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planetary probeA crewless spacecraft designed and equipped to study conditions on or in the vicinity of the planets, their satellites, and other members of the Solar System, as well as in the interplanetary medium itself, and to transmit the information it gathers back to Earth. Probes have now made successful visits to all the planets of the Solar System except Pluto.
Planetary and interplanetary probes may use solar panels to derive power for communications and internal operation. However, spacecraft that travel farther from the Sun than the orbit of Mars receive comparatively little solar energy and must rely on other power systems, such as the radioisotope thermoelectric generators carried by Pioneers 10 and 11 and Voyagers 1 and 2. Most probes carry a main rocket propulsion system for course corrections as well as smaller thrusters for attitude adjustments. An alternative source of power for forward motion is ion propulsion, pioneered by NASA's Deep Space 1 (1998–2001) and also to be used in the projected Dawn mission to the asteroids (1) Ceres and (4) Vesta (due for launch 2006).