http://messenger.jhuapl.edu/webcam/annotatedimages/annotated-20030912.html

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.