MESSENGER Mission News Portable Sun October 3, 2003

Last week the MESSENGER team carried out critical tests on the
spacecraft’s six Sun Sensors (or Digital Solar Attitude Detectors).
In this frame an engineer holds a Sun lamp in front of one of the four
sensors mounted on the sunshade. The lamp itself is an ordinary
halogen work light, painted black behind the bulb to simulate a narrow
sunbeam. Two other sensors are found on the backside of the spacecraft,
not visible from this viewpoint.

The Sun Sensors will continuously measure the angles to the Sun in
elevation and azimuth, and serve as a backup to the Star Trackers and
ephemeris models to provide Sun-direction information. The MESSENGER
guidance and control system requires knowledge of Sun direction to keep
the sunshade pointed at the Sun — and the spacecraft in the shade’s
cool shadow – when the spacecraft moves closer to the Sun. (For the
first three months after launch MESSENGER will actually fly with
its “back” toward the Sun, so the two sensors on that side of the
spacecraft will provide Sun-direction readings.)

MESSENGER’s flight software will constantly monitor the spacecraft’s
orientation relative to the Sun and the Sun Sensors are one source of
this information. If the system detects that the Sun is “moving” out
of a designated safe zone, it will initiate an automatic turn to
ensure proper orientation of the sunshade relative to the Sun. Once
that happens, ground controllers can analyze the situation while the
spacecraft patiently maintains a safe orientation relative to the Earth
and Sun – a condition otherwise known as “safe” mode.

Check out a time-lapse movie of the Sun Sensor test at:

http://cps.earth.northwestern.edu/DMOVIES/20030923-Sun%20Test.mpg