Solar Radiation Pressure Force acting on Hayabusa Station Keeping

Hayabusa has kept its position controlled since it arrived at the Gate Position, 20 km from Itokawa. The biggest disturbance worked is the Solar Radiation Pressure force. The force is simply Light Force acting on everyone everyday. From bright walls, also from heaters, we receive this small force that is never felt on the ground. It is due to the photons that come from the Sun or anything illuminating. The force becomes large when the projected area becomes large and also when the exposed surface has high reflectivity. It is 1/100 with respect to the ion engines thrust, but ten times larger than the gravity of Itokawa at the Home Position.
Fig. 1 shows the geometry of Hayabusa to the Sun and the Earth. Hayabusa does not have to fire its engines when it descends to the surface, while it fires to pop-up in the air to keep its position in the control box. A typical trajectory actually performed is drawn in Fig. 2.

The speed accelerated per day is about 1 cm/second that deserves the translation speed of about 1 km/day. As the Home Position control box is at 7 km from Itokawa, this inadvertent speed-up is significant and the spacecraft needs to effort to resist this drift flow. Fig. 3 draws the history of the distance to the Itokawa surface measured by the Laser Altimeter (LIDAR) aboard. It corresponds to the data shown in Fig. 2 above. On September 19th, Hayabusa ascends up by firing its engines aiming at returning back again taking this solar radiation effect into account. As planned, it returned to the position, 11 km point on September 26th. It is like the motion similar to the throwing and falling ball motion under the gravity on the ground.

During this parabolic flight, this solar radiation pressure force was accurately calibrated. On the ground, also in space around the Earth, this force is too small to be measured directly owing to the atmospheric drag, the gravity of the Sun and the Earth. However, while Hayabusa is hovering to Itokawa, the measurement is directly obtained and important in terms of technology. And in order to make Hayabusa descend for touching down to the surface, this effect must be estimated precisely before it. An example showing this acceleration is shown in Fig. 4.

This figure presents Doppler velocity difference along the line of sight direction connecting Hayabusa and Itokawa. Itokawa also receives the light force, but the mass is enormously huge relative to the area exposed to the Sun, and drift motion of Itokawa is negligible. Since this Doppler velocity difference builds up linearly and gradually, it is found a certain constant acceleration had been working. This acceleration is calibrated to
1.261x10-10[km/s2]= 1.25 [m/s/day]=4.5 [km/hour/day].
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.

October 25, 2005