The Relativity Mission, Gravity Probe B Experimental Design, Sources of Error, and Status Mac Keiser Snowmass 2001 July 4, 2001

GP-B Concept

Experimental Design Guidelines l Reduce Classical Torques on Each Gyroscope Less Than 0.3 mas/yr. l Measure Drift Rate of Each of Four Gyroscopes Relative to the Guide Star with an Accuracy of better than 0.3 mas/yr. u Use Optical Aberration as a Known Calibration Signal. l Using VLBI, Measure Proper Motion of Guide Star to an Accuracy of Better than 0.15 mas/yr. l Include Telemetry to Monitor Potential Experimental Errors and Commands to Change Experimental Conditions l Reduce Overall Experimental Error to Less than 0.5 mas/yr.

Gyroscope Drift Rate l Reduce Support Dependent Torques by u Drag-Free Satellite u Rolling Spacecraft u Careful Selection of Gyroscope Orientation and Orbit u Tight Manufacturing Tolerances l Reduce Support Independent Torques by u Monitoring and Controlling Charge on Gyroscope Rotor u Residual Magnetic Field Less Than 9 Microgauss u Residual Gas Pressure Less Than Torr u Rolling Spacecraft and Orbit Selection

Gyroscopes Fused quartz rotor  R/R<10 -6 Quartz housing  R/R<10 -5 Electrostatic suspension g - 1 g Capacitative positioning <0.3nm at roll He gas spin-up >150Hz UV charge control <5 pC Low temp. bake-out < torr Spin to roll alignment <1arcsec < 0.01 marcsec/yr (non supported gyro) <0.08 marcsec/yr (suspended gyro)

London moment read-out with dc SQUIDs Superconducting pickup loop on gyroscope housing London Moment Read-Out SQUID < 8  J/Hz (<50  0 /  Hz) 200marcsec/  Hz (5  G/  Hz )

Superconducting Magnetic Shields l Magnetic shielding: total ac attenuation

l Folded Schmidt Cassegranian l 150” focal length, 5.6” diameter l All quartz construction l Potassium hydroxide bonding l Image splitting with roof prisms for quadrant read-out information l Low temperature read-out using photodiodes and silicon preamplifiers at 70K l 4 telescope windows with 60% transmission in visible and IR and rf rejection Telescope 0.1 marcsec pointing precision

Quartz Block Metrology Frame l Four Gyroscopes Mounted 8.25 cm center-to-center l Telescope Bonded to End of Quartz Block l Orientation of Gyroscope Spin Axis Relative to Measured Direction to Guide Star Determined by Difference Between Gyroscope and Telescope Readouts l Stability of Quartz Block Metrology Reference Frame at Satellite Roll Frequency Better Than 1.3  arc sec u Low Thermal Expansion Coefficient u Small Variation in Temperature at Satellite Roll Freuquency

Probe and Dewar l Probe u Ultrahigh Vacuum < Torr Achieved Using Low Temperature Bakeout u Sintered Titatanium Cryopump Provides Equivalent of 50 m 2 of surface area l Dewar u Holds 2300 liters of superfluid helium u Lifetime on Orbit Greater Than 17 months

Main GP-B Systems GyroscopeTelescopeScience Instrument Cryogenic Probe PayloadSpace Vehicle

Uncertainty in Proper Motion of GP-B Guide Star HR 8703 (IM PEG) Visible and radio star Magnitude mv = 5.69 Declination= deg (close to equator) Proper motion calibrated by SAO using VLBI Expected accuracy by 2002 is about 0.09 marcsec/yr

GP-B Mission Timeline  Initial set-up and calibration  days, ~ 1,000 events  Science Mission data  months  Post Mission calibration  30 days, ~ 100 events  Data rates  26 kbit/s total  15 kbit/s science data  11 kbit/s snapshots, vehicle

Experiment Accuracy One Gyroscope

GP-B From Now to Launch l Complete Payload Verification 10/1/01 l Payload/Spacecraft Integration10/22/01 l Space Vehicle Testing 8/21/02 l Launch 10/30/02

Experimental Test of General Relativity in Space l Special Relativistic Effect - Aberration of Starlight - Used as Known Calibrating Signal l Gravitation Deflection of Light by the Sun Used as Experimental Cross-Check l Precise Measurement of Motional (Lense- Thirring) Effect l Most Accurate Experimental Test of General Relativity