The Japanese spacecraft Hayabusa, now in a ‘parking orbit’ above the asteroid Itokawa, is providing good evidence of just how useful the pressure of solar photons can be. Japan’s Institute of Space and Aeronautical Science (ISAS) reports that the force being experienced by the spacecraft is 1/100th of the thrust produced by its ion engines, but fully ten times larger than the gravity of Itokawa itself. The effect is consequential enough that it must be factored into Hayabusa’s descent close to Itokawa’s surface; the spacecraft will deploy a small surface ‘hopper’ called MINERVA to take measurements on the asteroid.

Hayabusa (once known as MUSES-C and renamed for a Japanese rocket pioneer) thus becomes both a testbed for current technologies and a reminder of a future one. Its electric propulsion or ion drive engines have met the challenge of asteroid rendezvous, although their performance was degraded by solar panel damage from solar flares in 2003. The spacecraft also carries an autonomous navigation system, a sample collection system and a sample capsule for return of asteroid material to Earth. As to that intriguing photon pressure, ISAS has this to say in a news release:

The solar radiation pressure ideally amounts to the gravity force acting on one yen coin per 1,000 square meters. But reflection is not perfect and actual light force is reduced. Hayabusa uses this light force for descending and station keeping purposes. However, contemporary research has studied the application to revolutionary new spaceships, Solar Sails. JAXA also has investigated the new generation spaceships utilizing hybrid propulsion combining electric propulsion with this light force.

Despite the failure of the Planetary Society’s Cosmos 1 mission, solar sail research continues in a number of venues because the cumulative acceleration offered by photons gives us the best understood method to reach the outer Solar System while leaving bulky chemical fuel systems behind. Solar sailing offers up a number of problems, including especially the difficulty of deployment of the large, thin surfaces needed for this work, but the theory behind it is absolutely sound and understood by any engineer who has studied the orbital behavior of large objects like communications satellites in space.

Ahead for Itokawa: surface samples are due to be collected in November with return to Earth in June of 2007. A useful backgrounder on the mission can be found here.