Fate of Japan's troubled asteroid probe still uncertain
BY STEPHEN CLARK
Posted: December 11, 2005
Japanese officials are struggling to fix a horde of problems plaguing the Hayabusa space mission in time to begin its journey back to Earth with or without a package of specimens that were supposed to have been collected from the surface of asteroid Itokawa late last month.
The issues have delayed the return trip to Earth that was due to begin within the first ten days of December, according to previous news reports. New information also suggests the last ditch effort to collect the world's first samples from an asteroid - first described as a success - may not have achieved its goal.
After a November 26 approach to gather samples from the potato-shaped space rock, the 1,000-pound probe encountered a series of difficulties with its chemical propulsion system that is responsible for controlling the craft's orientation.
First, as scientists rejoiced after an apparent successful end to a highly ambitious three-month stay in the vicinity of Itokawa, controllers noted a propellant leak in Subsystem B, one of two attitude control subsystems aboard Hayabusa. Commands were sent to shut latching valves aboard both subsystems, and the spacecraft was left in safe mode as conditions were slowly stabilized.
Hayabusa had earlier gone into safe mode last month, but officials worked several days to finally recover the probe to set up a pair of sampling attempts in the succeeding weeks.
However, this time the revival was not as trouble-free, and communications passes in the next few days were not able to bring back Hayabusa to normal operations. Control jets in Subsystem A were not producing enough thrust to point the craft's high gain antenna toward Earth. Investigations since then have concluded this was likely due to frozen propellant, while a "severe fuel leak continued," a status report said.
During this period, Hayabusa was also suffering a serious electricity shortage that almost fully drained the battery, engineers now believe. On November 28, no communications were received from the spacecraft at all, but a low bandwidth beacon signal was obtained the next day. By November 30, recovery operations began in earnest with the aid of the on-board computer than can work without help from the ground.
"It was unusually fortunate that the spacecraft recovered the attitude, power, and communications," a December 8 internal update said.
The mission has been relying more heavily than planned on chemical propellant quantities after two of three reaction wheels failed since this summer. Normally the assemblies would be responsible for attitude control, but thrusters must now take care of those activities.
Waning fuel numbers have drawn into doubt the chances for mission success since October, and the subsequent fuel leak certainly complicates the state of affairs.
Another setback struck the asteroid explorer on December 1 when controllers believe the spacecraft inexplicably pointed its solar panels away from the Sun, further depleting batter power reserves. This caused the instruments aboard Hayabusa to shut off, and controllers were then faced with the prospect of slowly restarting the systems one by one, lengthening the time it would take to downlink data that could explain that events that transpired to create the sticky situation, which officials continue to portray as not being optimistic.
Tests of the chemical propulsion system on December 2 showed it to still be sluggish, and engineers then set out to devise a new method of attitude control employing the xenon gas used by the ion engines that are needed for the trip back to Earth. The amount of xenon in pressurized tanks on Hayabusa is sufficient for both the required lengthy ion engine burns and attitude control, according to the project team.
The new technique proved to be a success, paving the way for the transmission of recorded data back to controllers on December 6 to be analyzed for information on the final outcome of the November 26 sample gathering maneuver.
Officials did not get the definitive verdict they were seeking because data gave inconclusive results. The ground team was feeling positive about the approach because initial reports indicated the order to fire the sampling bullet into Itokawa's surface was sent as expected.
Plans called for Hayabusa to deftly descend toward the asteroid for several hours before a funnel was to gently make contact with the surface. A small projectile made of tantalum metal was to be fired into the space rock at several hundred miles per hour, and the dust and rock kicked up by the impact was supposed to be gathered by the collecting horn and deposited in a chamber designed to ferry the specimens back to Earth by June 2007.
The firing command was indeed sent, but the confirmation of success was not yielded in the data downlinked to Earth last week. Instead, the records showed no indication the pyrotechnic explosives designed to shoot the tantalum pellet were fired, but a "no fire" indicator was also not included in the data.
A disarming command was also found to be incorrectly positioned, and engineers are unable to determine if it was triggered before last month's touchdown. However, other signs in the data - such as a temperature increase - would support the firing of the explosive device, though project manager Jun'ichiro Kawaguchi said the odds were "not high enough."
By the end of last week, conditions aboard Hayabusa were stabilized, and controllers were able to consistently communicate with the craft through the medium gain antenna. The probe was drifting away from Itokawa, and had reached a distance of around 500 miles from the space rock, which was located some 180 million miles from Earth.
Officials say the earliest possible date for the resumption of ion engine operations is Wednesday, but that hinges on no other problems impeding the progress of the ground team's quest to get the probe shipshape by then.
Despite the problems currently facing Hayabusa, the probe has successfully passed many previous tests and has achieved several firsts in space exploration, including its distinction was the only spacecraft to ever take off from an asteroid or any celestial body other than the Moon.
"We are now at the position to make another mission proposal of sample and return using advanced technology," Kawaguchi told Spaceflight Now. "No other country is so sure to make it than Japan, we think. Accessing and making a touchdown plus landing to (an) irregular-shape body (at a) pin-point landing site requires sophisticated technology, which we think now we have obtained in our hands."