Gravity Probe A

Gravity Probe A

Diagram of the Gravity Probe A experiment[1]: 17
Mission type	Astrophysics
Operator	NASA
COSPAR ID	GRAVR-A
Mission duration	1 h 51 m
Apogee	10,224 km (6,353 mi)
Spacecraft properties
Spacecraft	GP-A
Launch mass	60 kg (130 lb)
Power	22 W
Start of mission
Launch date	18 June 1976 (1976-06-18)
Rocket	Scout[2]
Launch site	Wallops Flight Facility[3]
End of mission
Disposal	Decommissioned
Deactivated	18 June 1976 (1976-06-18)

Gravity Probe A (GP-A) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity. It was performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration. The experiment sent a hydrogen maser—a highly accurate frequency standard—into space to measure with high precision the rate at which time passes in a weaker gravitational field. Masses cause distortions in spacetime, which leads to the effects of length contraction and time dilation, both predicted results of Albert Einstein's theory of general relativity. Because of the bending of spacetime, an observer on Earth (in a lower gravitational potential) should measure a slower rate at which time passes than an observer that is higher in altitude (at higher gravitational potential). This effect is known as gravitational time dilation.

The experiment was a test of a major consequence of Einstein's general relativity, the equivalence principle. The equivalence principle states that a reference frame in a uniform gravitational field is indistinguishable from a reference frame that is under uniform acceleration. Further, the equivalence principle predicts that phenomenon of different time flow rates, present in a uniformly accelerating reference frame, will also be present in a stationary reference frame that is in a uniform gravitational field.

The probe was launched on June 18, 1976 from the NASA-Wallops Flight Center in Wallops Island, Virginia. The probe was carried via a Scout rocket, and attained a height of 10,000 km (6,200 mi), while remaining in space for 1 hour and 55 minutes, as intended. It returned to Earth by splashing down into the Atlantic Ocean.^[4]