Hayabusa spacecraft had been on the way to Earth with the ion engines B and/or D powered on since April 2007, though it lost the functions of chemical thrusters and two of three reaction wheels at the proximity operation around the asteroid. At the rescue operation in 2006 the ion engine C was not confirmed to exhaust the plasma jet.
On July 28, 2007 a series of the restoration works and a new operation sequence revived the ion engine C again, which are accelerating Hayabusa on behalf of the engines B and D aiming for the Earth return on 2010. Sharing the delta-V duty among the multiple engines will secure a safety flight for the homeward journey.
Because the severe solar radiation seemed to increase the temperature beyond the safety zone to prevent the fuel leak, Hayabusa was operated with the attitude leaned against Sun so as to avoid solar heat flux in May. It passed through the perihelion at the solar distance 0.95AU on June 7 without any problems.
During this term the engine D accelerated continuously Hayabusa as scheduled. At the end of July after establishment of the safety temperature, the warming up of a power supply by electrical heaters during several days and a special operational sequence successfully ignited plasmas again in the engine C. Then the Hayabusa's acceleration was shifted from the engine D to C reserving the rest of operational life.
The turn-on of the engine C is good news for the homeward journey of Hayabusa, which is still frighten at additional malfunctions. Since May 2003 each ion engine has been operated in space as follows
- Engine A: stand-by
- Engine B: about 9,500 hours
- Engine C: about 7,000 hours
- Engine D: about 13,500 hours
- Total: 30,000 hours
Hayabusa will continue the powered flight using the engine C till November this year and then change to the ballistic flight with hibernation mode.
