A small Japanese asteroid probe riddled by a streak of bad luck began its slow limp home Wednesday, but officials still face a myriad of challenges to bring the craft back in 2010.

Controllers sent commands for the Hayabusa probe to start one of its four ion engines Wednesday, officially beginning its three-year journey to Earth.

The milestone came after months of tests to determine whether the 900-pound spacecraft was healthy enough to attempt the voyage. Hayabusa is running on a damaged battery and just one of its four ion engines is currently deemed ready for long-term operations, according to the Japanese Aerospace Exploration Agency, or JAXA.

Hayabusa also lost two of its three fast-spinning reaction wheels responsible for attitude control. After the failures, the craft was forced to exhaust all of its chemical propellant reserves.

Engineers devised a new attitude control scheme using excess xenon fuel used by Hayabusa's electric propulsion system. Officials estimate Hayabusa's tanks still hold more than 66 pounds of xenon, while only about 44 pounds are needed for the Earth-bound leg of its mission.

JAXA officials remain cautious about the chances of Hayabusa successfully reaching Earth.

"This is not an optimistic operation, but a very tough operation," said Junichiro Kawaguchi, Hayabusa project manager, in a February interview. "The spacecraft is not in a very healthy state."

The probe is still located in the vicinity of asteroid Itokawa, a small potato-shaped space rock that was the subject of three months of scientific scrutiny by Hayabusa in 2005. Ground teams believe the spacecraft is currently about 50 million miles from Earth.

Hayabusa will have to complete two more orbits around the Sun before reaching Earth in June 2010, when it is expected to separate its return capsule for a parachuted landing in southern Australia.

The reentry vehicle was designed to house small chunks of Itokawa retrieved as Hayabusa swooped down to the surface of the asteroid. A small pellet was to fire into the asteroid to force dust and rocks into the sample chamber, but reviews of data streaming back from the spacecraft later caused engineers to question whether the system worked as planned.

Officials will likely not know for sure if the capsule contains any samples until it lands.

The start of the return trip was postponed by a year after a fuel leak in December 2005 threw Hayabusa off course and cut off communications with the probe for six weeks.

On Tuesday, JAXA released a heap of catalogued raw science data from Hayabusa's mission. The data included more than 1,600 optical images, about 135,000 pieces of spectral data in the near-infrared and X-ray bands, and 1.7 million data points from a laser rangefinder.

Scientists also assembled a three-dimensional shape video of Itokawa, which is believed to have been formed by the collection of several smaller bodies linked together by loose material and weak gravity. See the movie clip here.