Orbiting relativity test gets slow start

Unexpected but necessary adjustments to a satelliteborne test of relativity theory have consumed more than a quarter of the 13-month period the mission had allotted to collecting data. On Aug. 27, the Gravity Probe B (GP-B) experiment, which was launched into Earth orbit on April 20, finally began what’s now expected to be a 10-month run of gravity measurements.

TO FOLLOW A STAR. This frame from an animation depicts the Gravity Probe B spacecraft locking its telescope onto the star IM Pegasi. A. Pozzer, N. Bartel/York Univ., Astronomy Films

Despite the anticipated shortfall of data, the experiment should achieve the measurement precision originally expected, says Bob Kahn, a mission spokesman. As a consequence of the delay, however, observations of a reference star won’t be as thorough and so won’t give scientists as much backup data as planned.

The GP-B spacecraft is essentially a large liquid-helium tank surrounding a tube containing four gyroscopes and a telescope (SN: 5/15/04, p. 316: Available to subscribers at After 40-year prep, gravity test soars). With these components, mission scientists expect to monitor two effects predicted by the general theory of relativity. One of them is that Earth’s gravity bends space-time in the planet’s vicinity. The other is that the planet drags the fabric of space-time with it as it rotates. Those effects ought to be discernible as a subtle drifting of the gyroscopes’ axes with respect to the reference star.

“We’re doing incredibly complicated positional control of the satellite,” notes Kahn. Most of the delay resulted from unanticipated challenges in fine-tuning the spacecraft’s trajectory and locking onto the reference star, he says.

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