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
Hayabusa will continue the powered flight using the engine C till November this year and then change
Engine B: about 9,500 hours
Engine C: about 7,000 hours
Engine D: about 13,500 hours
Total: 30,000 hours
to the ballistic flight with hibernation mode.