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Gravity Probe A (GP-A) was a space-based experiment to test the theory of general relativity, performed jointly by the Smithsonian Astrophysical Observatorymarker and the National Aeronautics and Space Administrationmarker. It sent a hydrogen maser, a highly accurate frequency standard, into space to measure the rate change of a clock in lower gravity with high precision.

The probe was launched on June 18, 1976 on top of a Scout rocket and remained in space for 1 hour and 55 minutes, as intended. It then crashed into the Atlantic Ocean.

Experimental setup

The 100 kg Gravity Probe A spacecraft housed the hydrogen maser system that ran throughout the mission, and a microwave repeater to measure the Doppler shift of the maser signal. The satellite was launched nearly vertically upward to cause a large change in the local gravity seen by the maser, reaching a height of 10,000 km (6200 miles). At this height, general relativity predicted a clock should run 4.5 parts in 1010 faster than one on the Earth.


The clock rate was measured from the ground by comparing the microwave signal from the clock to a maser on the ground and subtracting a signal from the spacecraft that measured the Doppler shift. The clock rate was measured for most of the duration of the flight and compared to theoretical predictions. The stability of the maser permitted measurement of changes in the rate of the maser of 1 part in 1014 for a 100-sec measurement.

The experiment was thus able to test the equivalence principle. Gravity Probe A confirmed the prediction that gravity slows the flow of time, and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.

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