Team trying to regain control of troubled asteroid probe
BY STEPHEN CLARK
Posted: March 11, 2006
The Hayabusa space probe that last year attempted to capture the first samples of an asteroid has regained stability after a mishap in December left the craft out of control. Japanese officials remain hopeful that the craft can return to Earth in June 2010, with or without the bits of rock coveted by scientists worldwide.
For over six weeks, Hayabusa - which means "falcon" in Japanese - was out of the reach of ground controllers after a sudden attitude disturbance caused its communication antennas to point away from Earth. The loss of signal from Hayabusa came on December 8, and engineers investigating the incident concluded the likely cause was a gas eruption involving leaked chemical propellants.
Ground stations re-acquired a beacon signal from Hayabusa on January 23, and officials found the spacecraft had shifted orientation during the six-week dark period. The last telemetry beamed down from the probe in December had shown it to be in a slow spin at one degree per second. By late January, Hayabusa was rotating seven times faster in the opposite direction, and the spin axis had swung almost 90 degrees. In addition, the high gain antenna was pointed approximately 70 degrees from the direction of Earth. The change can probably be attributed to the further leakage of propellant.
"The spacecraft was designed to settle to a pure simple spin motion around the antenna axis," explained Junichiro Kawaguchi, project manager for Hayabusa at the Institute of Space and Astronautical Science.
Evaluations from December predicted a sixty to seventy percent chance of restoring communications within one year, said Kawaguchi. "We are a little lucky, since the restoration came earlier than anticipated."
On January 26, systems aboard Hayabusa began responding to uplinked commands, and the craft had completely revealed its health information by the first week of February. Data showed that the lithium ion battery was out of commission, the spacecraft had suffered a power outage, and oxidizer tank was empty. Hydrazine fuel for the maneuvering thrusters leaked out in December.
A newly developed attitude control method was employed to gradually nudge the spin axis toward the Earth and Sun. The technique uses cold xenon gas as a low-thrust solution to continue reliable communications between the probe and ground stations. The use of xenon proved to be successful as the signal strength and bandwidth improved throughout February, and a link via the medium gain antenna was restored on March 4.
Range and Doppler measurements also supplied officials with Hayabusa's accurate position for the first time since December. The craft is leading Itokawa by about 8,000 miles in its orbit around the Sun, and is flying over 200 million miles from Earth on the opposite side of the solar system.
Despite the improvement of conditions aboard Hayabusa, managers are careful to point out the $100 million mission still has to clear many hurdles before it can reach Earth. Leaked chemical propellant could still be covering the craft and could cause another out-gassing event that would again disrupt communications.
Engineers will likely have to diligently plan a series of "baking" procedures using on-board heaters to disperse the chemical residue without causing another gas eruption that would send Hayabusa out of control. First, the ground team will attempt to clear the spacecraft and sample return capsule, and then another bake out will be carried out on the cruise home under the power of the ion propulsion system.
About 95 pounds of xenon gas remain inside Hayabusa - more than enough to get the probe back to Earth if nothing else goes wrong. The xenon will be used for both attitude control and to power the craft's four ion engines, which are due to be powered up in early 2007 to begin the three-year journey home. At most, up to three ion engines will be operated simultaneously.
"Now the spacecraft attitude is within our hand, but the spacecraft is threatened by another gas eruption," Kawaguchi said. "That is why we have to perform the baking operation to exclude those gases from the spacecraft. The xenon gas left is sufficient for the return cruise to the Earth."
Several key spacecraft systems remain untested following December's violent events. The ion engines have not been turned on since last summer, and critical electronics and the navigational star tracker still must undergo thorough checks before being declared healthy.
During its scientific campaign at Itokawa last fall, the small 1,000-pound probe inadvertently landed on the asteroid for up to 30 minutes before performing the first take-off from an asteroid in history. Mission plans had called for the spacecraft to fire a large bullet into the surface of Itokawa to eject bits of rock that were to be funneled into a collection chamber for return to Earth.
A subsequent attempt on November 26 was initially called a success after telemetry showed the command was sent to fire the tantalum metal projectile that was designed to impact the asteroid's crust. However, data sent down from Hayabusa ten days later put the successful outcome in doubt. The new information indicated the firing mechanism was likely disarmed, and controllers could not determine with certainty whether or not the pellet was fired.