Gravity Probe A
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Gravity Probe A (GP-A) was a satellite-based experiment to test Einstein's theory of general relativity performed by the Smithsonian Astrophysical Observatory. It used a MASER to measure the rate change of a clock in lower gravity with high precision.
It was launched on June 18, 1976 on top of a Scout launcher and remained in space for 1 hour and 55 minutes, as intended. It then crashed into the Atlantic Ocean.
Experimental setup
Gravity Probe A consisted of a MASER system that ran throughout the mission. It represented a "clock" with very high accuracy. The satellite containing the MASER was launched into a highly elliptical orbit, which caused a large change in the "local" gravity seen by the MASER. (Maximum height of the orbit was 6200 miles (10000 km)). At this height, a clock should run 4.5 parts in 1010 faster than one on the Earth. The clock rate was measured from the ground and compared to the theoretical predictions. Thus, the experiment was able to test the "equivalence principle" with an accuracy of about 150 parts per million.
Results
Gravity Probe A confirmed the prediction that gravity slows the flow of time.