23 March 2010—A small, severely damaged spacecraft named Hayabusa is limping back to Earth with a jury-rigged ion drive that would have made Star Trek’ s Scotty proud. Already seven years into its planned five-year mission to retrieve soil samples from an asteroid, the probe faces one last round of hazards before its midnight landing in Australia sometime in June with a sample canister.
If successful, the mission will mark the first retrieval of material samples from an object beyond the moon. (Two earlier NASA probes, Stardust and Genesis, retrieved nothing more than space dust from such distances.) Yet even if the canister turns out to be empty, the probe will still have brought back useful scientific observations, demonstrated the technology, and provided an inspirational example of ingenuity and perseverance.
Between 5 and 12 March, the Japan Aerospace Exploration Agency (JAXA) controllers turned off the ion drive to refine the craft’s navigation. The engine was then restarted for a final six-week burn to shift its Earth flyby path into a ”near miss” flyby. Early in May, the vehicle is supposed to ignite its hydrazine thruster to refine the course still further. No one knows whether the thruster, which had a major breakdown several years ago, will work at all.
The craft is supposed to aim itself at a corridor above the Indian Ocean and trend southeast into the Australian desert. It will briefly endure acceleration forces equal to 25 times the force of gravity; the heating loads will be much higher than the Apollo space capsules faced. If Hayabusa survives, it will pop a parachute, activate radio beacons, and land.
The probe carries four xenon-fueled ion engines, each capable of an acceleration of approximately three one-millionths of a gravity, or g. That may not seem like much, but the advantage of such engines is that they can push for tens of thousands of hours. The acceleration is slow but persistent. Other ion-drive probes, such as Europe’s SMART-1 lunar orbiter or NASA’s Dawn asteroid orbiter, are also using this technology.
Hayabusa (Japanese for ”peregrine falcon”) was launched 9 May 2003 and reached the half-kilometer-long potato-shaped asteroid Itokawa in September 2005. The arrival was delayed by several months when a solar flare damaged the probe’s solar arrays, reducing power for the ion engines, two of which also developed instabilities that required they be shut down. During the approach to the asteroid, two attitude-controlling reaction wheels also failed, requiring backup control from small thrusters. Several batteries also failed.
After mapping Itokawa’s surface in detail (including a dramatic photograph of the shadow that the probe itself cast on the face of the asteroid), Hayabusa approached the surface, opened a sample canister, and activated a soil-collection device. Several approaches and retreats were made with uncertain results. It is still not clear how much, if any, of the asteroid was scooped inside, but the controllers know that the canister lid was closed at the end when the probe pulled away.
As Hayabusa prepared to return to Earth, the probe endured a series of thruster and power problems that led to the loss of communications for several months. As a result, it missed the planned window for its return to Earth. The probe then coasted along near the asteroid for more than a year while ground controllers assessed its condition and planned an alternative return trajectory, which began in April 2007.
Over the next two and a half years, the probe’s engines delivered about 90 percent of the course correction required to achieve the new trajectory. But last November, the ion generator in one of the engines failed, leaving insufficient thrust to make the planned impact with Earth’s atmosphere.
Ground controllers then developed a scheme to activate a remaining component in another failed engine. The idea was to put the parts of two distinct engines together to form the equivalent of one ”virtual” engine. With one engine’s thruster and the other engine’s charge neutralizer, thrusting was resumed within two weeks.
JAXA officials remain tight-lipped about when in June the probe is to land. It seems to observers that it will happen sometime in the second week of the month. Aside from the recovery group, which hopes to find the capsule, another science team is awaiting the fireball entry to observe it as an ”artificial meteor.” An array of ground and airborne instruments will attempt to collect calibration readings with which to analyze other observations of natural objects hitting Earth along similar trajectories.
Amateur observers in Australia, from Nullarbor Plain to the Eyre Peninsula, may also be able to spot the fireball with the naked eye.
About the Author
James Oberg spent 22 years working as an engineer at NASA mission control; he is now a writer and consultant in Houston.