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GSAT0201 (GALILEO 5)

European Navigation Satellite (Galileo — Stranded, Used for Relativity Test) · ESA / European Commission

GSAT0201 (Galileo 5) is at the center of one of the best accidents in the history of satellite navigation. Launched in August 2014, a malfunctioning Fregat upper stage stranded it in the wrong orbit entirely — a highly elliptical path instead of the circular one Galileo satellites need to function normally, an apparent total mission failure. ESA engineers refused to give up: they used the satellite's own thrusters to partially correct the orbit, then discovered something remarkable. The satellite's constantly changing altitude, swinging between roughly 10,500 and 16,000 miles, made its atomic clock experience measurably different amounts of gravitational time dilation at each point in its orbit — exactly the effect Einstein's general relativity predicts. Scientists used GSAT0201 and its stranded twin to test relativistic time dilation with a precision four times better than any previous experiment, turning a launch failure into genuine fundamental physics.