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Thruster problem latest hurdle for asteroid probe BY STEPHEN CLARK SPACEFLIGHT NOW Posted: November 29, 2005 After weeks of confusion and setbacks, the Japanese Hayabusa probe finally met with success last week in its mission to retrieve the world's first assortment of asteroid specimens that will be bound for Earth within the next few weeks -- if controllers can put an end to a potentially ruinous problem with thrusters needed for the trip home. The spacecraft carried out its descent to the surface of asteroid Itokawa throughout Friday night and Saturday morning on its final chance to try to collect the precious samples to achieve one of the primary goals of the $100 million mission. An earlier attempt a week before had reached Itokawa, but had failed to gather samples due to a last-second abort ordered by an on-board sensor. Hayabusa finally touched the surface at a velocity of around four inches per second as scheduled in the early morning hours Saturday in Japan, and the retrieval procedure went according to plan. Data giving the first confirmation of the historic success reached the control room as the craft began to ascend away from Itokawa after spending just one second on the asteroid. The design of the sampling system called for a 16-inch diameter funnel to first make contact with the surface, followed by the firing of two bullet-like projectiles made of tantalum metal into the asteroid at several hundred miles per hour. The high-speed impact of the pellets is meant to blow bits of dust and rock through the collecting horn and into a chamber that will contain the specimens throughout the journey back to Earth that will culminate in a parachuted landing in June 2007 at the Woomera test site in central Australia. Two bullets were fired to increase the amount of samples to be transported back for study in laboratories, but the exact quantity will not be known until the return capsule arrives at Earth. Previous estimates of the total sum of material amassed over two separate passes were around one gram, or about two one-thousandths of a pound. However, the return to Earth hinges on a remedy for thruster problems that have plagued the mission since Saturday's sample retrieval. The trouble showed up during the ascent back to a staging position near the asteroid when the ground team noted a shaking and vibration in telemetry, reports said. The snag sent Hayabusa into a safe mode for the second time in a week, and controllers have struggled to regain control of the spacecraft over the past few days but so far to no avail. Next on the schedule for the intrepid space mission is the departure from the vicinity of Itokawa, which has lingered nearby for almost three months. If possible, that milestone is set to occur within the next ten days before an early December deadline, after which Hayabusa will be unable to make it back to Earth because it will be out of position in its orbit around the Sun. It took just over nine hours for the 1,000-pound probe to make its way from over 3,000 feet in altitude to the surface. During the descent, officials opted not to release the last remaining target marker, and chose instead to rely on guidance cues offered by an identical indicator already deployed during the first sampling attempt on November 20. At a distance of around 115 feet, the autonomous navigation system aboard Hayabusa began receiving precise measurements from a laser range finder. The device feeds range and closure rate data to the control system by using four laser beams to help determine not only the height but also information on the terrain below the probe. When Hayabusa reached around 45 feet in altitude, it began to adjust its orientation to match that of the slope of the surface of the asteroid directly below. Communications with the craft was also switched to a beacon mode as the high gain antenna was pointed away from Earth, which was located 179 million miles away on the far side of the solar system. This led to a reduction in the amount of telemetry reaching controllers in real time, and teams had to rely on Doppler data for updates on the status of the descent. During the last few critical moments of the approach to Itokawa, Hayabusa used a set of abort commands that limited the reasons to call off the attempt. The November 20 sample collection effort ended in an abort after a sensor detected potentially unsafe obstacles, although the spacecraft continued to descend to the surface, where it sat for up to thirty minutes before taking off again. That marked the first takeoff from an asteroid in the history space exploration. The new software package disregarded the sensor that caused the previous abort, but stipulated that the maneuver be stopped if the laser altimeter lost sight of the asteroid, the range finder ceased to function, or the probe's attitude varied beyond set restrictions. No issues arose as Hayabusa descended through the final feet to the surface, and touchdown occurred at 7:07 a.m. Japan Standard Time on Saturday morning (2207 GMT Friday). During the climb away from Itokawa, telemetry reached controllers indicating the on-board computer sent the signal to fire the bullets. This initial evidence of success came less than a half-hour after the momentary landing. Officials also pointed to signs that the sample funnel was somewhat warped following its soft collision with the surface of Itokawa. Other than the pressing thruster issue, Hayabusa has already countered several challenges during its mission, including a delay in the arrival at Itokawa after a solar flare damaged the capability to produce electricity, the loss of two reaction wheels, and aborts of a rehearsal and sampling pass earlier this month. These issues - though primarily the reaction wheel failures - have caused the craft to consume more chemical propellants than expected, leaving the future of the mission in the balance of dwindling fuel reserves. "Hayabusa may face more difficulties on its way back to Earth, but we should renew our determination and do our utmost to complete this challenging mission successfully," said President Keiji Tachikawa of the Japan Aerospace Exploration Agency, or JAXA. If returned, the samples will be the first material to be brought to Earth from any celestial body other than the Moon. The specimens will be analyzed for the precise chemical components of Itokawa to help scientists better string together the evolution of the early solar system, of which asteroids are believed to be ancient relics. The material will also help bridge a connection between the asteroid and certain types of meteorites that have fallen to Earth. Other scientists will be anxious to see the results to determine what minerals and elements on Itokawa would be commercially viable to mine if such ventures become reality in the coming decades. |
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