Gravity Probe A was a joint program of NASA-Marshall Space Flight Center and the Astrophysical Observatory of the Smithsonian Institution. It was also the first test in space to explore the structure of space and time. It is known to many scientists as the "Red Shift Experiment" or the "Clock Experiment." The Gravity Probe A (GP-A) payload was launched on June 18, 1976 at 7:41 A.M. Eastern Daylight Time from the NASA-Wallops Flight Center in Virginia. Unlike Gravity Probe B, GP-A was only in space for one hour and 55 minutes in an elliptical flight trajectory over the Atlantic. It attained a maximum height of 6200 miles above the earth before impacting into the Atlantic Ocean. Why was GP-A in space for such a short time? No accidents on the launch pad - it was part of the design of the experiment (unlike Gravity Probe B, which will be in a polar orbit over the Pacific Northwest for nearly two years). To yield an accurate and inexpensive experiment, GP-A required a flight path with a large change in the gravitational potential to provide a large gravitational redshift, and it required a flight path that kept the flight Hydrogen MASER in contact with the ground Hydrogen MASER during data collection.
NASA’s Gravity Probe B was launched successfully from Space Launch Complex 2 at Vandenberg Air Force Base on April 20 at 9:57:24 a.m. PDT. The solar arrays had a nominal deployment while still attached to the Boeing Delta II second stage. Once acquired by the tracking station in Kiruna, Sweden, onboard cameras confirmed that deployment of the four arrays had been completed 72 minutes after launch, followed by live video of spacecraft separation from the Delta II launch vehicle 75 minutes after liftoff. Gravity Probe B was launched into a 400-nautical-mile-high polar orbit for a 16-month mission.
All four gyro systems have been activated and are undergoing checkout. The full Initialization & Orbit Checkout phase of the Gravity Probe B mission is planned to last 45 to 60 days, after which the science data collection will begin.
Gravity Probe B was launched on April 20.
Gravity Probe B was originally scheduled for launch on December 6, 2003 but data obtained during spacecraft prelaunch testing showed electronic noise on an output channel associated with the No. 1 experiment gyro. This could have compromised the quality of data received from it. The problem was isolated to a component in the spacecraft's experiment control unit (ECU). The rescheduled launch on April 17 was postponed until April 19 when a short-circuit was found in early April in the ground based support equipment.
The Gravity Probe B mission is a relativity experiment developed by NASA's Marshall Space Flight Center, Stanford University and Lockheed Martin. The spacecraft will test two extraordinary predictions of Albert Einstein's general theory of relativity that he advanced in 1916: the geodetic effect (how space and time are warped by the presence of the Earth) and frame dragging (how Earth's rotation drags space and time around with it). Gravity Probe B consists of four sophisticated gyroscopes that will provide an almost perfect space-time reference system. The mission will look in a precision manner for tiny changes in the direction of spin.
Gravity Probe B will be launched into a 400-mile-high polar orbit for an 18-month mission.
Government oversight of launch preparations and the countdown management on launch day is the responsibility of NASA's John F. Kennedy Space Center. The launch service is provided to NASA by Boeing Expendable Launch Systems
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