The more important measurement conducted by the science instrument is that of the angle between the spin axis of our gyroscopes and the fixed reference line provided by our guide star (via our on-board telescope). Effectively, we determine the rate of change of the gyro spin directions in comparison to the direction of one particular star. By means of our specialized guiding telescope, the spacecraft is kept pointing at this "guide star" for almost the entire mission. But is this guide star an adequate reference point, fixed on the sky with respect to the galaxies lying behind it? The answer is "No!"  Like everything else in the universe, stars move, and are not fixed points on the sky. Our guide star (like all stars) is free to move slowly across the sky, and this motion cannot be ignored in the GP-B test of general relativity. In fact, the European space mission Hipparcos has shown that each year the chosen guide star moves across the sky about 35 milliarcseconds, almost as much as the entire relativistic gravitomagnetic effect in one year. Moreover, the uncertainty of this value is larger than the intended accuracy of the of the GP-B relativity tests. Some other method was needed to measure the motion of the guide star.

The solution to this problem was to choose a star that emits not only light that can be observed with the telescope of the GP-B spacecraft, but also microwave radio noise that can be recorded by large astronomical and deep space communication radio antennas on the ground. The chosen star is IM Pegasi--in Latin "Pegasi" means "of or in Pegasus," which is a constellation of stars easily seen high over head on autumn evenings throughout North America, Europe, and Asia. The designation "IM" is just a pair of letters chosen more or less sequentially to identify certain stars. This star IM Pegasi is barely bright enough to see with the naked eye under very dark and clear skies, but when observed in the microwave range, this star is often among the brightest in the sky. The microwave noise coming from this star makes it possible to use the methods of very-long-baseline interferometry (VLBI) to measure the position of the star in comparison to several quasars that lie nearby to the star on the sky. Quasars are galaxies with a quasi-stellar (i.e., nearly unresolved) appearance found in the distant universe millions of times farther away from the Earth than the stars visible to the naked eye. The wonderful thing about these quasars is that even though they are far enough away to be ideal reference points for the relativity tests, they emit so much microwave radio energy that they appear to radio antennas to be among the brightest objects in the sky, in many cases even brighter than the guide star.

To measure the motion on the sky of this particular star, since 1997 the GP-B program has obtained VLBI observations of this star and at least two quasars behind it about four times per year. Data is recorded with the VLA and VLBA radio antenna arrays of the National Radio Astronomy Observatory (NRAO), the 70 meter diameter dishes of NASA's Deep Space Network (DSN) and the 100 meter diameter dish of the Max-Planck-Institut für Radioastronomie (MPIfR) in sessions lasting up to 18 hours. The data from all the antennas are initially combined and processed by NRAO in Socorro, NM, before being sent to York University, in Toronto, Canada, and to the Harvard-Smithsonian Center for Astrophysics, in Cambridge, MA, for further analysis. The use of so many telescopes increases both the accuracy of the measurements and their ability to detect the the sometimes weak radio emission from IM Pegasi. The results of this analysis are a series of maps and positions from which the motions of IM Pegasi can be estimated. By the end of the GP-B mission, these observations should determine the mean (or so-called "proper") motion of IM Pegasi against the distant galaxies to an accuracy of about 0.1 milliarcseconds per year. This accuracy is comparable to the intended accuracy of the measurement of the mean rate of change of the gyro spin directions with respect to IM Pegasi. Combined together, these measurements will determine the mean rate of change of the gyro spin directions with respect to the distant universe. Together they will test, with much greater accuracy than ever before, the two predictions of general relativity for the behavior of gyroscopes near a massive body, and thus reveal something about the nature of space-time itself.

Some readers have asked for alternative naming conventions for our guide star, as it appears listed in different catalogs. These names can include: IM Peg, HR 8703, HD 216489, SAO 108231, BD +16 4831, FK5: 3829

Pages 18-20 of the Gravity Probe B Launch Companion contain information about the guide star and the science telescope. Furthermore, the ETH Institute of Astronomy in Zurich, Switzerland, is working with the Harvard-Smithsonian Center for Astrophysics to provide detailed optical information about the GP-B guide star, IM Pegasi. You can find out about the ETH Institute's work in monitoring magnetic activity on IM Pegasi and the Doppler Imaging Technique used for this purpose on the ETH Institute of Astronomy GP-B Web page.

The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto) are studying the guide star to provide crucial measurements of its motion relative to far away quasars. These measurements are needed to relate the tiny changes in the gyroscopes' spin direction to the distant universe, so that general relativity can be tested. Learn more about these measurements by going to the IM Pegasi web site at York University.