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( Space probe)
A space probe is a scientific space exploration mission in which a robotic spacecraft leaves the gravity well of Earth and approaches the Moon or enters interplanetary or interstellar space; approximately twenty are currently extant. The space agencies of the USSR (now Russia and Ukraine), the United States, the European Union, Japan, India and China have in the aggregate launched probes to several planets and moons of the solar system as well as to a number of asteroids and comets. A space probe destined for a planet or other astronomical body can be classified as a " flyby", an "impactor", an "orbiter" or a "lander" mission. Historically, flyby missions proved easiest to accomplish, as they did not require the precise navigation needed for an impact, nor the need for additional propulsion to conduct a maneuver to enter orbit. Upon landing some landers have released "rovers" which travel across the surface of the astronomical body upon which they have landed. Once a probe has left the vicinity of Earth, its trajectory will likely take it along an orbit around the Sun similar to the Earth's orbit. To reach another planet, the conceptually simplest means is to execute a Hohmann transfer orbit maneuver. More complex techniques, such as gravitational slingshots, can be more efficient, though they may require the probe to spend more time in transit. A technique using very little propulsion, but possibly requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network. Along with Pioneer 10, Pioneer 11, and its sister ship Voyager 2, Voyager 1 is an interstellar probe.
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