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Gravity Probe B



Gravity Probe B launched successfully at 16:57:25.870 UT (12:57 p.m. EDT) on 20 April.

The Gravity Probe B (GP-B) mission is designed to study certain aspects of Einstein's general theory of relativity through the use of precision gyroscopes in Earth orbit. The probe launched on 20 April 2004 into a near-circular 650 km polar orbit. The scientific objectives of the mission are to test the prediction of frame dragging, the effect caused by a rotating body slowly dragging space-time around with it and to measure the geodetic effect, a change in spin direction of the gyroscopes caused by the Earth warping space-time.

The probe consists of a 2.7 meter long cylinder shielded by a special iron alloy, wrapped in superconducting lead foil bags, and immersed in a large dewer containing 400 gallons of superfluid helium at a temperature of 1.8 K. The interior of the cylinder is kept at high vacuum and contains a telescope bonded to a 53 cm long block of fused quartz which holds four gyroscopes and a proof mass. The gyroscopes are 3.8 cm diameter spheres of uniform fused quartz each coated with a very thin, chemically pure layer of niobium. The gyroscopes are within 40 atomic layers (0.8 millionths of a cm) of a perfect sphere. The spheres are levitated electrically within spherical cavities and spun up to 10,000 rpm. The four gyroscopes are aligned parallel to the telescope axis, two spinning clockwise and two counterclockwise.

The proof mass floats within an evacuated cavity near the spacecraft's center of mass isolated from external accelerations. The mass tends to follow an ideal gravitational orbit, its position is tracked and the spacecraft thrusters respond to keep it at the center of the cavity, maintaining the spacecraft in a drag-free trajectory. The mean acceleration on the gyroscopes will be about 10^-9 m/s^2. The telescope is made of fused quartz and has an aperture of 14.2 cm and a focal length of 381 cm.

After launch and insertion into the 650 km orbit coplanar with the guide star IM Pegasi the gyroscopes were spun up by a stream of helium gas. Full speed was reached in half and hour, after which the gas was pumped out of the probe to a pressure of 10^-11 torr. After a 60 day checkout period the science observations began. The spin direction of the gyroscopes is measured with the Superconducting QUantum Interference Device (SQUID), which can detect a change of 0.1 milliarc-seconds within a few days. The frame-dragging effect on the gyroscopes should amount to about 42 milliarc seconds per year. The telescope is aligned with the spin axis of the spacecraft and is pointed towards IM Pegasi. The main structural element of the spacecraft is the dewer. Mounted on the axis of the spacecraft are four solar panel wings. The roll rate of the spacecraft will be 0.1 to 1 rpm, Data downlink will be at 2.5 Mbps.

Alternate Names

  • GP-B

Facts in Brief

Launch Date: 2004-04-20
Launch Vehicle: Delta II
Launch Site: Vandenberg AFB, United States

Funding Agency

  • National Aeronautics and Space Administration (United States)


  • Astronomy

Additional Information

Questions or comments about this spacecraft can be directed to: Dr. David R. Williams.



NameRoleOriginal AffiliationE-mail
Dr. C. W. EverittMission Principal InvestigatorStanford

Related Information/Data at NSSDCA

NASA Successfully Launches Gravity Probe B Mission (20 April 2004 Press Release)

Other Source of Gravity Probe B Information/Data

Gravity Probe B Home Page - Stanford University

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