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Asteroid probe fails to land for sampling mission
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 20, 2005

A crucial milestone in the Hayabusa asteroid mission failed this weekend after officials confirmed the craft did not succeed in its planned momentary asteroid touchdown that was supposed to deliver a crushing blow to a potato-shaped space rock to collect a small sampling of rock and dust for return to Earth in 2007.

 
An artist's concept shows Hayabusa during its planned touchdown to collect samples of the asteroid. Credit: JAXA
 
Under the control of an autonomous navigation and control system, Hayabusa fired its thrusters to nudge itself on a meticulous collision course with Itokawa early Sunday (Japan time). During the approach, the craft released a target marker at an altitude of 130 feet to aid in the delicate guidance and navigation process to steer the 1,000-pound probe to the surface. The marker contained the signatures of 880,000 people from across the globe.

Events proceeded as planned to a point just 55 feet from the asteroid, when attitude control was successfully switched to a system relying on the laser range finder to determine precise information about range and closing rate relative to Itokawa. At this time, mission managers were not able to follow the final descent through high gain communications, but were keeping tabs via Doppler data.

Hayabusa then stopped firing its thrusters, project manager Jun'ichiro Kawaguchi explained. The probe began inching its way toward the surface in free fall, but after a half-hour no touchdown had occurred. After analyzing available data, engineers concluded Hayabusa probably stalled at very low altitudes fewer than 30 feet, and never came in contact with the asteroid.

An abort was eventually ordered because the spacecraft likely drifted away from its planned landing site, and as Hayabusa began ascending again, it unexpectedly went into safe mode. Officials conjectured the cause of this was an attitude anomaly of some kind at a distance of around 30 feet.

The safe mode commanded the probe to fly away from Itokawa at a very high velocity, Kawaguchi said. Reports indicate Hayabusa flew up to 60 miles from the asteroid, all the while burning precious fuel.

Scientists want to downlink data and images gathered from the craft's sensors as soon as possible to help determine exactly what went wrong. Concern is also focused on the health of Hayabusa's four science instruments, which were subjected to extremely high temperatures from reflected solar radiation near the surface. "We need to make sure every instrument is still in order," Kawaguchi said.

Recovery operations to regain control of the probe were successful after a frantic 24 hours, and managers want to have Hayabusa return to a staging position a few miles from Itokawa in the next several days.

The setback throws into some doubt the future of the mission, which is relying on rapidly depleting chemical propellant reserves of hydrazine and nitrogen tetroxide. The increased use of fuel is due to the loss of two of three reaction wheels aboard Hayabusa, which govern the orientation of the spacecraft in three axes. However, thrusters must now take control of those duties in two axes.

The outlook for the fuel is "not optimistic," according to a senior project official before this weekend's botched descent. "But there is no choice (but) to perform sampling, we think."

On a positive note, guidance and navigation using the innovative autonomous control system was on the mark as it was during rehearsals in the past few weeks. Despite fuel worries, hopes are high that another touchdown attempt can be made.

"The project looks positively at the next opportunity, since almost every difficult step was now identified to function normally," Kawaguchi told Spaceflight Now.

A second touchdown was on the schedule for around November 25, and the departure of Hayabusa from the vicinity of Itokawa was to follow in early December with the command to fire up its microwave discharge ion drive engines for the return journey back to Earth. Together the powerplants - fed by xenon gas and electricity - amassed over 25,000 hours of operation to power the probe on the first leg of its voyage to Itokawa.

A final target marker remains aboard Hayabusa, and the last try to collect samples could be carried out before the December deadline passes.

Officials in charge of the $100 million Hayabusa mission saved the climax of the flight for the craft's last few weeks in the vicinity of asteroid Itokawa, a member of the Apollo-class of near-Earth asteroids that pose potential impact threats to our planet. Itokawa measures roughly 2,000 feet by 900 feet, and was discovered in 1998.

Controllers carefully selected the location to attempt the sampling after an extensive survey of the entire asteroid in the probe's two months near Itokawa. After some deliberation, a region known as MUSES Sea was chosen due to its lack of large boulders and other potential obstacles. The site is relatively smooth and is characterized by loose dust and rock.

The locale was named after the original designation for the Hayabusa mission. The probe, formerly known as MUSES-C, was launched in May 2003 from Kagoshima, Japan. After reaching space, the craft was renamed Hayabusa - the Japanese word for "falcon" in English.

In the sampling attempt, a 16-inch diameter funnel attached to the return capsule housed on Hayabusa was to make delicate contact with the surface of Itokawa. A small bullet-like projectile made of the tantalum metal was then to be fired into the surface at several hundred miles per hour, impacting the surface and blowing bits of rock through the collecting horn and into a chamber designed to carry the material back to Earth.

The 44-pound entry module is supposed to be released from the Hayabusa mothership shortly before arriving back at Earth, and the capsule is designed to plummet into the atmosphere at eight miles per second and parachute to a landing at the Woomera test site in the Australian outback in the summer of 2007.

The total amount of material recovered from Itokawa on two sample runs was expected to total less than one gram - or just two one-thousandths of a pound. Ground controllers did not expect to be able to directly determine the amount gathered until Hayabusa's return home, but officials said they would have been able to confirm samples were indeed collected using circumstantial evidence beamed back to Earth, almost 180 million miles away.

Controllers put Hayabusa through a rehearsal of the close approach on November 12, bringing the craft to within 180 feet of the asteroid. The maneuver verified the successful use of the laser range finder, a device that was critical to this weekend's operations.

During this practice run, the probe also released a tiny 1.3-pound rover called MINERVA (Micro/Nano Experimental Robot Vehicle for Asteroid). The robot was designed to slowly fall to the surface with a set of three cameras and six thermometers, and use Itokawa's ultra-low gravity field to hop around the asteroid during its mission.

However, MINERVA never had the opportunity to begin its trek across Itokawa due to a guidance miscue that caused the autonomous system controlling the altitude of Hayabusa to fall into an open-loop state, the mission's project manager Jun'ichiro Kawaguchi reported. This led to a situation known as "pseudo-hovering," he said, with no accurate information on the range rate between the craft and the asteroid.

Engineers have determined that Hayabusa was actually ascending away from the surface at a rate higher than Itokawa's escape velocity at the time of the rover's deployment. This caused the robot to likely miss the asteroid entirely, although cameras aboard MINERVA did operate as planned and captured images of part of Hayabusa's solar panels as it fell away.

The release also occurred at a point in time between a pair of two-way ground station communication passes, further complicating the operation.

Hayabusa has already completed a series of global observations of Itokawa with its payload of four scientific instruments, which includes a visible camera, laser altimeter, and near-infrared and X-ray spectrometers. The probe arrived at the asteroid in mid-September.

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