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For release: 04/28/04
Status report #: 04-127

Gravity Probe B mission status report

Gravity Probe B spacecraft circles above the Earth.

In its first week on orbit, Gravity Probe B — a NASA experiment to test two predictions of Albert Einstein's Theory of General Relativity — has accomplished many steps along the way in a smooth transition into the science phase of the mission. The spacecraft has already achieved a science mission orbit, within the plane of its guide star. Launched April 20 from Vandenberg Air Force Base, Calif., Gravity Probe B is managed by the Marshall Center.

Photo: The Gravity Probe B spacecraft orbits the Earth in this view, captured by a camera mounted on the Delta II rocket that carried it to space. (NASA/KSC)

In its first week on orbit, Gravity Probe B has achieved many successes that will ensure a smooth transition into the science phase of the mission and the best possible experimental accuracy. The spacecraft has already achieved a science mission orbit, within the plane of the Guide Star, IM Pegasi, and its inclination error is six times better than expected.

In the quiet environment of space, the gyro readout system is performing significantly better than it did during any ground testing. All four SQUIDs (Super-conducting Quantum Interference Devices) are fully functional and have detected calibration signals with high precision. Noise levels are below the allowable mission requirements.

The electrical power system is fully functional and is providing adequate power for all operating conditions. Eclipse operation (when the spacecraft is in the shadow of the Earth) meets all requirements. There is no evidence of solar array motion that might disturb the experiment.

All other spacecraft subsystems are fully functional. All four gyros have been checked out and are performing well. They have all been electrically suspended in analog mode; digital suspension activities will commence shortly. The team is in the process of updating data tables for the Gyro Suspension System (GSS) to aid in achieving digital suspension of the gyros.

The spacecraft's Attitude Control System (ATC) is maintaining a stable attitude (relative position in orbit—pitch, yaw and roll). We expect to lock onto the Guide Star, IM Pegasi, within a few days, after completing on-orbit re-calibration of t he spacecraft's 16 micro thrusters. During thruster re-calibration, it was observed that one of the redundant micro thrusters was stuck partially open. It was isolated and removed from the system, and the flow of helium to the remaining micro thrusters has been adjusted to compensate for its removal.

Overall, one week after launch, it appears that all of the spacecraft's subsystems are functioning well—meeting or exceeding mission requirements, in preparation for beginning the science experiment.

The spacecraft is being controlled from the Gravity Probe B Mission Operations Center, located at Stanford University. Mission operations have demonstrated that the hardware developed for the GP-B mission is performing as planned, and the Stanford-NASA-Lockheed Martin operations team is performing well.

The Initialization & Orbit Checkout (IOC) phase of the Gravity Probe B mission is planned to last 45-60 days, after which the 13-month science data collection will begin. This will be followed by a two-month final calibration of the science instrument assembly.

NASA's Gravity Probe B mission, also known as GP-B, will use four ultra-precise gyroscopes to test Einstein's theory that space and time are distorted by the presence of massive objects. To accomplish this, the mission will measure two factors -- how space and time are warped by the presence of the Earth, and how the Earth's rotation drags space-time around with it.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Gravity Probe B program for NASA's Office of Space Science. Stanford University in Stanford, Calif., developed and built the science experiment hardware and operates the science mission for NASA. Lockheed Martin of Palo Alto, Calif., developed and built the GP-B spacecraft.

For more information:
Status report
News release
More Gravity Probe B News

Don Savage
NASA Headquarters
(202) 358-1547

Steve Roy
Public Affairs Office
(256) 544-0034

Robert Kahn
Stanford University, Stanford, Calif.
(650) 723-2540

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