November, the ion engine face several shorter firings in the next few weeks that will exercise the system through multiple starts and stops, officials said.
Ion engines are highly-efficient propulsion systems that produce thrust by rapidly accelerating charged xenon gas particles.
"The spacecraft's ion engine is not in good condition," Kawaguchi said. "There is only one reaction wheel left available. Everything is cause for some concern."
Because its conventional thrusters are not operating, Hayabusa's ion engine will fire four times in May and June to put the probe on course for landing at the Woomera Test Facility in South Australia.
Two burns around May 5 and May 29 will bend Hayabusa's trajectory to aim for a point about 200 kilometers, or 120 miles, above Earth's surface. Two more lengthy firings in early June will put the spacecraft on a precise path toward landing.
"That final maneuver will take about 20 hours or more," Kawaguchi said. "It just cannot happen instantaneously."
About a day before re-entry, Hayabusa will turn its sample container toward the sun to heat the canister.
"We extended the flight time more than three years, so the remaining capacity [in the battery] is a concern," Kawaguchi said. "We want to heat it up and prevent it from becoming cold. We changed the separation time as late as possible to three hours before entry."
The capsule will be spring-ejected from the Hayabusa mothership as it flies within 25,000 miles of Earth.
"The spacecraft releases the capsule by pushing a spring," Kawaguchi said. "When the capsule is released, a spinning motion is applied to the capsule, so the capsule will have a spin at a very slow rate."
The 16-inch-wide canister will dive into the atmosphere at more than 25,000 mph. Temperatures around the capsule will reach about 4,900 degrees Fahrenheit, but the tiny craft will be protected by a carbon-fiber heat shield.
The capsule will fly over Antarctica on the approach to Australia, deploy its parachute after reaching the lower atmosphere, and float gently to Earth while emitting a navigation beacon signal to nearby recovery teams.
The Hayabusa mothership will also plummet into the atmosphere, but it will burn up during re-entry, according to Kawaguchi.