Gravity Probe B

Launch data: Designation  28230 / 04014A Launch date  20 Apr 2004 - 16:57:24 Ut Launch site  Va, SLC2W Launch vehicle  Delta 7920-10C(#D304) Mission  Fundamental Physics Earth orbit on 20 Apr: Perigee / Apogee  641 / 645 km Inclination  90,01° Period  97,59 min

Description:

• Gravity Probe B is a science payload, with the science package developed by Stanford University and Lockheed Martin for NASA, with Marshall managing the programme; to test two unverified predictions of Einstein's general theory of relativity. The satellite was finally launched on 2004 Apr 20 after 40 years of development. The 3145 kg spacecraft carries 4 gyroscopes kept at 1.8 Kelvin by a liquid helium dewar, laser retroreflectors and 2 GPS receivers for orbit determination, a drag compensation system, and a 14 cm aperture quartz telescope. The challenging physics experiment, developed by Stanford University together with Lockheed Martin, will observe the 5th magnitude star IM Peg for over a year, attempting to measure the tiny shifts in the gyroscopes' orientation caused by the Lense-Thirring gravitomagnetic (or `frame-dragging') effect predicted by general relativity. The frame-dragging caused by the Earth's rotation will make the orbital plane rotate by 40.9 millarcseconds per year. GP-B's polar orbital inclination of 90.01 degrees was chosen to minimize the orbital plane rotation due to the Newtonian effect of the Earth's polar flattening, which is proportional to the cosine of the inclination. GP-B will also make an accurate measurement of the well-established gravitostatic warping of spacetime due to the Earth's mass, which is a much larger effect of 6.6 arcseconds per year in a perpendicular (in-plane) direction.

Mission details:

• The Delta 7920 rocket entered a 167 x 652 km transfer orbit and then fired again to put GP-B in a 641 x 645 km x 90.01 deg orbit. The second stage then separated and two further burns put it in a 182 x 631 km x 94.56 deg orbit to ensure that it would not recontact GP-B and that it will re-enter rapidly.
• The spacecraft is spin stabilized (0.1 to 1 rpm). Attitude control thrusters use helium boiled off from the experiment's dewar. Experiment gyro used for attitude reference.
• A block of fused quartz 21 inches long holds four gyroscopes and a proof mass, all bonded to a quartz telescope. The package is inserted into a 1500 L helium dewar to keep at 1.8 Kelvin. The gyroscopes are constructed from ultrasmooth quartz balls coated with niobium which becomes a superconductor as liquid helium temperatures, allowing the gyroscopes to be suspended electrically. Very sensitive magnetometers detect any changes in the gyroscope's spin axis. The gyros spin at 10,000 revolutions a minute.