Gravity Probe B Testing General Relativity with Orbiting Gyroscopes - PowerPoint PPT Presentation

GP-B T0082 Gravity Probe B – Testing General Relativity with Orbiting Gyroscopes Int’l Workshop on Precision Tests and Experimental Gravitation in Space Galileo Galilei Institute, Firenze, Italy; Sep 28-23, 2006 William Bencze, GP-B Program Manager for the GP-B Team GGI Workshop 2006 1 GP-B T0083

Outline • Gravity Probe B – Description of the experimental concept – Difficult requirements and key enabling technologies. – Status of post-flight data analysis • STEP Mission Update GGI Workshop 2006 2 GP-B T0083

Testing GR with Orbiting Gyroscopes “If, at first, the idea is not absurd, then there is no hope for it.” - Albert Einstein ⎡ ⎤ 3 GM ( ) GI 3 R ( ) = × + ⋅ − Ω ω ω Leonard Schiff’s R v R ⎢ ⎥ ⎣ ⎦ 2 3 2 3 2 2 c R c R R relativistic precessions: Geodetic, Ω G Frame Dragging, Ω FD d s dt = × Ω s Spin axis orientation: GGI Workshop 2006 3 GP-B T0083

How Big is a 0.1 Milli-Arc-Second? 0.1 marc-sec = Angular width of Lincoln’s eye in New York seen from Paris! 0.1 marc-sec GGI Workshop 2006 4 GP-B T0083

Einstein’s 2 1/2 Tests Perihelion Precession of Mercury • GR resolved 43 arc-sec/century discrepancy. Deflection of light by the sun • GR correctly predicted 1919 eclipse data. • 1.75 arc-sec deflection: Present limit 10 -3 Gravitational Redshift: Equivalence Principle • Einstein’s “half test’ – Equivalence principle only • 1960 Pound-Rebka experiment (ground clocks) • 1976 Vessot-Levine GP-A (orbiting clocks): 2 × 10 -4 Tests of General Relativity to date rely on astronomical measurements, not a laboratory experiment under scientist's control. GGI Workshop 2006 5 GP-B T0083

Why a Space-based Experiment? 6614 Electrostatic vacuum 10 10 Geodetic effect gyro on Earth 10 3 <0.002% accuracy marc-s / yr uncompensated (10 -1 deg/hr) 10 9 10 2 Spacecraft gyros 10 8 Frame dragging (3x10 -3 deg/hr) 41 <0.3% accuracy 10 Best laser gyro 10 7 marc-s / yr (10 -3 deg/hr) 10 6 1 Electrostatic vacuum gyro GP-B requirement 0.5 on Earth (torque Single gyro expectation 0.21 modeling) (10 -5 deg/hr) 10 5 0.1 0.12 4 Gyro expectation (3x10 -10 deg/ √ hr) 10 4 Cold Atom Gyro (3x10 -6 deg/ √ hr) Expected GP-B 0.01 (Kasevich 2006) 10 3 performance on orbit Operation in 1g environment degrades mechanical gyro performance Laser gyroscopes and other technologies fidelity too low for GP-B GGI Workshop 2006 6 GP-B T0083

The “simplest experiment” “No mission could be simpler than Gravity Probe B. It’s just a star, a telescope, and a spinning sphere.” - William Fairbank, GP-B PI (ca. 1964) 1. “Spinning Sphere” Perfect Gyros Drift < 0.1 marc-sec/yr – Perfect mass balance < 20 nm mass unbalance – Roundest spheres < 20 nm p-v – Gentle gyroscope suspension 200 mV – Gyroscope centering control ~ 1 nm – Precise initial gyro orientation < 10 arc-sec ~ 10 -12 g cross-axis “drag free” – Cross axis force control – Spin down torques (gas drag) < 10 -9 Pa – Rotor electrical charge < 15 mV ~ 200 marc-sec/ √ Hz – Orientation readout: low noise SQUIDS – Magnetic Shielding 240 dB shielding – Cryogenics, superfluid He dewar 2500 liter @ 1.8K GGI Workshop 2006 7 GP-B T0083

The “simplest experiment” 2 2. Telescope – Accurate pointing < 0.1 marc-sec/yr – Precision vehicle pointing ~5 marc-sec ~ 34 marc-sec/ √ Hz – Low measurement noise – Mechanically “rock solid” Cryogenic quartz fabrication – Precise orbit Orbit trim with GPS monitoring 3. Guide Star – Inertial Reference < 0.1 marc-sec/yr – Optically “bright” 6 magnitude – Maximize frame dragging effects Near equator – Precise proper motion measurement VLBI – good radio source – Near extra-galactic radio source Quasar – distant inertial frame A “simple” experiment … Indeed! GGI Workshop 2006 8 GP-B T0083

The Overall Space Vehicle � Redundant spacecraft processors, transponders. � 16 Helium gas thrusters, 0-10 mN ea, for fine 6 DOF control. � Roll star sensors for fine pointing. � Magnetometers for coarse attitude determination. � Tertiary sun sensors for very coarse attitude determination. � Magnetic torque rods for coarse orientation control. � Mass trim to tune moments of inertia. � Dual transponders for TDRSS and ground station communications. � Stanford-modified GPS receiver for precise orbit information. � 70 A-Hr batteries, solar arrays operating perfectly. GGI Workshop 2006 9 GP-B T0083

GP-B Launch - 20 April 2004 Launch! Release from launch vehicle Fairing Installation GGI Workshop 2006 10 GP-B T0083

The Science Gyroscopes � Material: Fused quartz, homogeneous to a few parts in 10 7 � Overcoated with niobium. � Diameter: 38 mm. � Electrostatically suspended. � Spherical to 10 nm – minimizes suspension torques. � Mass unbalance: 10 nm – minimizes forcing torques. � All four units operational on orbit. Gyroscope rotor and housing halves Demonstrated performance: • Spin speed: 60 – 80 Hz. • 20,000 year spin-down time. GGI Workshop 2006 11 GP-B T0083

“Perfect” Mass Balance Needed! Mass Balance Requirements: ω s r Gyro spin δ r axis On Earth ( ƒ = 1 g) δ < 5.8 x 10 -18 r r CG (ridiculous – 10 -4 of a proton!) Standard satellite (ƒ ~ 10 -8 g) δ r f < 5.8 x 10 -10 r (unlikely – 0.1 of H atom diameter) External forces acting GP-B drag-free (ƒ ~ 10 -12 g δ r through center of force, < 5.8 X 10 -6 r cross- track average) different than CM (straightforward – 100 nm) δ r Drag-free eliminates < 3 x 10 -7 Demonstrated GP-B rotor: r mass-unbalance torque δ ω and key to r 2 r Requirement Ω < Ω 0 < Ω s understanding of other 0 ~ 0.1 marc-s/yr r 5 f support torques (1.54 x 10 -17 rad/s) Ω = τ ω I Drift-rate: s τ = δ mf r Torque: = 2 I ( Moment of Inertia: 2 5) mr GGI Workshop 2006 12 GP-B T0083

Sphericity Measurement Talyrond sphericity measurements to ~1 nm Typical measured rotor topology; peak-valley = 19 nm If a GP-B rotor was scaled to the size of the Earth, the largest peak-to-valley elevation change would be only 6 feet! GGI Workshop 2006 13 GP-B T0083

Flight Proportional Thruster Design • Cold gas (no FEEP!) proportional thruster; 16 units on space vehicle. • Operates under choked flow conditions • Pressure feedback makes thrust independent of temperature Thrust 3.5mm ia Thrust: 0 – 10 mN Propellant: Helium Dewar Boiloff I SP : 130 sec Supply: 5 to 17.5 torr Mdot: 6-7 mg·s − 1 Noise: 25 µN·Hz − 1/2 Location of thrusters on Space Vehicle GGI Workshop 2006 14 GP-B T0083

Drag-free Operational Modes • Suspended “accelerometer” mode – Measured gyro control effort nulled by space vehicle thrust. – Used during most of mission due to robustness, gyro safety. • Unsuspended “free float” mode – SV chases gyro; nulls position signal. 1 � 2 Ms U � R − � � ( ) r R � r 1 � ms 2 u GGI Workshop 2006 15 GP-B T0083

Drag Free Control for a Perfect Orbit Demonstrated performance Prime and Backup Drag Free operations, GP-B Gyro3 (VT=142273900) better than 10 -11 g residual 20 Gyro3 pos (nm) Position (nm) Xsv Normal gyro acceleration on drag free Ysv Gyro 0 Zsv gyroscope in measurement suspension band -20 (12.9mHz ± 0.2mHz) 0 1000 2000 3000 4000 5000 6000 Gyro control 0.1 Rejection ~ 10,000x Gyro3 CE ( μ N) effort ( μ N) Accelerometer Xsv Prime Ysv mode mode 0 Zsv Suspension Inertial space – Frequency domain Suspension ON OFF -0.1 Drag-free control effort and residual gyroscope acceleration (2004/239-333) 0 1000 2000 3000 4000 5000 6000 -7 10 SV trans force (mN) Polhode 5 Gyro CE inertial Gravity SV Thrust frequency SV CE inertial Xsv Gradient Ysv -8 (mN) thrust Roll rate 10 0 Zsv Acceleration (g) Thruster Control Effort (g) -9 -5 10 Force 0 1000 2000 3000 4000 5000 6000 seconds -10 10 Drag free modes in operation Residual gyro acceleration 5x10 -12 g in band -11 10 ~1.5x10 -8 (m/s 2) / √ Hz 0.02mHz – 80 mHz -12 10 -4 -3 -2 -1 0 10 10 10 10 10 Frequency (Hz) GGI Workshop 2006 16 GP-B T0083

Superconducting SQUID Readout The Conundrum: How to measure with extreme accuracy the direction of spin of perfectly round, perfectly uniform, sphere with no marks on it? The Solution: 2 mc = − e ω = − × − ω 7 ( ) M 1.14 10 Gauss London Moment Readout . A L s s spinning superconductor develops a magnetic “pointer” aligned with its spin axis. Magnetic field sensed by a SQUID, SQUID electronics in Niobium carrier a quantum limited, DC coupled magnetic sensor. Performance: measurement better Requirement than 200 marc-s/ √ Hz GGI Workshop 2006 17 GP-B T0083

Science Instrument Assembly Stanford-developed silicate Star bonding technique to join Guide star tracking block and telescope. IM Pegasi telescope ( HR 8703 ) Gyros 1 & 2 1 2 3 4 Quartz Mounting block flange Gyros 3 & 4 GGI Workshop 2006 18 GP-B T0083