MESSENGER Mission News September 17, 2003

Scanning the Stars

Similar to ancient mariners, the MESSENGER spacecraft will keep
track of its position by checking the location of stars and the Sun.
On September 12 the MESSENGER team re-installed the spacecraft’s two
Star Tracker Cameras, after removing them for testing and to work on
the spacecraft below the cameras’ location. The devices sit on
MESSENGER’s top deck – the portion of the spacecraft facing the
windows at left in this Webcam view – and work in conjunction with
six Sun Sensors and the Inertial Measurement Unit.

The cameras are amazing instruments; each compact device weighs
about 3.1 kilograms (about 7 pounds) and stores a complete map of
the heavens. Every 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. Accurate
spacecraft orientation is critical not only for obtaining the best
science data, but also for keeping the spacecraft steady and moving
in the right direction during course-correction maneuvers.