On the other side of the sunshield is the vital machinery that operates the observatory. The "spacecraft bus
" (the boxy protrusion in the middle) houses the equipment that runs the satellite, including the rockets that keep it in stable orbit, the star tracker that monitors where the telescope is pointing, the antennas that help it communicate with Earth, and the computers that act as the brains of the craft. The momentum flap (folded down on the far right) helps stabilize the observatory--the pressure of sunlight on the sunshield can actually cause the telescope to spin slowly, ruining its aim, so the flap acts like another sail to help counteract the pressure. Solar panels in between the spacecraft bus and momentum flap help power the craft.