Gravity Probe A - GP-A was the launch of a precise clock on a suborbital
rocket to explore the structure of space and time. The experiment
was performed by launching a highly accurate MASER atomic
clock, and recording how it measured time as it rose to
weaker levels of gravity, then fell back to the Earth.
stands for Microwave Amplification by Stimulation Emission
of Radiation, and it is one type of highly accurate clock
used to measure infinitesimal changes in the passage of
time. The purpose of the experiment was to determine whether
time progressed at a different rate in conditions where
gravity is weaker. This difference in the rate of passage
of time, predicted by Einstein's Theory of Relativity, is
called the "redshift."
primary objective of the mission was to test a portion of
Einstein's gravitation and relativity theories called the
"Principle of Equivalence," to an accuracy of
200 parts per million.
secondary objective was to demonstrate the operation of
a MASER clock in space. The Gravity Probe A payload was
launched in 1976 from the NASA-Wallops Flight Center in
Virginia. GP-A was only in space for one hour and 55 minutes,
in an elliptical flight trajectory over the Atlantic. However,
it attained an altitude of 6,200 miles above the Earth before
crashing into the Atlantic Ocean.
this short trip, GP-A transmitted accurate measurements
of slight changes in the clock's rate in lower gravity,
and provided the best test of this portion of Einstein's
theories that has been performed to date.