` Optical support of the Gravity Probe B Mission

Launch on April 20, 2004


Optical support of the Gravity Probe B Mission


Gravity Probe B is the relativity gyroscope experiment being developed by NASA and Stanford University to test two extraordinary, unverified predictions of Albert Einstein's general theory of relativity. The experiment will 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 will 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 of 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:

  • a drift-free gyroscope:
    		• a gyroscope having an absolute drift-rate (change in spin direction from nonrelativistic disturbances) less than 10-11 degrees/hour
  • a gyro readout:
    		• a method for determining changes in spin angle to 0.1 milliarc-second without disturbing the gyroscope
  • a stable reference:
    		• a means (telescope and mechanical structure) of referring the gyro readout to the guide star
  • a trustworthy guide star:
    		• a bright, properly located star whose motion with respect to inertial space is known
  • a technique for separating relativity effects:
    		• an orbit and a data processing method that together allow the frame-dragging and geodetic effects to be separated
  • a credible calibration scheme:
    		• a scheme of in-flight calibration tests to ensure that the gyroscopes - and the entire instrument - are free from errors that might masquerade as relativity signals


    All six prerequisites are met. Hardest is the fourth, which goes beyond Gravity Probe B into the world of astrometry and astrophysics. How a guide star was chosen can be found on Stanford's FAQ page.

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

    IM Peg has been 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:

  • The long-period RS CVn binary IM Pegasi. I. Orbital and stellar parameters
    Berdyugina S.V., Ilyin I., Tuominen I., 1999, A&A 347, 932

  • The long-period RS CVn binary IM Pegasi. II. First surface images
    Berdyugina S.V., Berdyugin A.V., Ilyin I., Tuominen I., 2000, A&A 360, 272

    The employed Doppler imaging technique is described in the paper:

  • Surface imaging by the Occamian approach. Basic principles, simulations, and tests.
    Berdyugina S.V. 1998, A&A 338, 97


    Here are the images of IM Peg for the previous years:

    Aug-Oct, 1996



    Nov-Dec, 1996



    June-Aug, 1997



    Oct-Dec, 1997



    July, 1998



    Oct-Nov, 1998



    May-July, 1999



    Sep-Nov, 1999




    Observations for the above images were obtained at
  • Nordic Optical Telescope, La Palma, Spain
  • Crimean Astrophysical Observatory, Ukraine
  • National Astronomical Observatory, Rozhen, Bulgaria


    Observations during the mission will be also carried out at
  • Tennessee State University 2.0m AST


    The project is supported by
  • Harvard-Smithsonian Astrophysical Observatory, Cambridge, MA
  • Stanford University, Stanford, CA
  • NASA Marshall Space Flight Center