Terminator orbits are spacecraft orbits about a small Solar System body in which the orbital plane is held approximately perpendicular to the body–Sun line, so that the spacecraft follows the body's terminator, the boundary between its lit and unlit hemispheres. They are also described in the astrodynamics literature as the terminator plane, a dawn–dusk orbit, or a "3 o'clock/9 o'clock" orbit. Because solar radiation pressure is a dominant perturbation in the weak gravity of a small body, this geometry provides passive stability that most other orbit orientations do not, and it has become a standard operational configuration for asteroid rendezvous missions.