The Gravity Probe B mission provided two new quantitative tests of Einstein’s theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earth’s orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

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Spherical gyroscope rotors for a fundamental experiment to test two predictions of General Relativity Theory, geodetic precession and frame dragging, were manufactured out of fused quartz and single-crystal silicon. These rotors required a mass unbalance and asphericity of less than 25 nm (1 μin), a fractional difference in the moments of inertia of less than 10−5, and a diameter within 0.5 μm (20 μin) of a standard sphere with a diameter of 37.996 mm (1.4959 in). We describe the manufacturing process and the associated metrology.