Encyclopedia Astronautica
Gravity Probe-B

American earth geodetic satellite. One launch, 2004.04.20, Gravity Probe B. Gravity Probe B was an experiment developed by NASA and Stanford University to test two unverified predictions of Albert Einstein's general theory of relativity.

The experiment would check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in a satellite in a 650 km polar orbit. The gyroscopes would measure how space and time were warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, had far-reaching implications for the nature of matter and the structure of the Universe. The spacecraft was spin stabilized (0.1 to 1 rpm). Attitude control thrusters used helium boiled off from the experiment's Dewar. The experiment gyro used for attitude reference. The payload included a block of fused quartz 21 inches long holding four gyroscopes and a proof mass, all bonded to a quartz telescope. The package was inserted into a 1500 l helium Dewar in order to maintain it at a temperature of 1.8 Kelvin. The gyroscopes were constructed from ultrasmooth quartz balls coated with niobium which became a superconductor as liquid helium temperatures, allowing the gyroscopes to be suspended electrically. Very sensitive magnetometers detect any changes in the gyroscope's spin axis. The gyros spun at 10,000 revolutions a minute.

Financial status as of 1997 was $ 236 million spent, $ 340 million to complete.

First Launch: 2004.04.20.
Number: 1 .

More... - Chronology...

Associated Countries
See also
  • Delta The Delta launch vehicle was America's longest-lived, most reliable, and lowest-cost space launch vehicle. Development began in 1955 and it continued in service in the 21st Century despite numerous candidate replacements. More...

Associated Launch Vehicles
  • Delta American orbital launch vehicle. The Delta launch vehicle was America's longest-lived, most reliable, and lowest-cost space launch vehicle. Delta began as Thor, a crash December 1955 program to produce an intermediate range ballistic missile using existing components, which flew thirteen months after go-ahead. Fifteen months after that, a space launch version flew, using an existing upper stage. The addition of solid rocket boosters allowed the Thor core and Able/Delta upper stages to be stretched. Costs were kept down by using first and second-stage rocket engines surplus to the Apollo program in the 1970's. Continuous introduction of new 'existing' technology over the years resulted in an incredible evolution - the payload into a geosynchronous transfer orbit increasing from 68 kg in 1962 to 3810 kg by 2002. Delta survived innumerable attempts to kill the program and replace it with 'more rationale' alternatives. By 2008 nearly 1,000 boosters had flown over a fifty-year career, and cancellation was again announced. More...
  • Delta 7920-XC Three stage vehicle consisting of 9 x GEM-40 + 1 x EELT Thor/RS-27A + 1 x Delta K with 3.05 m (10 foot) diameter composite fairing More...

Associated Manufacturers and Agencies
  • NASA American agency overseeing development of rockets and spacecraft. National Aeronautics and Space Administration, USA, USA. More...
  • Lockheed American manufacturer of rockets, spacecraft, and rocket engines. Lockheed Martin, Sunnyvale, CA, USA. More...
  • Stanford American manufacturer of spacecraft. Stanford University, USA. More...

  • McDowell, Jonathan, Jonathan's Space Home Page (launch records), Harvard University, 1997-present. Web Address when accessed: here.
  • JPL Mission and Spacecraft Library, Jet Propulsion Laboratory, 1997. Web Address when accessed: here.
  • NASA Report, Gravity Probe B Testing Einstein's Universe, Web Address when accessed: here.
  • NASA Report, Gravity Probe B Press Kit, Web Address when accessed: here.
  • NASA Report, Review of Gravity Probe B, Web Address when accessed: here.

Associated Launch Sites
  • Vandenberg Vandenberg Air Force Base is located on the Central Coast of California about 240 km northwest of Los Angeles. It is used for launches of unmanned government and commercial satellites into polar orbit and intercontinental ballistic missile test launches toward the Kwajalein Atoll. More...
  • Vandenberg SLC2W Delta launch complex. Originally a Thor 75 SMS launch pad. Upgraded to a space launch complex in 1966. More...

Gravity Probe-B Chronology

2004 April 20 - . 16:57 GMT - . Launch Site: Vandenberg. Launch Complex: Vandenberg SLC2W. LV Family: Delta. Launch Vehicle: Delta 7920-XC. LV Configuration: Delta 7920-10C D304.
  • Gravity Probe B - . Mass: 3,145 kg (6,933 lb). Nation: USA. Agency: NASA. Class: Astronomy. Type: X-ray astronomy satellite. Spacecraft: Gravity Probe-B. USAF Sat Cat: 28230 . COSPAR: 2004-014A. Apogee: 646 km (401 mi). Perigee: 640 km (390 mi). Inclination: 90.0000 deg. Period: 97.60 min. Gravity Probe B's mission was to confirm a prediction of Einstein's theory of relativity. The physics experiment, developed by Stanford University and Lockheed Martin, was to observe the magnitude 5 star IM Pegasi for over a year, attempting to measure the tiny shifts in the spacecraft gyroscopes' orientation caused by the Lense-Thirring gravitomagnetic (or `frame-dragging') effect. To accomplish this the spacecraft carried four gyroscopes kept at 1.8 deg Kelvin by a liquid helium dewar, laser retroreflectors and two GPS receivers for orbit determination, a drag compensation system, and a 14 cm aperture quartz telescope. The satellite was also to make an accurate measurement of the already-established gravitostatic warping of spacetime due to the Earth's mass.

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