Space probe rehearses landing dance with asteroid
BY STEPHEN CLARK
Posted: November 12, 2005
With over a month of detailed remote study behind it, Japan's Hayabusa asteroid probe is ready to reach out and touch a small space rock hidden in the black depths of the solar system after a critical weekend test run confirmed all was ready for the high stakes operation that will gather the first ever asteroid samples destined for return to Earth.
The craft is consuming more fuel than expected due to the loss of two of three reaction wheels in the past few months. The reaction wheels normally control the orientation of Hayabusa in three axes. Control teams were forced to devise a plan to reduce fuel use after the failures.
On its two-month anniversary in the vicinity of asteroid Itokawa, the Hayabusa science platform swooped within 100 meters, or around 330 feet, of the potato-shaped object that measures 2,000 feet by around 900 feet. Its orbit stretches from inside Earth's out to a distance of 157 million miles from the Sun, making it a member of the Apollo class of near-Earth asteroids that pose potential impact threats to our planet.
The plunge began early Saturday, Japan time, as Hayabusa methodically descended primarily under the control of its chemical thrusters fueled by nitrogen tetroxide and hydrazine-derived propellants.
The probe eventually maneuvered to its hovering position under 330 feet
from Itokawa, where it released a tiny rover with an Earth weight of just
over one pound. An update of the project's web site confirmed that officials received communications from the craft after its release, but the Japanese Kyodo news agency later reported the explorer was likely lost.
At such close ranges, MINERVA could resolve minerals in rocks and pebbles, along with individual grains in loose regolith believed to be covering parts of the surface. A group of six thermometers were also fitted to the top and bottom of the rover to measure temperatures.
In the early parts of the mission's pre-launch development, NASA had planned to build a slightly more advanced 2.2-pound rover for inclusion on Hayabusa. That vehicle would have carried a camera along with infrared and X-ray spectrometers, but it was canceled five years ago this month after rising costs above the original $21 million estimate.
Hayabusa began ascending again after a short stay stationkeeping at the low altitude, and was expected to return to a position four miles away.
Saturday's descent also served as a dry run for two attempts later this month that will finish the trip to Itokawa's surface, where a 16-inch funnel will come in contact with the object and a metal pellet will be fired at up to several hundred miles per hour into the rocky crust.
The high-speed impact will blow rubble and dust through the funnel to be
captured in a chamber, which will carry the cargo on the 18-month ride
back to Earth and through the fiery re-entry back into the atmosphere. The
total amount gathered is anticipated to be around one gram - or about two
one-thousandths of a pound.
The sampling rehearsal was aborted in its first try on November 4 after an anomaly was detected in the navigation computer at an altitude of about 2,300 feet. Once the craft returned to its start position, engineers found that the image processor aboard Hayabusa was at fault.
"The asteroid is seen usually (as) a single object, but depending on the aspect angle with the Sun's direction, the object is seen comprised of multi- or many objects, since boulders and terrain are partially illuminated," said Jun'ichiro Kawaguchi. "If many objects are identified aboard, (the) processor cannot determine aboard what the direction should be."
The problem was subsequently fixed, and Hayabusa confirmed the patch on Wednesday when the spacecraft came within 230 feet of Itokawa to test navigation and guidance systems. Images from both Wednesday and Saturday showed the shadow of the 1,000-pound probe against the asteroid.
Named after the Japanese word for "falcon," Hayabusa arrived 12 miles from Itokawa on September 12, after a journey from Earth that stretched over two years in length. The probe, formerly known as MUSES-C, was launched in May 2003 from Kagoshima, Japan. Its four cutting edge microwave discharge ion engines - fed by xenon gas - accomplished over 25,000 hours of operating time during the craft's trip through the solar system, before handing propulsion control over to Hayabusa's chemically-fueled thrusters.
The arrival at Itokawa had been delayed several months after a large solar flare in 2003 degraded electrical production aboard Hayabusa, which curtailed the operation of the ion engines for a period of time.
The rendezvous sequence was delicately controlled largely by an autonomous navigation system that reduces human intervention from ground teams on Earth 180 million miles away. This technology, along with the ion engine and others, can be incorporated into future robotic space missions.
After almost three weeks in what officials call the "gate position," Hayabusa maneuvered closer to Itokawa to the "home position" just over four miles from the asteroid's surface.
Shortly after this milestone, Hayabusa experienced the loss of its second reaction wheel in early October. The failure came after the probe lost its X-axis wheel in July, and combined the malfunctions caused the navigation system to burn more precious chemical propellant than expected.
Scientists observing Itokawa have been surprised by the wide variety of surface features found by the visiting probe. Contrary to earlier hypotheses that small near-Earth asteroids would have relatively uniform characteristics, images show both massive boulders and protrusions cover parts of the space rock, while smooth areas mark other regions.
Hayabusa also worked to locate potential sampling sites, and used its spectrometers to analyze the areas to remotely determine composition and other data.
Scientists narrowed the list of attractive targets to just two - a region of smooth and loose rock known as the MUSES Sea, and a broad, flat, and rocky area dubbed Woomera. After higher resolutions pictures of the latter site were beamed back to Earth, officials opted to attempt both sample runs at the MUSES Sea locale because of concern with potentially hazardous rocks and boulders at Woomera, named after the central Australian test site that will receive Hayabusa's return capsule.
Data has also been used to determine mass and density estimates for Itokawa. "This may indicate there is substantial porosity for this body, and forces conventional views of these small objects to be changed drastically," the Japan Aerospace Exploration Agency, JAXA, said in a statement.
"The scientific discoveries reported here will redefine scientific notions and views of asteroids from the pre-Hayabusa era, and are a remarkable accomplishment that Japan has contributed to planetary exploration."
Asteroids offer scientists a glimpse back in time to the early solar system, when planets and other objects had just formed. Current information is confined only to remote studies and ground analysis of meteorites, and Hayabusa's returned samples will fill in the gap with a pristine source directly from space.