Gravity Probe B is the relativity gyroscope experiment developed by NASA and Stanford University to test two extraordinary, unverified predictions of Albert Einstein's general theory of relativity. The aim of the experiment is to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 650 km altitude directly over the poles. The gyroscopes should measure how space and time are 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, have far-reaching implications for the nature of matter and the structure of the Universe. The details and updates on the project can be found on the Stanford's Gravity Probe B page.

Measurement of the two effects demands far more than placing a gyroscope in a satellite. Six distinct technical requirements have to be simultaneously satisfied (see Stanford's FAQ page). Among them is a trustworthy guide star which must have been a bright, properly located star whose motion with respect to inertial space is known. The requirement is the hardest as it goes beyond Gravity Probe B into the world of astrometry and astrophysics.

Our participation in the mission is to provide the best knowledge on the guide star in the optics.

IM Pegasi was chosen as a guide star for the mission telescope, which must remain fixed on the optical center of the star. Any nonzero trend during the mission in the offset between the optical centroid of the light of the primary and its center of mass should be known with a standard error less than 0.05 milliarcseconds/year. The star shows, however, a high level of magnetic activity which is expressed, for instance, in large cool magnetic regions on the stellar surface (see our page on Stellar Activity). Cool regions on the stellar surface radiate lower continuum flux as compared to the hotter photosphere and, thus, distort the position of the optical center of the stellar disk. Moreover, due to stellar rotation and spot evolution, the offset of the optical center varies on time scales of one stellar rotation, months, and years. In order to provide estimates of the optical offset due to spots, the spot distribution on the surface of IM Peg will be monitored during the mission with the help of the Doppler imaging technique, which provides images of the stellar surface, from which the offsets of the optical center will be calculated.

The most accurate stellar and orbital parameters of IM Peg and the first Doppler images of the star are published in our papers: