Update on the Gravity Probe B Experiment

When Leonard Schiff was Chair of the Physics Department in 1960, three years after Sputnik, he suggested that it should be possible to make two new tests of Einstein's General Theory of Relativity by observations on gyroscopes in a satellite in orbit around the Earth. Gravity Probe B is a collaborative program between members of the Physics and Aero-Astro Departments at Stanford, being developed in the Hansen Experimental Physics Laboratory with NASA support.

Schiff calculated that, according to General Relativity, a gyroscope in a 650 km polar orbit, should experience two distinct relativistic precession effects: (1) in the orbital plane a geodetic precession of 6.6 arcsec/yr; (2) at right angles to the orbital plane a frame-dragging precession of 0.042 arcsec/yr due to the Earth's rotation. GP-B aims at measuring each of these effects in each of four independent gyroscopes to an accuracy of 0.0002 arcsec/yr per gyroscope. This minute number (0.0002 arcsec) corresponds to the width of a human hair seen at a distance of 15 miles.

Performing an experiment to this extreme precision requires a gyroscope with a drift rate many orders of magnitude better than that of the very best inertial navigation gyroscopes. The GP-B gyroscope shown in the illustration is an extremely round, extremely uniform sphere 38 mm in diameter, coated with a thin film of superconductor and electrically suspended in a vacuum. It spins at approximately 10,000 rpm. The direction of the spin is measured by means of superconducting circuits connected to a SQUID (Superconducting QUantum Interference Device). The four gyroscopes with a reference telescope are all mounted together in a quartz block structure within a dewar vessel, containing 2,300 liters of superfluid helium at a temperature of 1.8 K. The experiment will remain operating in space for approximately 19 months. The flight dewar and many of GP-B's fascinating technologies may be seen in HEPL.

Gravity Probe B has been remarkable for the richly varied contributions of graduate and undergraduate students to its development. To date, there have been 64 doctoral dissertations in five Stanford departments and 8 at other universities. Currently, there are 16 doctoral candidates and 21 undergraduates from 6 Stanford departments. Several of the undergraduates have won University awards for their accomplishments.

GP-B is scheduled to be launched from the NASA Western Test Range at Vandenberg Air Force base by a Delta II launch vehicle in March 2000. The dewar and spacecraft are built by Lockheed Martin. GP-B has a launch mass of 3,000 kg.