Slowly pulling alongside a space rock the size of several typical city blocks, a Japanese probe is preparing to begin scooping the first dusty samples of material from the surface of an asteroid this fall for an eventual return to Earth.

An artist's concept of MUSES-C. Credit: ISAS

Japan's Hayabusa spacecraft is halfway through a four-year $100 million mission to chase down a small celestial target, retrieve pieces of its rocky crust, and return them safely to Earth in a capsule designed to survive the intense heat as it enters the atmosphere and parachutes to a safe landing.

Formerly known as MUSES-C, the Hayabusa craft was launched in May 2003 aboard a solid-fueled M-5 rocket to embark on its journey to visit asteroid 1998 SF36. The asteroid, named Itokawa, is in an orbit that brings it close to Earth and is classified as a near-Earth asteroid, meaning its closest approach to the Sun is inside a point 1.3 times further than Earth's orbit.

En route to Itokawa, Hayabusa completed one orbit of the Sun and conducted a high-speed flyby of Earth in May 2004, using Earth's gravity to "slingshot" the 1,000-pound probe on an arcing trajectory outward from the Sun to intercept its small target this year.

Hayabusa is busy this month calibrating instruments and cameras in advance of its crucial arrival in the vicinity of Itokawa in the middle of September. Its star tracker sighted the asteroid in late July and took a series of 24 images through August 12 to refine the craft's course to the object. Higher resolution imagery will be gathered in the next few weeks using the optical navigation camera. As of August 15, the asteroid had reached magnitude four when viewed from Hayabusa.

One of the probe's three reaction wheels responsible for controlling the orientation of the spacecraft failed on July 31 when friction levels went out of limits. Software governing modes of operation for the reaction wheels can support the use of only two of the assemblies, and officials are confident the fix will work. The wheels were built by the Ithaco company based in upstate New York.

Operated by the Japanese Aerospace Exploration Agency's space science division, Hayabusa has been propelled chiefly by three ion drive engines relying on xenon gas and electricity to tweak its path toward Itokawa. The high-efficiency propulsion system generates very low levels of thrust, and therefore it must fire nearly continuously for thousands of hours to produce optimal results.

A hands-free navigation system is also aboard Hayabusa, demonstrating key technologies that allow the probe to precisely approach Itokawa without intervention or commands from ground teams. During the high-stakes approach, the hybrid system will combine optical and laser data from on-board assets with radio data obtained from ground telescopes on Earth.

With the craft now within 6,000 miles of its destination and closing at 82 feet per second, the most critical phase of the mission will soon begin. By Thursday, range will be less than 4,800 miles and the closing rate will be near 50 feet per second.

Once it reaches a point around 2,200 miles from the asteroid near the first of September, the ion engines will be turned off to leave Hayabusa coasting toward its target at about 33 feet per second. That closing rate will then be slowed by smaller thrusters using chemical propellants.

Although exact dates remain elusive, Hayabusa should come to a relative stop in what officials call the "gate position" a little over 12 miles from Itokawa in mid-September. There it will begin to study the asteroid as it completes one rotation every 12 hours.

Hayabusa will not enter orbit, but instead it will remain in a fixed position after the tedious rendezvous sequence. The spacecraft could push onward to the "home position" within five miles of the asteroid's surface before October if all goes as planned.

The arrival at Itokawa was delayed two months from earlier this summer after a violent solar flare in 2003 caused damage to the craft's solar array panels that produce electricity, which is needed to power the probe's ion propulsion system needed during the approach.

Itokawa is roughly potato-shaped and its dimensions measure about 2,000 feet by 942 feet by 866 feet. It is a member of the Apollo class of near-Earth asteroids that pose potential impact threats to our planet because they often cross paths with Earth during their trip around the Sun. The asteroid travels in an elliptical solar orbit stretching from a furthest point of over 157 million miles to a closest approach to the Sun of around 88 million miles.

The asteroid was discovered in September 1998 by a telescope operated by a joint team of U.S. Air Force and MIT scientists under the Lincoln Near-Earth Asteroid Research, or LINEAR, program.

From its static positions five and 12 miles out, the probe will exhaustively survey the asteroid using its science payload comprised of a high resolution imager, near-infrared and X-ray spectrometers, and light detection and ranging equipment to map the surface in resolution as high as three feet as Itokawa rotates below.

"Taking advantage of asteroid rotation, remote sensing is possible," said Jun Kawaguchi, Hayabusa's project manager.

Studies from several ground-based telescopes during Itokawa's last close pass by Earth in 2001 revealed the object is brighter and more reflective than would be expected with most asteroids of its type and size.

An extrapolation of these results was used to determine hypothesized surface characteristics, which indicates a strong likelihood that Hayabusa will find a relatively smooth but rocky surface. However, other data suggests a complicated surface with rough terrain and larger boulders widely strewn due to collisions with other space materials. Officials say the surface is "enigmatic" and awaits the arrival of Hayabusa, according to a report presented at 2004's Lunar and Planetary Science Conference.

Scientists hope to gain an even better understanding of the physical properties and chemical make-up of the asteroid using Hayabusa's instruments to locate potential locations for sample retrieval, scheduled to get underway in November.

When landing sites are chosen, the Hayabusa spacecraft can make up to three passes to attempt to capture a total of one gram of surface samples, or about two one-thousandths of a pound.

The approaches will heavily lean on the autonomous navigation system aboard the probe, which must operate correctly with little input from engineers 200 million miles away on Earth and cope with the extremely weak gravity field of Itokawa. A target marker will be released on the surface as the spacecraft closes in.

During each opportunity, a 16-inch funnel will first make contact with the asteroid, followed by the firing of a small metal projectile into the surface at well over 600 feet per second. Rocks and dust kicked up by the impact will be gathered by the funnel and fed into the sample collection capsule to be returned to Earth.

"It breaks the surface and ejecta climbing up through a funnel-like device are collected by a sample catcher," Jun Kawaguchi explained.

On the first pass, it is planned for Hayabusa to deploy a tiny rover called MINERVA to move across the asteroid for up to two days by leaping from place to place in Itokawa's near-microgravity environment. The 1.3-pound rover will take stereo images using three cameras, and six thermometers will gather temperature measurements.

A larger rover -- provided by NASA -- was originally part of Hayabusa's payload, but the project was cancelled due to funding issues.

Due to positioning of Itokawa and Earth in their solar orbits, Hayabusa must depart the asteroid by December to make its trip home. The delay in the craft's arrival shortened the length of time available for operations at Itokawa, but officials are hopeful all goals of the mission will be achieved in the three-month stay.

Hayabusa will fire its ion drive propulsion system once more during the cruise back to Earth, culminating with the return capsule's fiery re-entry to the planet in June 2007 after separating from the mothership. Streaking into the atmosphere at almost eight miles per second, the canister will be slowed by friction before making a parachuted touchdown in the Australian outback.

Scientists will then access the capsule to recover the first materials ever returned from an asteroid, or from the surface of any body other than the Moon. The samples 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 data 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.