A Nov. 7 article incorrectly said a landing by Japan's Hayabusa spacecraft on an asteroid would be the first space mission to land on a celestial body and bring samples back to Earth since the 1972 Apollo lunar mission. The unmanned Soviet Luna 24 mission collected samples from the moon and brought them back in 1976. (Published 11/9/2005)
Like a hummingbird hovering over a tantalizing blossom, a small but virtuoso Japanese spacecraft is poised to touch down in the coming days on the surface of an asteroid 180 million miles away, stir up a small cloud of dust and gather in a sample.
Barring a mishap, Hayabusa, or "Falcon," is expected to make two and possibly three touch-and-goes on the asteroid Itokawa, then return to Earth with a tenth of an ounce of asteroid dust.
If it succeeds, Hayabusa will be the first spacecraft to land on a celestial body and bring something back from it since U.S. astronauts Eugene A. Cernan and Harrison H. Schmitt collected samples from the moon during the last Apollo lunar mission, in 1972.
On Friday, engineers at the Japanese Aerospace Exploration Agency aborted a practice run when they detected an "anomalous signal" from Hayabusa. But scientists said they will prepare a new schedule, with a practice descent and two sample collecting runs, to be completed by the end of the month.
Hayabusa is the latest in a series of missions to the solar system's smaller residents -- asteroids and comets. In July, NASA crashed a projectile into one comet and has a spacecraft on its way back with dust scooped from the gas cloud around another. NASA and the European Space Agency have missions underway to survey two other asteroids and a third comet.
Hayabusa is also one of the few missions to use charged atomic particles as its main source of propulsion, following NASA's Deep Space 1 in 1998 and Europe's current Smart-1 mission to the moon. Its "ion propulsion" engine bombards atoms of xenon gas with electrons to create positively charged ions that are propelled out the stern of the spacecraft.
This tiny trail of blue exhaust has about the same thrust as a piece of paper resting on someone's hand, but the force is constant and cumulative, and in the weightlessness of space the ions eventually deliver 10 times the thrust of a conventional rocket. Project Manager Junichiro Kawaguchi said Hayabusa and Itokawa are traveling around the sun about the same speed as Earth -- 67,000 mph.
"We are excited about attempting something that has never been tried before," Kawaguchi said in an exchange of e-mails. But he cautioned that "it will be very tough, because the asteroid has very few smooth surfaces to land on."
Hayabusa, weighing only 1,100 pounds, was launched May 9, 2003, atop a Japanese M-V-5 rocket from the Uchinoura Launch Center on Kyushu Island. Once aloft, JAXA christened the spacecraft and named the asteroid Itokawa, after Hideo Itokawa, the father of the Japanese space program.
Hayabusa has endured considerable adversity. A solar flare damaged its electrical storage capacity, and two of its three variable-speed gyroscopes failed, making it necessary for engineers to use auxiliary thrusters to keep the spacecraft pointed in the right direction.
The thrusters and their chemical rocket propellant are also needed for the asteroid touch-and-goes, but Kawaguchi said engineers have "no concern" about running out of fuel, and they expect to finish the mission without mishap.
Since its Sept. 12 rendezvous with Itokawa, Hayabusa has been hovering about 12.5 miles above the asteroid, surveying it. Itokawa, about 1,800 feet long by 590 feet wide, is a potato-shaped near-Earth object whose elliptical orbit crosses both those of Mars and Earth. (There is no danger that Itokawa will collide with Earth.)
Itokawa is strewn with boulders and shows surprisingly little of the celestial soil known as regolith. This is an advantage, Kawaguchi said, in that scientists are getting a close-up look at the original surface of the asteroid, but it is also a disadvantage because safe landing surfaces are scarce.
Engineers have chosen a small expanse of regolith in the middle of Itokawa for the first descent, and a broad, flat plain on the asteroid's tip for the second. On the practice run, they intend to drop a coffee-can-size "hopping" robot called Minerva, which will bound around the asteroid for 36 hours taking pictures and collecting temperature data.
Hayabusa has been using ion propulsion to fly alongside Itokawa but will switch to its chemical thrusters during the touchdown maneuvers. "Ion propulsion will give you exquisite control," said physicist Marc Rayman of NASA's Jet Propulsion Laboratory, "but it will take a very long time."
Time is something engineers will not have once maneuvers begin. Steering Hayabusa in real time is impossible, since it takes 17 minutes for communications to reach Earth. So controllers will upload commands for the entire sequence prior to each approach, and the spacecraft will maneuver almost autonomously, including ordering emergency "burns" if necessary.
"Ion propulsion provides acceleration with patience," Rayman said. "Hydrazine [chemical propellant] gives you acceleration with 'adrenaline,' which is what you need here."
Kawaguchi said Hayabusa will begin its approach 1.8 to 2.4 miles above Itokawa, dropping slowly to the surface and steering for the landing target with optical reference points taken from its collected images.
Somewhere between 160 feet and 100 feet from touchdown, Hayabusa will drop a "target ball" reflector into the landing zone. The spacecraft will use a laser range finder to home in on the reflector during the final approach.
Hayabusa's sample collector is a trumpet-shaped appendage extending from the bottom of the spacecraft, bell-end down. Once the horn touches Itokawa, an air gun inside will fire a 10-millimeter tantalum bullet, punching a hole in the asteroid's crust and kicking up a plume of dust into a compartment in the bottom of Hayabusa.
"The spacecraft stays about one or two seconds," Kawaguchi said. "The sample run itself is made within one second." Once the job is completed, Hayabusa will pull away to an altitude of four miles to send back information and await instructions.
"Hayabusa is a very cool mission and very challenging," said Bruce Betts, director of projects for the Planetary Society, a space advocacy organization. "Dealing with a small, low-gravity object like Itokawa is almost like docking with another spacecraft."
Hayabusa must finish its final sample run by the end of the month or the heavens will no longer be in alignment for the 19-month return trip to Earth. Hayabusa's reentry capsule, with samples, is due to parachute into the Australia's southern desert in June 2007.