Abstract:  The NASA/Stanford Gravity Probe B (GP-B) mission is designed to measure two effects predicted by general relativity that result in the steady drift in orientation of the spin axes of four gyroscopes housed in a spacecraft in a drag-free, low-Earth polar orbit.  The changes in direction of the spin axes of the gyroscopes over time is measured relative to the direction to the binary star IM Pegasi (HR 8703).  To measure the relativistic drifts accurately, any changes in the apparent position of the star relative to an inertial (i.e., extragalactic) frame must be accounted for.  In particular, any error in the assumed proper motion of the star will map directly into the estimated relativistic drift rates.  I will present results from multiple types of optical observations of IM Peg that were intended to measure or provide upper bounds on particular contributions to the apparent motion of the star as seen by the spacecraft's on-board star-tracking telescope.  I will then focus on our very-long-baseline radio interferometry (VLBI) observations of IM Peg, which we used to measure the proper motion of the star with an accuracy goal of 0.14 milliarcseconds/yr (standard error) in both the "north-south" and "east-west" directions relative to an extragalactic frame.

Light refreshments available 4pm; Presentation begins 4:15pm. Open to all.