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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.



Fig. 1 Nominal Geometry of Hayabusa to Sun and Earth

Fig. 2 Illustrative Trajectory Experienced

    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.


Fig. 3 Altitude History to the Itokawa Surface

    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.


Fig. 4Direct Measurement of Solar Radiation Acceleration


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.

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