This is the main component of the engineering verification unit for the hydrogen maser oscillator (clock) developed by Vessot and Levine at the Smithsonian Astrophysical Observatory (SAO). The unit was designed to measure how varying gravitational fields effect the rate of passage of time as predicted by Einstein's general theory of relativity. Einstein's theory of general relativity proposes that time and gravity are closely related. In order to test this proposal, the SAO designed and built a hydrogen maser oscillator, more commonly known as an atomic clock, to be placed into a microgravity environment. This space-qualified instrument was sent up to 10,000 kilometers on a two-hour sub-orbital flight on a Scout launch vehicle in 1976. By 1979, Vessot concluded that this experiment, named Gravity Probe A, indeed verified Einstein's predicted gravitational red shift effect.

This object was transferred to NASM by SAO in December 1978.