In Einstein’s theory of general relativity (GR) spacetime is distorted in a gravitational field like that of Earth. Earth’s mass curves spacetime and Earth’s spin even warps it. Gravity Probe B (GP-B) measured the curving and warping of spacetime around Earth. For this experiment, gyroscopes in a spacecraft were used. According to GR, the gyroscopes were predicted to precess, known as the geodetic and frame dragging effects. These effects needed to be measured with respect to the distant universe. Our research was to provide an inertial reference frame in the distant universe within which the precessions could be measured. GP-B measured the precessions with a small on-board telescope relative to the bright guide star, IM Pegasi. We determined astrometrically with VLBI IM Pegasi’s motion, first relative to reference sources including the core of the quasar, 3C 454.3, and then relative to the International Celestial Reference Frame 2 (ICRF2). The ICRF2 is defined by the positions of ~4000 other quasar and radio galaxies and is the most stable astronomical reference frame that exists at present. IM Pegasi is a binary dominated in luminosity by the giant in the system. We determined the giant’s position at epoch, proper motion, parallax, orbital motion and jittery radio emission locations. Of relevance for GP-B was our proper motion:  -20.83±0.09 mas/yr in RA and -27.27±0.09 mas/yr in dec in the ICRF2. This is an important result since any error would directly translate into an error of the gyroscope precessions with respect to the distant universe and falsify the results of the GP-B mission. Our results met the pre-launch requirements. The geodetic and frame dragging effects could be measured and were found to be consistent with GR. The GP-B mission came to a successful conclusion. We published our results in seven papers in 2012 and summarized our results in an invited review for a Special Issue of the journal Classical and Quantum Gravity (Bartel et al. 2015).

 IM Pegasi, the guide star for Gravity Probe BVLBI arrayProper motion and reference sourcesParallax ellipseOrbit of primaryRadio images of IM PegasiRadio image peaks relative to the disk of the primarySimulation of scatter of radio emission peaksmovie of IM PegasiGP-B resultsGP-B posterresults