Daring asteroid probe on course to reach Earth in June
BY STEPHEN CLARK
Posted: January 11, 2010
Japan's gritty Hayabusa probe isn't the first mission to be called the little spacecraft that could, but the small robot is on the verge of concluding a remarkable journey through the cosmos.
Hayabusa was primarily conceived as a demonstration mission to test satellite technologies, including an innovative and highly-efficient ion propulsion system that consumes xenon gas. The mission's secondary, but more visible, objective was to fly to an asteroid and scoop samples off its rocky surface for return to Earth.
Japanese space officials are expected to announce this week that the probe's return to Earth is becoming more likely. Hayabusa's ion engines will put the craft on a path by about Wednesday to be captured by Earth's gravitational pull sometime in June, according to mission officials.
"We think, as a technology demonstrator, Hayabusa has a big mission of accomplishing a round trip to asteroid," said Junichiro Kawaguchi, Hayabusa's project manager. "And from that point of view, we are about to complete the mission."
Kawaguchi is referring to the Hill sphere, the region of space where Earth's gravity is the dominant force affecting nearby objects. Earth's Hill sphere extends about 1.5 million kilometers, or 932,000 miles, in all directions from the planet.
Entering the Hill sphere does not mean Hayabusa is on course to intercept Earth.
Hayabusa's sole operating ion engine will continue thrusting until March to guide the spacecraft on a razor-thin trajectory to release a hardened capsule for re-entry over Australia. The rest of the refrigerator-sized probe will burn up in Earth's atmosphere.
Officials won't reveal a specific date for the arrival, saying they wish to prepare for the recovery away from the glare of publicity. Japan has a mixed history with robotic deep space missions, including the failed Nozomi mission to Mars and the ambitious and successful Kaguya probe that studied the moon.
The Nozomi mission was afflicted with problems similar to Hayabusa, and Japan maintained the probe could reach Mars until the spacecraft missed the Red Planet and flew into solar orbit in 2003.
The Hayabusa spacecraft is now limping through space, propelled by a makeshift ion engine using components from two powerplants previously declared failed. Ground teams rigged the new thrusting technique after Hayabusa's last fully operational engine stopped working in November.
Hayabusa is also down to one reaction wheel to maintain the probe's orientation in space. The craft's other two reaction wheels failed within two years of launch.
"We have devised a (plan) to lower the rotation speed and also relaxed torque level for Hayabusa," Kawaguchi said. "However, it is kind of a miracle that the wheel is still alive and available."
Engineers are also worried that residual propellant from a 2005 fuel leak could still coat the spacecraft's outer skin. When Hayabusa travels closer to the sun as it approaches Earth, the fuel could heat up and evaporate, causing an "eruption" that may send the spacecraft in an out-of-control tumble, according to Kawaguchi.
Assuming Hayabusa survives those toils, the spacecraft's return capsule must survive a fiery-hot re-entry into the atmosphere with a heat shield two years beyond its design life.
That's because a series of problems after Hayabusa's 2005 reconnaissance of asteroid Itokawa forced managers to postpone the probe's return from 2007 until 2010.
If Hayabusa successfully returns to Earth, Japanese engineers will have accomplished a feat unmatched by the world's other space agencies -- the return of a spacecraft from the surface of an asteroid.
Hayabusa inadvertently spent about 30 minutes on the surface of Itokawa in November 2005 during a botched attempt to gather samples from its gravelly surface.
The sample collection system was designed to fire a projectile into the asteroid's surface, breaking loose bits of rock and funneling the material into a chamber.
According to an analysis of telemetry recorded from the spacecraft, Hayabusa never fired a pellet during two sample collection attempts, deflating the hopes ot scientists.
Itokawa is a potato-shaped asteroid with a very low density. Scientists describe such objects as rubble piles.
Kawaguchi said there is a low chance fine grains from Itokawa were transported into the collection chamber as Hayabusa bounced on the asteroid's surface. But there would be "no surprise" if the container is empty, Kawaguchi said.
Despite long odds, Hayabusa has overcome every obstacle thrown in its path since its launch in May 2003. Only five months stand between Hayabusa and history, but the mission's toughest challenges may still be ahead.
Kawaguchi gives a 60 percent chance of Hayabusa completing its journey and returning its re-entry capsule to the ground, with or without a cache of precious samples.