A spacecraft that is in orbit about a planet (usually the Earth) or a moon. Spacecraft are devices intended for observation, research, or communication in space. Even those spacecraft which are on the way to probe the outer reaches of the solar system usually complete at least a partial revolution around Earth before being accelerated into an interplanetary trajectory. Devices such as sounding rockets follow ballistic (approximately parabolic) paths after fuel exhaustion, but they are not satellites because they do not achieve velocities great enough to avoid falling back to Earth before completing even one revolution.