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.

See also

• Timeline of gravitational physics
• Gravity Probe B