David S.F. Portree

The Japanese Aerospace Exploration Agency (JAXA) announced on April 25 that Hayabusa (formerly called MUSES-C) has begun its long-delayed return to Earth. Though the spacecraft still has a long voyage ahead of it - it won’t reach Earth until June 2010 - JAXA engineers and flight controllers deserve praise for reaching this milestone. Through heroic efforts, they pulled the wounded spacecraft back from the brink.

Hayabusa - the name means “peregrine falcon” - left Earth on May 9, 2003. The spacecraft began to suffer malfunctions in July 2005, when one of three reaction wheels - used for orienting Hayabusa without using fuel - stopped spinning. Nevertheless, the 510-kilogram spacecraft arrived successfully at Itokawa, a bean-shaped near-Earth asteroid just half a kilometer long, on September 12, 2005.

A second reaction wheel failed early in October, forcing JAXA controllers to turn the spacecraft using small thruster rockets. In November 2005, after mapping Itokawa and studying its composition, Hayabusa released a small lander called MINERVA, but it missed the asteroid and was lost in space.

Hayabusa landed on Itokawa on November 19 to grab a sample, but its sample collection system did not operate. JAXA believes that a second landing on November 25 managed to collect a sample of dust, even though the collection system may again have failed to operate.

Then Hayabusa’s real troubles began. The spacecraft’s steering thruster system began to leak. The leak acted as a thruster, nudging Hayabusa in random directions. The spacecraft turned its solar panels away from the Sun and began to lose power. Because it could not reliably point its antenna, it broke radio contact with Earth on November 25. The radio link was restored by November 30, but Hayabusa continued to drift. An attempt to use the thrusters on December 2 failed.

Fortunately, Hayabusa has two propulsion systems. In addition to the balky steering thrusters, which use chemical fuel, it has four electric thrusters. Electric propulsion is the wave of the future. Electric thrusters typically use very little propellant and produce very little thrust, but can operate continuously for months.

Hayabusa was launched from Earth on a rocket too weak to send it directly to Itokawa. After the rocket pushed it out of Earth orbit and separated, controllers activated Hayabusa’s electric thrusters, which use the heavy gas xenon as fuel. The thrusters gradually changed Hayabusa’s course and speed over more than a year so that it could intercept Itokawa. This tactic saved tons of chemical fuel and millions of yen.

On December 4, desperate JAXA controllers commanded Hayabusa’s electric propulsion system to vent xenon gas into space. This acted as a makeshift thruster and slowed the spacecraft’s spin. Then, on December 8, Hayabusa suffered another sudden change of orientation and broke radio contact with Earth.

By then, however, controllers understood Hayabusa’s motion. They predicted that the spacecraft would stabilize and rotate enough that radio contact could be restored in about two months. On January 23, 2006, they detected Hayabusa’s radio beacon, and on February 6 they vented more xenon. On March 7, JAXA announced that it had restored contact with the spacecraft.

Controllers then checked out Hayabusa’s systems. Two of the electric thrusters had failed, as had four of Hayabusa’s 11 batteries. Nevertheless, JAXA engineers remained hopeful that controllers could begin Hayabusa’s return to Earth when an Earth-return opportunity occurred in April 2007. Now the Hayabusa team’s hard work, patience, and ingenuity have paid off.

Hayabusa’s recovery is a triumph for spacefaring humankind. Even if it retrieved no sample, the experience gained through this “near-death experience” will be applied to future space exploration missions. If Hayabusa did collect a sample of Itokawa, and if it succeeds in reaching Earth as planned, then the mission will be a scientific triumph, too.