space probe
space probe {n.}, {Space English}
An unmanned spacecraft other than an Earth satellite fitted with instruments which gather and transmit information about other planets in the solar system (e.g., Venus, Mars, and Jupiter) on what are called fly-by missions, i.e., without the craft landing on any of these bodies.
Both the U.S.A. and Russia have sent up many a space probe in the past decade.
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Source: A Dictionary of American Idioms