MESSENGER's digital Sun sensors and Inertial Measurement Unit (IMU) during spacecraft development
MESSENGER is well protected against the heat, but it must always know its orientation relative to Mercury, Earth, and the Sun and be “smart” enough to keep its sunshade pointed at the Sun. Attitude determination – knowing in which direction MESSENGER is facing – is performed using star-tracking cameras, digital Sun sensors, and an Inertial Measurement Unit (IMU, which contains gyroscopes and accelerometers). Attitude control for the 3-axis stabilized craft is accomplished using four internal reaction wheels and, when necessary, MESSENGER’s small thrusters.
The IMU accurately determines the spacecraft’s rotation rate, and MESSENGER tracks its own orientation by checking the location of stars and the Sun. Star-tracking cameras on MESSENGER’s top deck store a complete map of the heavens; 10 times a second, one of the cameras takes a wide-angle picture of space, compares the locations of stars to its onboard map, and then calculates the spacecraft’s orientation. The Guidance and Control software also automatically rotates the spacecraft and solar panels to the desired Sun-relative orientation, ensuring that the panels produce sufficient power while maintaining safe temperatures.
Five Sun sensors back up the star trackers, continuously measuring MESSENGER’s angle to the Sun.
If the flight software detects that the Sun is “moving”
out of a designated safe zone it can initiate an automatic turn to ensure that the shade faces the Sun. Ground controllers can then analyze the situation while the spacecraft turns its antennas to Earth and awaits instructions – an operating condition known as