[meteorite-list] Solar Radiation Pressure Force acting on Hayabusa Station Keeping

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Oct 25 12:25:50 2005
Message-ID: <200510251624.j9PGORI12503_at_zagami.jpl.nasa.gov>

http://www.isas.ac.jp/e/snews/2005/1025.shtml

Solar Radiation Pressure Force acting on Hayabusa Station Keeping
Japan Aerospace Exploration Agency (JAXA)
October 25, 2005

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.

[image1]
Fig. 1 Nominal Geometry of Hayabusa to Sun and Earth

[image2]
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.

[image3]
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.

[image4]
Fig. 4 Direct 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.
Received on Tue 25 Oct 2005 12:24:26 PM PDT