PAGEOS (Passive Geodetic Earth-Orbiting Satellite) was a 100-foot-wide chrome sphere launched into space in 1966, for use as a surveying tool. By referencing its position from stations on the surface, scientists were able to improve our measurements of the positions of the continents by roughly an order of magnitude. The last large fragment of PAGEOS deorbited in 2016.
A pair of half-tonne brass orbs just whipping around the Earth on an extremely stable orbit, they've provided laser calibration for ground-based tracking systems for nearly fifty years.
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