Mu Space Engineering Spacecraft-C (MUSES-C), as the project is known, is a unique effort by Japan's Institute of Space and Astronautical Science (ISAS). Japan's space agencies, including ISAS, have recently united under a new institute called JAXA. The first to make a two-way trip to an asteroid, Hayabusa will also be the first craft to complete a sample-return mission to an asteroid.

Although the project is not the only sample-return mission worldwide, NASA project scientist Don Yeomans called it an achievement to rival NASA's Stardust mission - which plans to retrieve dust particles as it passes through the tail of a comet. "MUSES-C has been a remarkably successful mission to date", said Yeomans, who oversees the American members of the mission.

Hayabusa originally targeted asteroid (10302) 1989 ML. But the project's original launch date in July 2002 on an ISAS M-5 three-stage solid-fuel rocket was delayed when an M-5 failed to hoist Japan's Astro E x-ray observatory into orbit in February 2000.

The estimated $170 million mission was stalled for a year while the rocket's engine failure was investigated. When it finally launched, the MUSES-C spacecraft was named Hayabusa - "Peregrine Falcon." This bird of prey is known for its ability to swoop down and snatch up its prey in its talons - a maneuver that some see as similar to the plans for Hayabusa, to seize samples of the asteroid's surface.

The launch delay allowed 1989 ML to move beyond Hayabusa's grasp, so a new target - asteroid 1998 SF36 - was selected. Current estimates measure its size at 490 by 180 meters. A near-Earth asteroid, its orbit around the Sun brings it to within 1.8 million kilometers of Earth's orbit.

"That's pretty close," Yeomans said, noting that any asteroid orbiting within 7.5 million kilometers of Earth's orbit is considered potentially hazardous because the gravity of the sun and other planets can easily alter its course by that amount.

By July 2003, the International Astronomical Union gave the asteroid a permanent number and name. In honor of the father of Japanese rocketry, it is now known as (25143) Itokawa.

As a Japanese "technology test mission," Hayabusa has unique technological experiments onboard. The craft is equipped with several high-tech components that, according to the project Web site, "will provide us with valuable technical data that will promote and enable ambitious sample-return missions in the future in the world."

Yeomans said the craft's four ion-drive electric propulsion engines, "like the tortoise and the hare," slowly and steadily propel the craft to Itokawa in an energy efficient way.

Hayabusa is not the first craft to use such engines - NASA's Deep Space 1 had that distinction - but it is the first Japanese craft to employ the technology, and the first to use microwaves to ionize the xenon fuel. The engines propel the craft forward at 12 meters per second per day, a force 20,000 times less than traditional thrusters that operate once for only a few seconds.

Once Hayabusa has reached Itokawa in July 2005, the craft will hover above the asteroid until November and collect more data about its physical aspects, including dimensions, shape, rotation axis and length of rotation cycle, topography, surface composition and structure.

Freefall experiments conducted with a laser altimeter (LIDAR) will allow Hayabusa to determine Itokawa's mass and estimate its volume, composition, bulk density and porosity. Optical navigation cameras will be used in combination with the LIDAR to maneuver the craft around Itokawa.

Because the spacecraft will approach the target with several minutes necessary for light-time communication with Earth, it must use its high-tech instruments to autonomously "decide" how best to move around and land on the asteroid.

Several devices onboard Hayabusa will enable scientists to make a more complex analysis of Itokawa. As the craft moves around the asteroid, an altimeter device will constantly measure the distance between the spacecraft and the asteroid to determine the asteroid's shape and volume.

Mineral composition and the wavelength-dependence of the light-scattering properties of the asteroid surface will be measured by a near-infrared spectrometer (NIRS) and compared with Earth-based meteorites.

Yeomans said a third device, the asteroid multi-band imaging camera (AMICA), "will do a great deal" - measuring the asteroid's size, shape, volume and rotation characteristics as well as mapping satellites around Itokawa (should any exist) and noting slight differences in surface colors.

The fourth high-tech device, an x-ray fluorescence spectrometer (XRS), detects elements on the asteroid's surface and provides elemental abundance ratios, information that scientists hope will clarify the link between Itokawa and meteorites.

MINERVA, a small, ISAS-developed hopping robot, will use three onboard cameras to image the asteroid's surface from very close range. The solar-powered cylindrical craft will autonomously explore Itokawa, relaying images and temperature data back to Hayabusa. MINERVA replaces a NASA-built rover that was to have accompanied the spacecraft. NASA withdrew the rover in 2000 when risk assessments and cost over-runs, caused partly by the target change, exceeded allowances for the mission.

Japanese and American mission scientists in Japan will use data obtained in the first month to optimize selection of the three sampling sites. Before executing a touchdown, Hayabusa will release a softball-sized target marker made of highly reflective material, and use a flashlight-like device to illuminate the surface marker and safely navigate a landing. The first target marker carries the names of 880,000 individuals who contributed their identities in response to announcements by the Japanese Planetary Society.

Collecting the samples is the mission's primary challenge. An asteroid sample return has never been attempted. Hayabusa and its scientists must overcome numerous obstacles, including the asteroid's low surface gravity (750,000 times less than Earth's), 12-hour rotation period and unknown surface structure and composition (current density estimates range from 2.5 to 4.0 g/cc). "This is an extremely bold and ambitious mission," Yeomans said.

In yet another technological experiment, Hayabusa will carry a sample horn that will be brought into contact with the asteroid's surface. A small pyrotechnic charge will fire a projectile into the surface, ideally smashing part of it. The resulting impact fragments will be sucked through the funnel into a sample container. The sample container will then be hermetically sealed and prepared for return to Earth.

Hayabusa will only remain on the surface of the asteroid long enough to fire a single projectile. Once it has done so, its engines will restart and it will return to a hovering position 100m away from the surface. Yeomans said the process will be repeated at least once more for a total of two to three samples.

After a two-year flight back toward Earth, sample canisters will be ejected and, after re-entry, parachute down near the town of Woomera in Southern Australia. The samples will be opened in Japan under controlled conditions at a new national facility built for the purpose.

Scientists hope retrieving samples of Itokawa will increase knowledge and understanding of asteroid surfaces, directly improving our knowledge of the connections between asteroids and meteorites, and helping to shape our understanding of solar system formation in the asteroid regions. As "leftovers" of the inner planets, asteroid samples may also give scientists clues to early solar system formation.

"Meteorites are thought to be asteroid collision fragments, and the holy grail of asteroid science is to find out the chemical composition of an asteroid and link it to the chemical composition of a particular type of meteorite on Earth," says Yeomans.

Likewise, knowing more about asteroid composition could help deflect an NEA on a collision course with Earth. "In terms of mitigating an Earth-threatening object, it will make a big difference whether we are dealing with a wimpy ex-cometary fluff ball or a slab of solid iron."

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