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Planetary News: Asteroids and Comets (2005)Hayabusa: Itokawa Beckons as Japan's Spacecraft Searches for Places to Touch DownBy A.J.S. RaylSeptember 16, 2005 Earlier this week, Hayabusa -- Japan's mission to the near-Earth asteroid Itokawa -- completed a 1-billion mile voyage and successfully settled into the heliocentric, parking orbit just above the big rock where it is now, conducting the world's first low-thrust rendezvous with a near-Earth asteroid despite being jolted just days before by an intense solar flare. The spacecraft is currently hovering about 12.5 miles/20 kilometers from the asteroid and is operating normally, reported Project Manager Jun'ichiro Kawaguchi. "We are very happy," said Kawaguchi, of the Institute of Space and Astronautical Science (ISAS) at the Japan Aerospace Exploration Agency (JAXA), in a telephone interview with The Planetary Society late Wednesday from Yoshinodai, Japan, not far from Tokyo. Hayabusa arrived at Itokawa on 10:00 a.m., Monday, September 12 Japanese Standard Time/1 a.m. Greenwich Mean Time/6 p.m. September 11, Pacific Daylight Time. Just getting to the asteroid is a major achievement that warrants a place in space exploration history, according to Bruce Murray, co-founder of The Planetary Society and former director of the Jet Propulsion Laboratory (JPL). "From a technological point of view, this is a remarkable achievement, and one that every space faring nation admires," he said yesterday from his office at the California Institute of Technology. "It is especially remarkable when you consider that Japan does not have the military aerospace infrastructure that the United States, Russia, and Europe have." But Hayabusa's journey is really just beginning, and its greatest challenges are still to come. The goal of JAXA's 4.5 year, $170 million mission is to touch-down twice on the surface of this revolving rock, making 'soft' landings for just long enough to collect two samples from two different spots on the asteroid, and then return the bounty to Earth. It is something that no mission that has gone before has done, and even though there is little gravity there to challenge its objective -- making the touch-downs more like dockings than actual landings, Hayabusa is confronting an incredibly complicated undertaking. For one thing, current estimates put the asteroid's size at 490-by-180 meters -- that's just 1600 feet by 590 feet -- and, from the images that Hayabusa has been sending home this past week, the team has discovered that Itokawa is quite a bit different than they assumed based on radar data collected from the ground at JPL. Although it is "ellipsoid," said Kawaguchi, the asteroid appears "like two bodies united with each other," as if fused together. Moreover, he added, "there are few craters on the surface and many boulders and rocks." "The paucity of craters is surprising to me," said Murray, "and suggests processes that have either buried or eroded the craters that surely must have formed early in its history." Indeed, the Hayabusa team had expected more flattened plains areas, Kawaguchi confirmed, and now "the big discussion is [about] where to land." In order to determine that, Hayabusa will spend the next month and a half, said Kawaguchi, intensely studying its prey and taking topographical measurements with various instruments onboard, including a laser altimeter, x-ray spectrometer, radar, and capturing more precise stereo images of the surface with its high-performance camera. Once the mission team selects the target areas on the asteroid, it will conduct a "rehearsal," Kawaguchi informed, "probably in early November," just to make sure all the systems are operating according to design. Then, Hayabusa will drop down to those sites, one at a time, snatch one gram's worth of dust samples, hop off, and return to its parking orbit. When both samples have been collected, the plan calls for Hayabusa in December to hit the space highway and head for home. Although the samples will be tiny, the bounty of knowledge to be learned from them will be great. That's because rocks contain the records of environments in which they have resided. And scientists believe asteroids preserve in their make-up the pristine materials that went into the formation of the solar system. While NASA's Apollo mission in the 1960s returned samples of Moon rocks, our companion planet has undergone thermal alteration, as have all the other larger bodies in our solar system, and so those samples could not provide us with the earliest records of the solar system. Asteroids, however, can -- and that is what makes them so alluring. JAXA designed the mission with the future in mind, and specifically to develop and test state-of-the-art technology required for sample-return missions. By the time the mission is over, Hayabusa will have tested four major space engineering technologies:
Hayabusa's four ion engines were designed to be activated throughout the flight to and from the asteroid. Developed by the electric propulsion engineering section at the ISAS division of JAXA, the engines ionize the propellant, xenon, by microwave, then accelerate the ions in a strong electric field and expel them at high speed, which provides the reactionary energy, or plasma jet, that propels the spacecraft. Although NASA's Deep Space 1 holds the distinction of being the first spacecraft to use ion drive, Hayabusa is the first to use microwaves to ionize xenon fuel, and the first Japanese craft to employ the technology. When Hayabusa flew by Earth for a gravity assist in May 2004, the event marked another important milestone for Japan and the world when it became the first spacecraft to perform a swing-by maneuver using an ion engine as the main thruster. Many space exploration scientists and engineers view the high efficiency of ion drives to be an essential technology for deep-space cruising on future planetary missions. Compared to the conventional chemical propulsion engines, electric propulsion engines are extremely efficient and promise long life, as well as high reliability. On the other hand, their thrust levels are quite low; consequently, they must operate continuously for a longer period of time to perform the same level of orbital maneuvers as that of the chemical propulsion engines. As a result, ion drives propel spacecraft in a slow and steady manner unlike the chemical propulsion engines, with the tortoise-vs.-hare comparison often noted as analogy. Like most space missions, Hayabusa has had its share of fits and starts and tense moments. Originally dubbed MUSES-C, it was first slated to target asteroid 4660 Nereus -- with 10302 1989 ML as backup -- and the mission was supposed to have carried a NASA/JPL rover, with the sample return landing to have been in Utah. But NASA backed out in late 2000. The project's original launch date in July 2002 on an M-5 three-stage solid-fuel rocket was delayed when a similar M-5 rocket failed to deliver Japan's Astro Ex-Ray Observatory into orbit. That delay meant that 1989 ML would be beyond the mission's grasp by the time the spacecraft could launch again. So, a new target -- asteroid 1998 SF36 -- was selected. Shortly thereafter, the International Astronomical Union (IAU) bestowed 1998 SF36 with its permanent name -- Itokawa, after the late rocketeer Hideo Itokawa, father of Japan's space program. The mission was finally launched by Japan's M-V-5 rocket from the Uchinoura Launch Center at Kagoshima on Kyushu Island, on May 9, 2003, in what controllers described as a picture perfect launch, into "blue skies." Once it was successfully on its way, the Japanese christened it Hayabusa. Six months later, in November 2003, the spacecraft encountered one of the biggest solar flares in recorded history. It was a major 'white-knuckle' moment, but the little 1100-pound/500 kilogram Hayabusa came through relatively unscathed, with only some solar cell performance degraded. Just before its arrival at Itokawa, on July 31, there was an incident on X-axis wheel, one of three reaction wheels onboard that control the Hayabusa's orientation, and are generally used to point instruments, antennas, or subsystems at chosen targets. The X-axis wheel [gyroscope] encountered increasing friction as it spun, to the point where it had to be shut down, Kawaguchi explained, and it is currently not functioning. However, since the spacecraft was designed to function just fine with the two remaining reaction wheels, he pointed out, it was able to resume attitude stability right away and has been operating normally since. When in recent days, the spacecraft got blasted again with another, intense solar flare, fate seemed still to be on Hayabusa's side. "It looks like no significant damage occurred to the spacecraft," Kawaguchi said. The project team is confident about the upcoming series of events, but is understandably concerned that every instrument onboard, and the spacecraft itself operate as they were designed, Kawaguchi said. Since it takes radio waves about 17 minutes to traverse the distance from the spacecraft back to Earth, once the action begins Hayabusa will essentially be on its own, relying on a highly autonomous navigation system. It will be assisted in its tasks by the Optical Navigation Camera and Light Detection and Ranging (ONC/LD&R) instrument, which will measure the distance to, and the shapes of the asteroid surface. Although mission control cannot maneuver Hayabusa in real-time, the team can send an order to stop the descent if it assesses danger, but that is about all it can do. Most of the critical decisions have to be made by the spacecraft itself. "This is an impressive demonstration of the confidence the Japanese have in their autonomous spacecraft control capabilities," noted Murray. "The descent is really a very sophisticated and challenging maneuver." After the mission team makes its final decisions on where and when to touch-down and gather its samples, Hayabusa will makes its first complete descent to Itokawa, measuring the relative position and attitude to the surface with its ONC/LD&R instrument, autonomously adjusting the propulsion engine as it transmits an image of the whole process to Earth. Prior to the actual touch- down, Hayabusa will shut down the propulsion engine, at about 164-196 feet/50-60 meters altitude, and enter into a free-fall descent, to prevent the jets from contaminating the asteroid surface by coming too close to it. At about 32.8 feet/10 meters altitude from the asteroid, the spacecraft will fire a 'target marker' -- a softball-sized, artificial sphere -- onto the asteroid. Then, guided by the 'target marker' from the asteroid's surface, which is reflecting light from a flash beam transmitted from the spacecraft, Hayabusa will continue descending to make a soft landing. The next task -- collecting a sample - will be Hayabusa's greatest challenge and will take place in a matter of seconds. The surface gravity of the asteroid is less than 1/100,000 of that of the Earth, and so anything that is not secured by a strong anchor into the surface may be pushed away. Since no one knows exactly what the asteroid is really made of, there is no way of knowing exactly how hard the surface will be or how difficult this collection of surface dust will be. The Japanese are employing a simple, direct strategy -- to "break the surface" by using a tiny pyrotechnic device to fire a "sampler horn," a cylindrical megaphone-shaped collection device, into the targeted site on the asteroid. Once the collection of surface dust is completed, Hayabusa won't waste any time starting its engine, lifting-off, and flying back to its hovering or parking position at 20 kilometers altitude, where it will wait for the next order from mission control. After the collection of the second sample, and following the spacecraft's final observations from its parking orbit, the team will bring Hayabusa home. The mission will leave behind its marks -- and the names of hundreds of thousands of Earthlings that were etched on an aluminum foil sheet and tucked inside the 'target markers' -- ostensibly for eternity, or the next visitors to Itokawa. The Planetary Society of Japan launched a worldwide campaign in 2002 to collect the names in a campaign promoted in Japan with the theme, "Let's fly to meet your star prince," an allusion to the title character in Antoine de Saint-Exupery's famous book, The Little Prince. [The Little Prince makes his home on an asteroid called B612.] In the final phase of the mission, Hayabusa will send a small reentry capsule carrying the asteroid samples into the Earth's atmosphere at a velocity more than 7,500 miles/12 kilometers per second. The maximum heating rate of the capsule in the reentry is much, much higher than that of the Space Shuttle, and several times higher than the Apollo recovery module, and so the Hayabusa re-entry capsule's heat shield is made of specially-designed, state-of-the-art material. After being fully decelerated by the atmosphere, the Hayabusa capsule is designed to deploy a parachute for a soft landing in June 2007, in, as it stands now, south Australia. And if all that goes as planned, Hayabusa will be the first mission in history to return samples from an asteroid. "It will be difficult," Kawaguchi admitted. "But it will be very exciting." |
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