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).
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