The Gravity Probe B spacecraft is in NASA's Payload Processing Facility 1610 on North Vandenberg Air Force Base in California and preparations are on schedule for a launch on Saturday, April 17.

Powered-on testing of the spacecraft with the reworked Experiment Control Unit (ECU) reinstalled is complete and a detailed data analysis is now underway. The ECU appears to be performing fully as intended. Functional testing of the remainder of the spacecraft continues and is on schedule. No problems have been revealed.

In other planned spacecraft processing, the Gas Management Assembly (GMA) "rate of rise" testing has been completed satisfactorily. This testing checked leakage rates and amounts. The GMA provides the helium gas required to spin up the gyroscopes. It also performs magnetic flux reduction, or "flux flushing," to minimize noise or reduce the trapped magnetic field within each gyro's housing.

Reconditioning of the spacecraft's cryogenic helium dewar back to a temperature of 1.65 Kelvin has been completed and the dewar was sealed. This was essentially a topping-off process that also cooled the helium in the tank to a superfluid state near absolute zero. The topping continued until 95 percent of the helium in the dewar was in a superfluid condition. Since the dewar has now been closed out, the launch nominally would need to occur within about 90 days. A final top-off is set to occur at the launch pad to assure the helium will last the planned 16-month duration of the mission.

Operations to reinstall the solar arrays will begin on March 8. The spacecraft is currently scheduled to be transported to Space Launch Complex 2 on April 1 and mated to the Delta II rocket.

Meanwhile, the Boeing Delta II rocket is at Space Launch Complex 2, enclosed within the gantry-like mobile service tower. It has successfully completed its testing to date and will remain there until the GP-B spacecraft arrives. The solid rocket booster inspections performed as a precaution after the recent earthquake in central California have been completed with no anomalies observed. There are no Delta II launch vehicle issues or concerns at this time.

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 precise manner for tiny changes in the direction of spin. Gravity Probe B will be launched into a 400-nautical-mile-high polar orbit for a 16-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 Launch Services.