The Cassini-Huygens spacecraft is one of the largest, heaviest and most complex interplanetary spacecraft ever built. The main body of the orbiter is a nearly cylindrical stack consisting of a lower equipment module, a propulsion module and an upper equipment module, and is topped by the fixed, four-meter diameter high-gain antenna. Attached about halfway up the stack are a remote sensing pallet, which carries cameras and other remote sensing instruments, and a fields and particles pallet, which carries instruments that study magnetic fields and charged particles. The two pallets carry most of the Cassini orbiter's science instruments. In general, the entire spacecraft must be turned to orient the instruments in the correct observing direction, although three of the instruments possess their own single-axis articulation capability.