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
Last updated: October 3, 2003