A celestial “smash-and-grab” space mission that could become the greatest triumph in the history of the Japanese space program is entering its most challenging stage in deep space.
Together with NASA's comet-crashing Deep Impact mission, the Stardust mission to bring cometary material back to Earth and the European Space Agency's comet-chasing Rosetta spacecraft, Japan's Hayabusa probe is helping inaugurate a new era of interplanetary exploration: the physical examination of the surfaces of small bodies such as comets and asteroids.
The Hayabusa probe is slowly closing in on a distant asteroid named Itokawa. Within a few months, after surveying the asteroid thoroughly from a safe distance, Hayabusa will swoop down to its surface and grab samples of the dirt for return to Earth, like a spacefaring bird of prey. In fact, the spacecraft's name comes from the Japanese word for "peregrine falcon."
Already closer to the asteroid than the distance between Earth and its moon, Hayabusa is approaching at a relative speed of 225 mph (100 meters per second) and is firing its gentle but exceedingly persistent engine to further slow its speed.
The probe uses an ion-drive system pioneered by NASA’s Deep Space 1 comet scout, but with a distinctive design innovation. It is the first probe to use microwaves to ionize the xenon fuel. Through electrostatic deflection — like charges repel each other — the four engines can produce a slow-but-steady gain or loss in total speed amounting to as much as 27 mph (12 meters per second) per day.
That’s about the same velocity gain that a rising rocket booster racks up in half a second. But traditional thrusters operate only sporadically for a few seconds or a few minutes at a time. In contrast, Hayabusa has been thrusting for two years, and now it has essentially reached its distant goal.
“The distance is within 400,000 kilometers [250,000 miles],” project manager Junichiro Kawaguchi of Japan’s Institute of Space and Astronautical Science reported in an e-mail exchange with msnbc.com.
Although Hayabusa took detailed pictures of Earth during a high-speed flyby last year, it has yet to turn its camera onto its target. “The asteroid is small,” Kawaguchi explained, “and also the attitude constraint [for firing its braking engines] restricts the camera to be oriented to the object.”
The probe and nearby asteroid will pass behind the sun next month, restricting radio contact. Images should start coming in once the radio interference clears up, Kawaguchi said. “In mid-August, it should be seen at the magnitude of 5 or so,” he explained, initially only as a dim speck of light that will grow gradually brighter. (As seen from the spacecraft's point of view, a magnitude-5 object would be slightly brighter than the dimmest light sources that can be seen from Earth with the naked eye.)
When Hayabusa was launched two years ago, mission planners expected the probe to be already at the asteroid by this time — but the schedule was delayed because of a crippling event that occurred during the voyage.
“It is due to the heavy solar cell damage caused by the historical largest solar flare that occurred in 2003,” Kawaguchi explained. The consequent reduction in electrical power from the craft’s solar cells caused Hayabusa's electrical propulsion system to lose thrust.
The probe is now expected to arrive at its "home station" stand-off position early in September.
The delay has been worrisome, however, because there’s only one safe route home with the samples, and that requires the probe’s departure from the asteroid in mid-November. For the kind of trajectory that the craft’s ion-powered engines can achieve, it’s a matter of leaving on time, or becoming a permanent asteroid resident.
Consequently, all the scouting and remote-sensing operations that had originally been planned to take three or four months must now be accomplished in half the time. This advance reconnaissance is essential if the actual landings — and several are planned — are to have any reasonable chance of success.
Hayabusa is aimed at a small piece of space debris once known as “Asteroid 1998 SF36” — signifying that it was discovered only a few years ago and was too small for anybody to bother to name. But once it had been selected as a sampling target, the International Astronomical Union acceded to Japan’s request to name it after Hideo Itokawa, the father of Japanese rocketry.
The potato-shaped asteroid measures about a quarter-mile (500 meters) wide, with a gravitational pull hardly more than a millionth of Earth’s. Itokawa's gravity is so faint that the probe won’t even bother to orbit the asteroid. Instead, it will hover about 12 miles (20 kilometers) away, surveying the surface both from the full sunlit side and then later from above the boundary between day and night.
Itokawa's shape and density are uncertain, and its 12-hour rotation period creates extra navigation hazards during the Hayabusa probe's slow approach.
After several weeks of surveying, the probe would begin its main task: retrieving about a gram (0.036 ounce) of dirt from up to three points on the surface, and then returning the samples to Earth two years later. The probe is too far away for real-time remote control from Earth, so to perform this delicate operation the probe has a sophisticated autopilot.
How it will be done
Using small liquid-fueled engines (rather than the low-thrust ion drive that serves during interplanetary cruising), Hayabusa will approach a pre-selected touchdown point. It will use a ranging laser to measure its approach range and speed, and half an hour before contact will deploy an optical sensor into the soil so that its camera system can sense any horizontal drift rates. The first sensor, about the size of a softball, will carry almost a million names of people who supported the project.
The probe will contact the surface with a large "collection horn," and then it almost immediately will fire a bullet into the dirt. Some of the material that scatters from the impact will make its way into a collection chamber, which will then be sealed. This process can occur up to three times at different locations.
Hayabusa will also deploy a small hopper robot named Minerva. This solar-powered mini-spacecraft will relay images from its three cameras to Hayabusa whenever the two vehicles are in direct line-of-sight contact.
Assuming the craft’s power system and four ion engines continue to function, the craft will return to Earth in July 2007. A special capsule will hit the atmosphere at 29,000 mph (13 kilometers per second), about the same speed as returning Apollo spacecraft, and undergo forces of about 25 G’s before touching down in central Australia.
The material will be brought to a new national laboratory in Japan. There it will be analyzed, and some of it will be shared with foreign investigators.
James Oberg, space analyst for NBC News, spent 22 years at the Johnson Space Center as a Mission Control operator and an orbital designer.