Gravity Probe B mission enters science phase

On Aug. 27, with its initialization and orbit calibration phase behind it, Gravity Probe B (GP-B) entered the science phase of its mission to test Einstein’s General Theory of Relativity.

“This is the moment we have been waiting for,” said Stanford Research Professor Francis Everitt, co-principal investigator of GP-B with Professor Emeritus Brad Parkinson. “It represents a magnificent effort by the entire Stanford–NASA–Lockheed Martin team.”

From its April 20 launch from Vandenberg Air Force Base in Southern California until it entered the science phase, the NASA spacecraft’s instruments—including four ultra-precise gyroscopes—have been undergoing systems checkouts and fine-tuning from an orbit 400 miles above Earth.

“All systems are meeting or exceeding the requirements of the mission,” said Stanford program manager Gaylord Green.

At launch, GPB’s dewar contained about 650 gallons of superfluid helium—enough to maintain the gyroscopes in a cryogenic state for 16 months. The mission timeline originally specified two months for initialization, checkout and instrument tuning, 13 months for relativity data collection and one final month for instrument re-calibration. Although the four months needed for initialization and orbit calibration result in a shorter data collection period than planned, GP-B will still “significantly surpass its mission performance requirements,” according to a NASA statement.

During the science phase, data are collected twice a day about vehicle and instrument performance. Extremely precise measurements of the gyroscopes’ spin-axis alignment relative to a guide star convey the amounts by which the Earth dents and drags local timespace.