Gravity Probe Beinstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf · Gravity Probe B in a...

GravityProbeB

Photos: (Left) Russ Underwood� Lockheed Martin Corporation; (Right) Boeing Corporation

Launch Companion

Gravity Probe B Launch Companion 1

Gravity Probe B in a NutshellIn 1916, in what has been called one of themost brilliant creations of the human mind,Albert Einstein formulated his general theory ofrelativity, which stands as one of thefoundational theories of modern physics. Thestuff of both hard science and science fiction,Einstein's theory weaves together space, time,and gravitation, and predicts such bizarrephenomena as black holes and the expandingUniverse, yet it remains arguably the leasttested of scientific theories. Now, 88 years later,a team from Stanford University, NASA, andLockheed Martin is poised to launch theGravity Probe B spacecraft to measure two ofEinstein’s oddest predicted effects.

The first of these effects, known as the geodeticor curved spacetime effect, postulates that anybody in space warps or curves its localspacetime. This is Einstein’s theory that gravityis not an attractive force between bodies, asIsaac Newton believed, but rather the productof bodies moving in curved spacetime. One wayto visualize this effect is to think of localspacetime as a flat bedsheet and the Earth abowling ball lying in the middle. The heavyball warps or puts a dent in the bedsheet, sothat a marble (another celestial body) movingalong the bedsheet will be inexorably drawndown the warped slope towards the massiveball. The geodetic effect being measured byGravity Probe B is the amount of the tiny angleby which the Earth is warping its localspacetime.

The other effect, known as “frame-dragging,”was postulated by Austrian physicists JosefLense and Hans Thurring two years afterEinstein published his general theory of

relativity. It states that as a celestial body spinson its axis, it drags local spacetime around withit, much like a spinning rubber ball in bowl ofmolasses drags around some of the molasses asit spins.

Particularly intriguing, the frame-draggingmeasurement probes a new facet of generalrelativity—the way in which space and time aredragged around by a rotating body. This noveleffect closely parallels the way in which arotating electrically charged body generatesmagnetism. For this reason it is often referred toas the "gravitomagnetic effect," and measuringit can be regarded as discovering a new force innature, the gravitomagnetic force.

The experiment aboard the Gravity Probe-Bspacecraft is designed to measure these effectswith unprecedented precision and accuracy.Basically, the spacecraft consists of a polar-orbiting satellite containing four ultra-precisespherical gyroscopes and a telescope—which islike saying that an aircraft carrier consists of asome sophisticated fighter planes and afloating runway. In other words, there’s awhole lot more going on.

The gyroscopes must be maintained in apristine environment, in which they can spin in

2 Gravity Probe B Launch Companion

a vacuum, unhindered by any external forces,magnetic disturbances from Earth, ordisturbances from the satellite itself. At thebeginning of the experiment, the telescope (andsatellite) are aligned with a distant “guide”star. The gyroscopes are aligned with thetelescope, so that initially, their spin axes alsopoint to the guide star. The gyroscopes are spunup, and over the course of a year, while keepingthe telescope (and satellite) aligned with theguide star, the gyroscopes’ spin axes aremonitored to detect any deflection or drift dueto the geodetic and frame-dragging relativisticeffects.

If the predictions based on Einstein’s theory arecorrect, the gyroscopes’ spin axes should slowlydrift away from their initial guide staralignment—both in the satellite’s orbital plane,due to the curvature of local spacetime, andperpendicular to the orbital frame due to theframe-dragging effect. While physicists believethat the effects of relativity—especially theframe-dragging effect—are enormous in thevicinity of black holes and other massivegalactic bodies, around a small planet likeEarth, these effects are barely noticeable. Forexample, the predicted angle of spin axisdeflection due to the frame-dragging effectcorresponds to the width of a human hair asseen from 1/4 mile away!

When Stanford physicist Leonard Schiff (andindependently, George Pugh at the Pentagon)first proposed this experiment in 1960, Americahad just created NASA, launched its firstsatellite, and entered the space race. Landingmen on the Moon was still 10 years away. Atthe time, this experiment seemed rather simple,but it has taken over four decades of scientificand technological advancement to create aspace-borne laboratory and measurement

instrument sophisticated and precise enough toquantify these minuscule relativistic effects.

Gravity Probe-B’s measurement of the geodeticeffect, the larger of the two, will be to anaccuracy of 0.01%, which is far more accuratethan any previous measurements, and willprovide the most precise test ever of generalrelativity. The frame-dragging effect has neverdirectly been measured before, but GravityProbe-B is expected to determine its accuracy towithin 1%.

At least nine new technologies had to beinvented and perfected in order to carry out theGravity Probe B experiment. The sphericalgyroscopes have a stability more than a milliontimes better than the best inertial navigationgyroscopes, and the magnetometers, calledSQUIDs (Super-Conducting QuantumInterference Devices), that monitor the spinaxis direction of the gyroscopes, can detect achange in spin axis alignment to an angle ofapproximately 1/40,000,000th of a degree.These advances were only possible through GP-B’s unique combination of cryogenics, drag-freesatellite technology and new manufacturingand measuring technologies.

Over its 40+ year lifespan, spin offs from theGravity Probe B program have yielded manytechnological, commercial, and social benefits.The technological benefits include cryogenicproducts used in other NASA missions, GlobalPositioning Satellite (GPS) products used inaviation and agriculture, optical bonding andfused quartz technologies that have commercialapplications, and photo diode detectors thathave ramifications for digital cameraimprovements.

Less tangible, but perhaps most important, theGravity Probe B program has had a profoundeffect on the lives and careers of numerousfaculty and students—both graduate andundergraduate, and even high school students,at Stanford University and other educationalinstitutions. Nearly 100 Ph.D. dissertationshave been written on various aspects of thisprogram, and GP-B alumni include the firstwoman astronaut, the CEO of a majoraerospace company, professors at Harvard,Princeton, Stanford and elsewhere, and a recentNobel Laureate in Physics.


Gravity is the most fundamental force innature; it affects all of us all the time. But,gravity is still an enigma—we don’t completelyunderstand it. Einstein’s 1916 general theory ofrelativity forever changed our notions of spaceand time, and it gave us a new way to thinkabout gravity. If the Gravity Probe-Bexperiment corroborates the two predictions ofgeneral relativity, then we will have made themost precise measurement of the shape of local

spacetime, and confirmed the mathematics ofgeneral relativity to a new standard ofprecision. If on the other hand, the resultsdisagree with Einstein's theoretical predictions,then we may be faced with the challenge ofconstructing a whole new theory of theuniverse’s structure and the motion of matter.Whatever the result, Gravity Probe B willprovide us another glimpse into the sublimes t r u c t u r e o f o u r u n i v e r s e .

Gravity Probe B Quick FactsSpacecraft

Length 6.43 meters (21 feet)

Diameter 2.64 meters (8.65 feet)

Weight 3,100 kg (3 tons)

Power Total Power: 606 Watts (Spacecraft: 293W, Payload: 313 W)

Batteries (2) 35 Amp Hour

Dewar

Size 2.74 meters (9 Feet) tall, 2.64 meters(8.65 feet) diameter

Contents 2,441 liters (645 gallons) superfluid helium@ 1.8 Kelvin (-271.4°C)

Telescope

Composition Homogeneous fused quartz

Length 35.56 centimeters (14 inches)

Aperture 13.97 centimeter (5.5-inch)

Focal length 3.81 meters (12.5 feet)

Mirror diameter 14.2 centimeters (5.6 inches)

Guide Star HR 8703 (IM Pegasi)

Gyroscopes (4)

Shape Spherical (Sphericity < 40 atomic layersfrom perfect)

Size 3.81 centimeter (1.5-inch) diameter

Composition Homogeneous fused quartz (Purity within2 parts per million)

Coating Niobium (uniform layer 1,270 nanometersthick)

Spin Rate Between 5,000 – 10,000 RPM

Drift Rate Less than 10-11 degrees/hour

LaunchVehicle

Manufacturer &Type

Boeing Delta II, Model 7920-10

Length 38.6 meters (126.2 feet)

Diameter 3 meters (10 feet)

Weight 231,821 kg (511,077 lbs or 255.5 tons)

Stages 2

Fuel 9 strap-on solid rocket motors; keroseneand liquid oxygen in first stage; hydrazineand nitrogen tetroxide in second stage

Mission

Launch Date April 19, 2004

Site Vandenberg Air Force Base, Lompoc, CA

Duration 12-14 months, following 40-60 days ofcheckout and start-up after launch

Orbit

Characteristics Polar orbit at 640 kilometers (400 miles),passing over each pole every 48.75 min.

Semi-major axis 7027.4 km (4,366.8 miles)

Eccentricity 0.0014

Apogee altitude 659.1 km (409.6 miles)

Perigee altitude 639.5 km (397.4 miles

Inclination 90.007°

Perigee Arg. 71.3°

Right Ascensionof asc. node

163.26°

Measurements

Predicted Drift-Geodetic Effect

6,614.4 milliarcseconds or 6.6 arcseconds(1.83x10-3 degrees)

Predicted Drift -Frame-Dragging

40.9 milliarcseconds (1.14x10-5 degrees)

RequiredAccuracy

Better than 0.5 milliarcseconds (1.39x10-7

degrees)

Program

Duration 43 years from original conception; 40years of NASA funding

Cost $700 million dollars


The Space VehicleAll of the Gravity Probe B technologies areintegrated into one of the most elegant andsophisticated satellites ever to be launched intospace. Over four decades in development, theGP-B space vehicle is a marvel of engineeringand truly a beautiful sight to behold.

From it largest to smallest parts, it is filled withthe cutting edge technologies and materialsdescribed in the previous section, many ofwhich were invented specifically for use in theGravity Probe B mission.

Inside the DewarA Dewar is a sophisticated Thermos bottle forholding cryogenic liquids. The Gravity Probe BDewar will be one of the largest and mostsophisticated ever put into space. It is nine-feettall and forms the main structure of the spacevehicle. The vacuum area just inside theDewar’s shell contains multiple reflectivesurfaces that cut down on heat radiation. TheDewar also contains vapor-cooled metal shieldsthat help maintain its internal cryogenictemperature and slosh baffles that helpsuppress tidal motions in the superfluid heliuminside. When cooled to almost absolute zerotemperature, liquid helium transforms into astate called “superfluid,” in which it becomes acompletely uniform thermal conductor. Onlyhelium exhibits this, and other specialproperties of superfluidity.

Inside the neck area of the Dewar is adoughnut-shaped tank called the “guard tank.”Once the Dewar has been conditioned forlaunch—an iterative process of successiveevacuations and fills that transforms the liquid

helium inside the Dewar to the superfluidstate—the guard tank keeps the Dewarsupercooled for weeks, without the need foradditional conditioning, while the space vehicleis being readied for launch.

Science Instrument Assembly—Quartz Block and TelescopeThe Science Instrument Assembly (SIA)contains the quartz block and the telescope.The SIA is located at the center of mass of theDewar, along its main axis. It is mountedinside a vacuum canister called the probe(described in the next section).

The quartz block houses the four gyroscopesand SQUID readout instruments (SuperConduc t ing Quantum Inte r f e renceDevices—the magnetometers that read thegyroscopes’ spin axis orientation). Eachgyroscope is enclosed in a quartz clamshellhousing, mounted in the quartz block andsurrounded by antimagnetic shielding. Thegyroscopes are electrically suspended with only0.001 inch clearance from the housing walls,and they spin at up to 10,000 rpm during thescience phase of the mission.

Optically bonded to the top of the quartz blockis the quartz reflecting Cassegrain astronomicaltelescope, which focuses on the guide star, IMPegasi. Optical bonding is a patented methodof fusing together quartz parts, without the useof any “glue” or fasteners to ensure that the SIAdoes not distort or break when cooled tocryogenic temperatures. The line of sight of thetelescope is rigidly aligned to the SQUIDreadout loop of each gyroscope. As such, thequartz telescope provides the frame of referencefor measuring any drift in the spin axis of thegyroscopes.


The Gravity Probe BSpace Vehicle


The ProbeThe SIA is mounted in a cigar-shaped canister,called the “probe,” which is inserted into theDewar. The probe is an amazing feat ofcryogenic engineering, designed by LockheedMartin Space Systems in Palo Alto, California.It provides both mechanical and structuralstability for the SIA. The probe is designed toprovide a free optical path for the telescope toview distant space through a series of fourwindows, mounted in its upper section. Thesewindows also serve to reduce thermalconductivity into the Dewar.

The inside of the probe is maintained at anextremely high vacuum—much greater thanthe vacuum of space. The probe is surroundedby a superconducting lead bag between it andthe Dewar. The superconducting lead bagprovides an impenetrable shield fromelectromagnetic signals that could disturb thegyroscopes.

Taken together, all of these measures create anultra pristine, cryogenic environment, free ofany external forces or disturbances, in whichthe gyroscopes spin.

At the upper end of the probe, capping off theDewar, is the “top hat.” The top hat serves as athermal interface for connecting over 450plumbing and electrical lines, that run from

various electronics and control systems,mounted on the space vehicle’s truss systemoutside the Dewar, to the cryogenic vacuumchamber inside the probe and Dewar.

Before Gravity Probe B was completelyintegrated, each component went throughyears of testing and construction. Some partseven had to be de-constructed and rebuilt. Theentire probe was assembled in a Class-10 cleanroom, as any particles larger than a singlemicron would disrupt the precise structure.

Outside the DewarOutside the Dewar are all of the systems thatprovide power, navigation, communication,and control of the space vehicle.

Sun ShieldThe sun shield is a long, conical tube that keepsstray light from entering the telescope. Inside


the sun shield are a series of black, metalbaffles that absorb incoming stray light beforeit can reach the telescope. In addition toblocking out stray light from the Sun, the sunshield also blocks stray light from the Earth,Moon, and major planets.

Proportional Micro ThrustersThe proportional micro thrusters on the spacevehicle provide a very means of controlling itsattitude or orientation in space. In the case ofGravity Probe B, an unprecedented amount ofon-orbit control is required for the vehicle tomaintain its drag-free orbit. This isaccomplished by harnessing the helium gasthat continually evaporates from the Dewar’sporous plug and venting it as a propellantthrough eight pairs of opposing or balancedproportional micro thrusters.

These micro thrusters continually meter out aflow of helium gas—at the rate of about 1/100th

the amount of a human “puff” exhalation thatone might use to clean eyeglasses. This meteredflow of helium keeps the space vehicle’s centerof mass balanced around one of the gyroscopes,called a “proof mass”—a predetermined testmass that serves as a reference formeasurement. The thrusters are set up in pairs,so that they counterbalance each other. As longas the same amount of helium is flowing fromtwo opposing thrusters, the space vehicle willnot change its position along that axis.However, if the telescope or the SQUID readoutfor the proof mass gyroscope requires the spacevehicle’s position to change, it is simply amatter of unbalancing the flow, ever so slightly,in the appropriate thruster pair to move thevehicle in the desired direction. Theseproportional micro thrusters also control theroll rate of the GP-B space vehicle.

Solar ArraysSolar arrays convert energy from the Sun intoelectrical power that is stored in the spacevehicle’s two batteries and then used to run thevarious electrical systems on board. Theposition of each solar array can be controlled tomaximize its power output.

GPS Sensors & AntennaeGPS (Global Positioning System) sensorscalculate and transmit information about thespace vehicle’s position. In the case of the

Gravity Probe B space vehicle, the number andplacement of the GPS sensors providepositioning information that is over 100 timesmore accurate than traditional ground-basedGPS navigation systems. For example, a highquality handheld GPS sensor on Earth canlocate your position to within about a meter,whereas the GPS sensors onboard the GP-Bspace vehicle can locate its position to within acentimeter.

Telemetry & CommunicationsAntennaeThese antennae enable both inbound andoutbound communications with the spacevehicle—this includes communications withground stations and with orbitingcommunications satellites in the Tracking DataRelay Satellite System (TDRSS). Telemetry datafrom the space vehicle and science data fromthe experiment are transmitted to groundstations using these communications systems.These systems also enable the GP-B MissionOperations Center (MOC) to send daily batchesof commands to the space vehicle.Communications between the space vehicleand orbiting satellites is limited to a 2K dataformat, whereas communications with groundstations uses a 32K data format, enabling farmore data to be transmitted per unit of time.

Star TrackersA star tracker is basically a camera and patternmatching system that uses constellations andstars to determine the direction in which asatellite is pointing. The Gravity Probe Bsatellite contains two star trackers—one widefield and one narrow field (called the starsensor). The wide field star tracker is used tolocate the general region of the heavenscontaining the guide star, and then the narrowfield star tracker helps align the space vehiclewith the guide star.

The Gravity Probe B on-board telescopebasically performs the same function, but ituses a different technique, and it is orders ofmagnitude more precise and more accurate.The narrow field star tracker has a field of viewon the order of one degree (60 arcminutes), andit can focus to a position within perhaps onearcminute—about the same as the whole fieldof view of GP-B on-board telescope, which canpinpoint the guide star’s position to within amilliarcsecond.


With such a small field of view, it would benearly impossible to locate the guide star usingonly the onboard telescope, so the star trackersfunction like “spotting scopes” for initiallypointing the space vehicle towards the guidestar. Once the narrow field star tracker hasfocused on the guide star, the onboard telescopetakes over the job of maintaining the precisealignment required for measuring gyroscopedrift.

Navigational GyroscopeGP-B uses a standard, flight-qualifiedgyroscope, equivalent to those found on otherspacecraft (and also airplanes, ships, and othervehicles). This gyroscope is not part of therelativity experiment, but rather it is part of thegeneral navigation system used for monitoringthe general direction and position of the spacevehicle.

Electro-mechanical Control SystemsSurrounding the Dewar, is a lattice of trussesthat forms the structure of the space vehicle.Attached to these trusses are a number ofelectrical and mechanical systems that controlthe operation of space vehicle and enable therelativistic measurements to be carried out.These control systems include the following:

• Attitude Control System (ATC)—Controlsall of the proportional micro thrusters thatdetermine and maintain the space vehicle’sprecise positioning.

• Mass Trim Mechanism (MTM)—A systemof movable weights that can be adjustedduring flight to restore rotational balance ofthe space vehicle (similar to spin balancingthe tires on an automobile)

• Gyro Suspension System (GSS)—Theelectronics that levitate and preciselycontrol the suspension of the fourgyroscopes at the heart of the Gravity ProbeB experiment. The GSS control boxes aremounted in the truss work, outside theDewar. The wiring goes through the top hatsection of the probe and down to eachgyroscope.

• Gas Management Assembly (GMA)—Avery complex set of valves, pipes, andtubing, that runs from a triangularassembly on the truss work, through the tophat and down the probe to each of thegyroscopes. The critical job of the GMA is tospin up each of the four gyroscopes byblowing a stream of 99.99999% purehelium gas over them, through a channelbuilt into one half of each gyroscope’squartz housing.

• Experiment Control Unit (ECU)—The ECUcontrols many of the systems onboard thespace vehicle, including the GMA, the UVsystem, and various thermal devices.

The MissionThe GP-B Space Vehicle will be launched fromthe Vandenberg Launch Facility at VandenbergAir Force Base (VAFB) in California. GP-B willbe launched on a Delta II 7920-10 rocket fromlaunch pad SLC-2W into a polar circular orbitof 640 km (400 mile) altitude. The SpaceVehicle is launched with the Command/ControlComputer Assembly, Command and DataHandling subsystem, and Attitude ControlElectronics powered on; command andtelemetry links should be established from theMission Operations Center (MOC) at StanfordUniversity as soon as the fairingseparates—about 5 minutes after launch. TheLaunch Vehicle will release the Space Vehicle inthe desired attitude and at the desired roll rate.Solar arrays are deployed soon after launch.

The Space Vehicle is configured for sciencemission operations during the Initial OrbitCheckout (IOC) phase, which is scheduled forthe first 40-60 days after launch. Manyimportant things happen during IOC.Subsystem boxes are powered on and verified.As soon as the orbit is determined, the Attitudeand Translation Control subsystem (ATC) iscommanded to begin making the necessaryorbit corrections. The low thrust of thepropulsion system requires that corrections bestarted as soon as possible so that the correctiondoesn’t delay the start of the science mission.The attitude is confirmed using the telescope.The science gyros are turned on and SQUIDstested. The drag free operation of the spacecraftis tested and verified, and the spacecraft is


balanced using the mass trim mechanisms.Calibrations are made, and the gyros are spunup. The orbit is finalized, the spacecraftrebalanced, and gyros are then precisely set fornominal science operations. The 60-dayinitialization schedule contains 20 days ofcontingency, and funding allows for anadditional 15 days if required.

Once the spacecraft and payload areconfigured, with the gyros suspended andspinning, and the ATC autonomouslyproviding attitude and drag-free orbit control,the science mission begins. Nominally three tofour Ground Network tracks per day will allowa daily upload to the flight computer to controlthe spacecraft and payload configuration andreturn data. Occasional data base updates maybe made. The space Network TDRSS willprovide two passes a week for orbitdetermination and as required for contingencyoperations.

When it is determined that there is only a 1-2months of helium left to maintain the requiredsuperconducting temperatures, the mission willshift to the post science calibration phase.During this phase, recalibrations will beperformed and intentional disturbances will beintroduced to ensure that the flight data is wellunderstood and valid. The nominal totalmission time is 18 months.

Launch Site & VehicleThe spacecraft will be launched fromVandenberg Air Force Base, CA, on a Delta II7920-10 launch vehicle. Overall, the rocket is38.6 meters (126.5 feet) long, 3 meters (10 feet)in diameter and weighs 231,821 kilograms(511,077 pounds or 255.5 tons). It has ninestrap-on solid rocket motors. The Delta’s firststage burns kerosene and liquid oxygenpropellant, and produces 890,000 newtons(200,000 lbs.) of thrust. The second stage burnshydrazine and nitrogen tetroxide propellant,producing a thrust of 43,400 newtons (9,750pounds).

Launch PeriodThe daily launch period opens on Monday,April 19, 2004. If launch does not take placethen, GP-B can be prepared to launch on twoconsecutive days, but needs each third day forre-servicing of its helium guard tank.

Launch opportunities from April 19-21 will usean inclination of 90.011 degrees; from April 22-30, the inclination would be 90.007. Bothinclinations are almost due South.

Daily Launch TimeThere is one 1-second launch opportunity eachday during the launch period. On April 19, thelaunch time is at 10:01:20 a.m. PDT. (On April20 and later, the launch time is earlier byalmost 4 minutes a day.

Television CoverageThe Delta rocket will carry two video cameras,which will record the separation of the 2nd stageof the launch vehicle from the GP-B SpaceVehicle.

Mission OperationsMission operations will be conducted from theGravity Probe B Mission Operations Center(MOC), located in the Gravity Probe B buildingat Stanford University, Stanford, California.Spacecraft communications will be conductedthrough NASA’s Space Network (TDRSS).

Mission Time LineThis mission time line is divided into four mainphases:

1. Launch and Early Orbit (L&EO) Phase

2. Initialization and Orbit Checkout (IOC)Phase

3. Science Phase

4. Post-science Calibration Phase.


Each of these phases is described in more detailbelow.

Launch and Early Orbit PhaseThis phase covers the period of timeimmediately prior to launch through the firstday of flight operations. The Gravity Probe Bspace vehicle will lift on a Delta II 7920-10rocket from launch pad SLC-2W at VandenbergAFB, CA. Into a polar circular orbit of 640 km(400 mile) altitude.

The Delta’s main engine cutoff will occur atabout 263 seconds. The second stage enginewill ignite about 277 seconds, and cut off forthe first time at about 11 minutes after launch.After coasting, the second stage will restart atlaunch plus about 62 minutes, and will burnfor about 16 seconds. At 75 minutes afterlaunch, the Gravity Probe B Space Vehicle willseparate from the second stage. The secondstage will perform a maneuver to move awayfrom the Space Vehicle to avoid future contactand possible contamination. After separationfrom the second stage, the Space Vehicle willorient itself towards the guide star.

The Mission Operations Center (MOC) atStanford University will be able to commandand monitor telemetry from the space vehicleat VAFB. The baseline plan is for the pre-launchstored program commands and launch time tobe loaded by the computers at the launch site,with telemetry monitoring at the MOC. As soonas the fairing separates, the stored programcommands will power on the space vehicletransmitter, starting a telemetry link through atracking data relay satellite (TDRS) to the MOC.Commanding is also possible at this time, ifrequired.

The nominal timeline below summarizes themain events of the launch/ early orbit phase.

Gravity Probe B — Launch & EarlyOrbit Nominal Timeline

Time fromLaunch

Event

0 Launch on Delta II at VAFB

48 sec Maximum Dynamic Pressure

1 min 26-27sec

First 6 solid rocket booster motorsseparate

2 min 12 secLast 3 solid rocket booster motorsseparate

4 min 24 sec Main Engine Cutoff (MECO)

4 min 31 sec Stage II separates from Stage I

4 min 37 sec Stage II ignition

4 min 41 sec Jettison Fairing

5 min 5 secPower on star sensors; set AttitudeControl (ATC) I/O directives

11 min 16 sec First Cutoff — Stage II (SECO-1)

1 hr 1 min 38sec

Stage II Restart Ignition (orbitcircularization)

1 hr 1 min 54sec Second Cutoff — Stage II (SECO-2)

1 hr 6 min 50sec

Begin solar panel deployment

1 hr 11 min 40sec

End solar panel deployment

1 hr 15 min 0sec

GP-B Spacecraft separation from Stage II

1 hr 15 min 1-42 sec

Stage II Retro

1 hr 16 min 37sec Begin attitude capture

1 hr 17 min 32sec

Enable survival heaters for battery, ECU,SRE, & GSS

1 hr 26 min 43sec

GPS Side A/B power on

1 hr 31 min 45sec Third Cutoff — Stage II (SECO-3)

1 hr 38 min 20sec

Restart 3 — Stage II

1 hr 44 min 19sec

Aft SQUID Readout Electronics (SRE)power on

1 hr 57 min 18sec

Electronic Control Unit A (ECU-A) poweron

5 hr 0 min 6sec

Proton Monitor on

5 hr 9 min 6sec Telescope Detectors on

6 hrs 33 min28 sec

Gyro Suspension System 2 (GSS-2)power on


Initialization and OrbitCheckout (IOC) PhaseThe IOC phase is planned to last between 40and 60 days. Payload activities during thisphase of the mission include calibration andmeasurements of on-orbit performance of thepayload instrumentation and uncaging,levitation, and a slow spin (~1 Hz) of thegyroscopes. Spacecraft activities includedeployment of the solar arrays, acquisition ofthe guide star, and final orbit trim. During thisphase extensive testing of the on-orbitperformance will require frequent commands tothe spacecraft and careful evaluation of thetelemetry. The initialization phase willconclude when the gyroscopes are spun up tofull speed and aligned with the satellite rollaxis and the satellite is commanded to its mainscience phase roll rate. The table belowprovides a nominal timeline of the IOCactivities.

Gravity Probe B — Nominal Timelineof IOC Activities

TimeFromLaunch

Activity

2 hours Dewar vent, Orbit Acquisition (Completion oftask)

1 day Start Orbit trim (Completes many days later)

2 days Power-On and Set-Up Of Payload ElectronicsBoxes

2 days Flux lock SQUIDs

2 days Guard tank runs out of helium

2 days Acquire guide star (Confirmation: 4 days)

3 days Drag Free (Gyro #2 acts as “proof mass”)

8 days Gyro checkout: Low voltage on 4 gyros, Highvoltage gyro 3

10 days Start Dewar pressure control

18 days Mass Trim Sequences (balancing vehicle)Completed

18 days Space Vehicle Spin Up & Helium BubbleWrap

20 days Trapped flux measurement/reduction/re-measurement

22 days 1 Hz spin all gyros (plus High Voltage checkgyros 1, 2, 4)

23 days 5 Hz spin gyros 3, 4

31 days Gyro health check, practice spin alignment &post science calibrations completed

32 days 5 Hz spin gyros 1, 2

36 days High speed spin gyros 4, 3, 1, 2

38 days Spin axis alignment polarity

43 days Spin axis alignment completed

43 days Low Temperature Bakeout

44 days Transition to Science Phase of Mission

Following is a more detailed description of themain IOC activities.

Orbit Trim Maneuver (L+1 Day)Because the launch vehicle does not place thespace vehicle exactly into the orbit planerequired for the science phase of the mission, itwill be corrected during IOC. Ground generatedcommand parameters will be used toautomatically command thruster firings. Theorbit trim process includes review of orbit dataas determined by Doppler and GPS data,calculation of the required parameters for theorbit trim flight software, transmission of theseparameters, and initiation of the automaticorbit trim maneuver. For large (3-sigma)launch errors, the time required to correctplanar errors may be the longest activity in theIOC phase, so an early determination of thelaunch orbit and initiation of its correction willbe important. Nominally, orbit correctionsequences are expected to begin within 24hours after achieving orbit. A one-sigma errorcan be corrected in 10-14 days.

Guide Star Acquisition (L+2 Days)About 24 hours after launch, the flight softwareautomatically begins the process of acquiringthe guide star. No real-time commanding orspecial telemetry requirements are expected tocomplete this activity, although the sequencecan be broken into steps requiring initiationcommands from the ground, if required. Guidestar acquisition consists of determining theinitial attitude using the star sensors,measuring gyro biases, performing a courseattitude maneuver using thrusters, and thenrefining the attitude based on data from theonboard science telescope.

Mass Trim Sequences (CompletedL+18 Days)After launch, the center of mass and productsof inertia of the space vehicle can be correctedby moving some or all of the seven mass trimmechanisms (MTM). The process to operate theMTMs includes analysis on the ground of ATCdata (to see what forces and torques are


required to hold attitude), generation ofcommands, and then execution of special masstrim sequences. The baseline timeline showsseveral MTM events throughout the IOC periodto allow refinement of the mass properties,beginning with a coarse trim on day 6. Finalmass trim cannot be completed until thevehicle is operating in drag-free mode. It isexpected that mass trim refinements will berequired occasionally throughout the missionas helium is depleted or thermal propertieschange.

Space Vehicle Spin Up / HeliumBubble Wrap (Completed L+18Days)The space vehicle is separated from the launchvehicle rolling at 0.1 rpm. During IOC the rollrate is sequentially increased to 1.0 rpm fortwo reasons:

1. Get better insight into adjusting the MassTrim Mechanisms (MTM)

2. Mass balance the space vehicle bywrapping the liquid helium bubbleuniformly around the Dewar chamber.

The space vehicle is then spun down to 0.6 rpmfor the first gyro spin up interval. Just prior tocommencing the science phase of the mission,the final roll rate will be determined and set(the value is expected to be between 0.3 rpmand 1.0 rpm).

Drag Free Operations (Starts L+ 3Days)Drag free operations are practiced beginningon Day 3 of IOC. Towards the end of IOC, thevehicle is commanded to fly drag free for theduration of the science phase of the mission.There is no real-time commanding required forthis process. The Attitude Control Electronics(ACE) uses position data from science gyro #2,which has been designated as a “proof mass,”and commands the thrusters to maneuver thevehicle to maintain this gyro in the center of itshousing.

Gyro Spin Up (Starts L+22 Days)Spinning up the Gravity Probe B gyroscopes is adelicate and complex process that requiresmore than half of the IOC mission phase. Eachof the four science gyros undergoes a series offour spin-up and testing sequences, gradually

increasing its speed to the final spin raterequired for the science phase of the mission.

The first spin up sequence involves using theGyro Suspension System (GSS) to levitate andposition each gyro—one at a time—in itsnominal spin up position. For each gyro, theGas Management Assembly (GMA), whichflows ultra pure helium gas through the spinup channel in one half of the gyro’s housing, isoperated; however no gas is flowed over thegyro during this first spin up sequence.

The second spin up sequence repeats the firstone; however in this case, a small amount ofgas is flowed over each of the gyros—again,one at a time. This results in each gyroreaching a spin rate of approximately 1 Hz (60rpm).

In the third spin up sequence, the GMA flows asomewhat larger amount of helium gas overeach gyro, resulting in a spin rate ofapproximately 5 Hz (300 rpm).

The fourth and final spin up sequence increasesthe gyro’s spin rate to full speed. This requiresthe GMA to flow helium gas over each gyrocontinually, for a period of 1-2 hours. At theconclusion of this sequence, each gyro will bespinning at up to 150 Hz (9,000 RPM).

Spin Axis Alignment (Starts L+38Days)After gyro spin up, the spin axis direction ofeach gyro must be closely aligned to the guidestar. This is accomplished by torquing ortwisting the rotor in a methodical manner,using the Gyro Suspension System (GSS)electronics. There are two phases to thisalignment process—coarse and fine. During thecoarse alignment, as much as one degree ofspin axis orientation correction is achieved. Thefine axis alignment then provides the finalorientation accuracy of better than 10arcseconds relative to the guide star.

Low Temperature Bakeout (L+43Days)Also following gyro spin up, is the lowtemperature bakeout process. The gyro spin upprocess leaves residual helium gas in andaround each of the science gyros. If notremoved, this helium would reduce theaccuracy of the experimental data collected


during the Science Phase of the mission. Thelow temperature bakeout process involvesapplying a very mild heat cycle to the Scienceinstrument Assembly, briefly raising itstemperature from 1.8 Kelvin to approximately7 Kelvin. The net effect of this procedure is toremove nearly all of the residual helium. Acryogenic pump, comprised of sintered titaniumin a sponge-like form, getters the remaininghelium from the gyro housings. After lowtemperature bakeout, the SIA temperature isrestored to 1.8 Kelvin.

Science PhaseAfter the gyroscopes are spun up to full speed,the mission enters a phase where the essentialscience data are collected. During this periodthere will be a minimal number of commandssent to the satellite, and the acquisition andtelemetry of the data will follow a routinepattern. The telemetry during this phase willinclude health and safety data, basic science

data, and additional data necessary for theevaluation of potential systematicexperimental errors This phase is expected tolast between 13 and 15 months.

Post-Science CalibrationPhaseAn essential part of the mission is the post-science calibration phase where the disturbingtorques on the gyroscopes and potentialsystematic measurement errors are deliberatelyenhanced. During this period, commands willbe sent to the satellite more frequently. Theinformation collected during this period will beused to place limits on the systematicexperimental errors. This phase is expected tolast between two and three months.Measurement of the remaining superfluidhelium will be used to time the transition fromthe Science Phase to the Calibration Phase.

The Amazing Technology of GP-BTo test Einstein’s theory of general relativity,Gravity Probe B must measure two minusculeangles with spinning gyroscopes, floating inspace. While the concept of Gravity Probe B isrelatively simple in design, the technologyrequired to build it is some of the mostsophisticated in the world. Scientists fromStanford University, NASA’s Marshall SpaceFlight Center, and Lockheed Martin SpaceSystems have drawn from a diverse array ofphysical sciences, and have invented much ofthe technology that makes the mission possible.In fact, much of the technology did not evenexist when Leonard Schiff (and independently,George Pugh) conceived of the experiment inthe early 1960’s.

The GP-B ScienceInstrumentThe Gravity Probe B science instrument takesthe shape of a long rectangular block with fourgyroscopes lined up behind a telescope thatpeers out the top of the Gravity Probe Bsatellite. Each gyroscope is suspended in aquartz housing, surrounded by a metal loopconnected to a SQUID magnetometer (SuperQuantum Interference Device) to monitor itsspin axis orientation. The fused quartz

gyroscopes sit in a fused quartz block that isbonded to the fused quartz telescope. Thesethree components make up the ScienceInstrument Assembly (SIA).

World’s Most PerfectGyroscopesTo measure the minuscule angles that LeonardSchiff predicted— 6,614.4 milliarcseconds (6.6areseconds) per year and 40.9 milliarcsecondsper year— Gravity Probe B needed to build anear-perfect gyroscope—one whose spin axiswould not drift away from its starting point by


more than one hundred-billionth of a degreeeach hour that it was spinning. This was anespecially stiff challenge, given that the spinaxes of all gyroscopes tend to drift slightlywhile they are spinning. Even the spin axis driftin the most sophisticated Earth-basedgyroscopes, found in missiles and jet airplanes,is seven orders of magnitude greater than GP-Bcould allow.

Three physical characteristics of a gyroscopecan cause its spin axis to drift:

1. An imbalance in mass or densitydistribution inside the gyroscope

2. An uneven, asymmetrical surface on theoutside of the gyroscope causing air friction

3. Friction between the bearings and axle ofthe gyroscope.

This means the GP-B gyroscope has to beperfectly balanced and homogenous inside,cannot have any rough surfaces outside, andmust be free from any bearings or supports.

After years of work and the invention ofnumerous new technologies, the result is ahomogenous 1.5-inch sphere of pure fusedquartz, polished to within a few atomic layersof perfectly smooth. It is the most sphericalobject ever made, topped in sphericity only byneutron stars.

Inside, the gyroscope is solid fused quartz. Itwas carved out of a pure quartz block grown inBrazil and then baked and refined in alaboratory in Germany. Its interior parts are allidentical to within two parts in a million.

On its surface, the gyroscope is less than threeten-millionths of an inch from perfectsphericity. This means that every point on thesurface of the gyroscope is the exact samedistance from the center of the gyroscope towithin 0.0000003 inches.

Here are two ways to imagine how smooth thisis. First, compare the GP-B gyroscope’ssmoothness with another smooth object—acompact disk. CD’s and DVD’s both appear andfeel incredibly smooth. The pits on the compactdisk’s surface, which carry the digitalinformation, are less than 4/100,000ths of aninch deep (one millionth of a meter). However,compared to the GP-B gyroscope, the surface ofa CD is like sandpaper. The bumps and valleyson the surface of the GP-B gyroscope are 100times smaller than those on a CD. Viewed atthe same magnification, one could barely seeany imperfections on the gyroscope’s surface.

Alternatively, imagine a GP-B gyroscopeenlarged to the size of the Earth. On Earth, thetallest mountains, like Mount Everest, are tensof thousands of feet high. Likewise, the deepestocean trenches are tens of thousands of feetdeep. By contrast, if a GP-B gyroscope wereenlarged to the size of the Earth, its tallestmountain or deepest ocean trench would beonly eight feet!

Finally, the gyroscope is freed from anymechanical bearings or supports by levitatingwithin a fused quartz housing. Six electrodesevenly spaced around the interior of thehousing keep the gyroscope floating in thecenter. A brief stream of helium gas spins thegyroscope up to 10,000 rpm. After that, thegyroscope spins in a vacuum, a mere 0.001inches from the housing walls, free from anyinterfering supports.


Spinning SuperconductivityEach Gravity Probe B gyroscope is nearlyperfectly spherical and nearly perfectlyhomogenous. While this ensures that thegyroscope will spin with near-perfect stability,its “near-perfect-ness” creates a dauntingchallenge—GP-B scientists cannot mark thegyroscope to see exactly which direction its spinaxis is pointing.

For GP-B to “see” the shape and motion of localspacetime accurately, the scientists must beable to monitor the spin axis orientation towithin 0.5 milliarcseconds, and spot the polesof the gyroscope to within one-billionth of aninch.

How can one monitor the spin axis orientationof this near-perfect gyroscope without aphysical marker showing where the spin axis ison the gyroscope? The answer lies in a propertyexhibited by some metals , cal led“superconductivity.”

Superconductivity was discovered in 1911 bythe Dutch physicist H. Kammerlingh Onnes. Hefound that at temperatures a few degrees aboveabsolute zero, many metals completely losetheir electrical resistance. An electric currentstarted in a superconductor ring would flowforever, if the ring were permanently kept cold.But, superconductors also have otherinteresting properties. In 1948, the theoreticalphysicist Fritz London predicted that a spinningsuperconductor would develop a magneticmoment, exactly aligned with its instantaneousspin axis. In 1963, three different groups,including a GP-B graduate student,demonstrated the existence of this Londonmoment experimentally.

What is remarkable about this phenomenon(and most fortunate for Gravity Probe B) is thatthe axis of this magnetic field lines up exactlywith the physical axis of the spinning metal.Here was the “marker” Gravity Probe B needed.GP-B scientists coated each quartz gyroscopewith a sliver-thin layer of a superconductingmetal, called niobium (1,270 nanometersthick). When each niobium-coated gyroscoperotates, a small magnetic field surrounds eachgyroscope. By monitoring the axis of themagnetic field, Gravity Probe B knows preciselywhich direction the gyroscope’s spin axis ispointing.

The magnetic field axis is monitored with aspecial device called a SQUID (SuperconductingQuantum Interference Device). The SQUID isconnected to a superconducting loop embeddedwithin the quartz housing. When the gyroscopetilts, the London moment magnetic field tiltswith it, passing through the superconductingloop. The SQUID detects this change inmagnetic field orientation. The SQUID is sosensitive that a field change of 5 x 10-14 gauss(1/10,000,000,000,000th of the Earth’s magneticfield), corresponding to a gyro tilt of 0.1milliarcsecond, is detectable within a few days.

Using the London moment to monitor thegyroscope’s orientation is the one readoutscheme perfect for Gravity Probe B: extremelysensitive, extremely stable, applicable to aperfect sphere, and—most importantly—exertsno reaction force on the gyroscope at all.

Cement Mixer-sizedThermos BottleOne of the greatest technical challenges forGravity Probe B is to keep its science instrumentconstantly supercooled. For the relativityexperiment to operate properly, the scienceinstrument must be kept just above absolutezero, at 1.8 Kelvin (-271.4° Celsius or –456.5°Farenheit) constantly for at least a year.


The science instrument is kept at this supercoldtemperature, by placing it in a special 2,441litre (645-gallon) Dewar, or Thermos, about thesize of the mixing tank on a cement truck, thatis filled with liquid helium in a superfluid state.This nine-foot tall Dewar is the main structureof the GP-B satellite itself. In its 640-kilometer(400-mile) high polar orbit, the GP-B satellite islow enough to be subjected to heat radiatingfrom the Earth’s surface, and it is also subjectedto alternating hot and cold cycles, as it passesfrom intense sunlight into the Earth’s shadowevery ninety-seven minutes. Throughout the lifeof the mission, key portions of the scienceinstrument must be maintained at a constanttemperature to within five millionths of adegree centigrade.

The Dewar’s inner chamber is a vacuum, whichlimits the amount of heat penetrating throughthe outside wall into the inner chambercontaining the science instrument. In addition,it includes several other devices for maintainingthe necessary supercold temperature:

1. Multilayer insulation—multiple reflectivesurfaces in the vacuum space to cut downradiation

2. Vapor-cooled shields—metal barriers,suitably spaced, cooled by the escapinghelium gas

3. Porous Plug—invented at Stanford, andengineered for space at NASA MarshallSpace Flight Center in Huntsville, AL andthe Jet Propulsion Laboratory in Pasadena,CA, this plug allows helium gas toevaporate from the Dewar’s inner chamber,

while retaining the superfluid liquid heliuminside.

Virtually no heat can penetrate from theoutside wall of the Dewar through the vacuumand multilayer insulation inside. However, asmall amount of heat (about as much as isgenerated by the message indicator lamp on acell phone) leaks into the Dewar from twosources:

1. Conduction of heat flowing from the top ofthe Dewar into the liquid helium

2. Radiation leaking down through thetelescope bore into the liquid helium.

The porous plug controls the flow of thisevaporating helium gas, allowing it to escapefrom the Dewar, but retaining the superfluidhelium. The plug is made of a ground upmaterial resembling pumice. The evaporatinghelium gas climbs the sides of the inner tanknear the plug and collects on its surface, whereit evaporates through the pores in the plug,much like sweating in the human body.

The evaporating helium provides its own kindof refrigeration. As the helium gas evaporatesat the surface of the porous plug, it draws heatout of the liquid helium remaining in theDewar, thereby balancing the heat flow intothe Dewar. You can feel this effect on your skinwhen you swab your skin with water. As theliquid evaporates off your skin, it draws heatenergy with it, leaving your skin a tiny bitcooler than before.

The helium gas that escapes through theporous plug is cycled past the shields in theouter layers of the Dewar, cooling them (thusthe name, “vapor-cooled shields”), and then itis vented out into space through eight pairs ofthrusters that are strategically located aroundthe spacecraft.

Based on data from the on-board telescope andthe gyroscope that is used as a proof mass, theflow of the escaping helium gas is carefullymetered through these thrusters in order toprecisely control the spacecraft’s position. Infact, the position of the entire spacecraft isbalanced around the proof mass gyroscope byincreasing or decreasing the flow of heliumthrough opposing thrusters, creating a drag-freeorbit. Thus, the Dewar and liquid helium servetwo critically important functions in themission:


1. Maintaining a supercold temperaturearound the science instrument.

2. Providing a constant stream of gaspropellant for precisely controlling theposition and attitude of the entire satellite.

Drag-Free SatelliteAs the GP-B satellite orbits the Earth with itsfour gyroscopes inside, one may assume thatthe satellite carries the gyroscopes around theEarth in polar orbit. After all, the gyroscopesspin inside four quartz housings that are rigidlyattached inside the spacecraft. Whatever orbitthe spacecraft follows, the gyroscopes will beforced to follow as well.

In fact, one of the gyroscopes inside the satelliteis following an orbit of its own—a near-perfectgravitational free-fall orbit around the Earth.This gyroscope acts as a “proof mass,” showingGP-B the path of a near-perfect orbit. Thisgyroscope spins inside its housing a meremillimeter from the edge. The satellite usessensors inside the housing to keep thespacecraft perfectly oriented around thespinning gyroscope and, therefore, follows the“proof mass” around the Earth in a near-perfectorbit.

For the most part, the satellite stays on courseby following the same free-falling orbit.However, outside the satellite, two factors canalter the satellite’s path. The solar radiationstreaming from the Sun is enough to knock thesatellite askew, and friction from atmosphericgases can slow the satellite down.

GP-B needs extremely sensitive thrusters to re-orient the satellite and keep it on its proper

path. Here’s where the escaping helium gasthat slowly boils off from the liquid heliumcomes in handy. Minute amounts of gas, 1/10th

of a human breath or a few millinewtons offorce, provide just the right amount of thrustnecessary to adjust the satellite’s position. Thethrust force of the escaping helium gas providesplenty of force to keep the GP-B satellite in itsprecise position (within 10 milliarcseconds of aperfect Earth orbit, pointing to within 1milliarcsecond of the exact center of the guidestar).

The first drag-free satellite was the U.S. Navy’sTriad transit navigation satellite launched in1972. Transit satellites enable ships to locatetheir positions on the Earth’s surface byreference to orbit data stored on board thesatellite, but earlier ones had been limited byuncertainties in orbit prediction arising fromatmospheric drag. Triad overcame thisuncertainty by incorporating a drag-freecontroller, the DISturbance CompensationSystem (DISCOS), developed by Stanford Aero-Astro Department as an offshoot of GravityProbe B research. Drag-free technology is nowstandard on transit satellites.

Telescope & Guide StarIn the GP-B science instrument, enclosed withinthe Probe, along the central axis of the Dewar,a 36 centimeter (14 inch) long Cassegrainreflecting astronomical telescope, with a focallength of 3.8 meters (12.5 feet), is opticallybonded to the end of the quartz block thathouses the gyroscopes. Together, the telescopeand the quartz block form the ScienceInstrument Assembly (SIA). Optical bonding isa patented method of fusing together quartzparts, without the use of any “glue” orfasteners. This is necessary for the SIA not todistort or break when cooled to the cryogenictemperatures required for superconductivityused by the gyroscopes. The telescope’s line ofsight provides a frame of reference formeasuring any drift in the gyroscopes’ spin axisover the life of the experiment.

The telescope must be focused on a distantstable reference point—a guide star—and itmust remain fixed on the center of this guidestar within a certain range (+/- 20milliarcseconds) throughout the mission. Theresulting telescope signal is continuouslysubtracted from the gyroscope signal at the 0.1


milliarcsecond level to determine the amount ofspin axis drift in each gyroscope.

Ideally, the telescope would be aligned with adistant quasar (massive bodies, located in themost distant reaches of the universe, which putout powerful radio emissions), because theyappear to be fixed in their position and wouldthus provide an ideal, stable reference point formeasuring gyroscope drift.

However, quasars are too dim for any opticaltelescope this size to track. So, instead, we thetelescope must be focused on a brighter, nearbystar. But, like the Sun, nearby stars moverelative to the other stars in our galaxy, andtheir light diffracts or scatters as it travelsthrough the universe. This situation posed twodifficult challenges to the experiment:

1. Choosing a guide star whose motion can bemapped relative to quasars separately, sothat the Gravity Probe B gyroscopemeasurements can be related to the distantuniverse.

2. Creating a means for the telescope to findand remain focused on the exact center of astar whose light is widely diffracted.

Choosing and Mapping a Guide Starwith VLBI

In order to precisely map the motion of a starrelative to a quasar, it was necessary to find astar that meets all of the following criteria:

• Correct position in the heavens for trackingby the on-board telescope (for example, thesun never gets in the way)

• Shines brightly enough for the on-boardtelescope to track

• Is a sufficiently strong radio source that canbe tracked by radio telescopes on Earth

• Is visually located within a fraction of adegree from the reference quasar

It so happens that stars that are radio sourcesbelong to binary star systems. Because almosthalf the star systems in the universe are binary,it initially seemed that there would be manygood candidates for the guide star. However,out of 1,400 stars that were examined, onlythree matched all four of the necessary criteria.The star that was chosen as the GP-B guide staris named IM Pegasi (HR 8703).

IM Pegasi moves around its binary partner in aspiraling pattern, rather than a linear path.The total motion of IM Pegasi in one year aloneis 100 times larger than the smallest gyroscopespin axis drift measurable with Gravity Probe B.Clearly this motion has to be determined withhigh accuracy for Gravity Probe B to besuccessful.

Because IM Pegasi is also a radio source, itsmotion can be tracked by a sophisticatedsystem of radio telescopes, operating inconjunction with each other. This system iscalled the Very Long Base Interferometry or

IM Pegasi -- HR8703

IM Pegasi Facts

Distance: ~300 light years

Right Ascension: 22.50’34.4”

Declination: 16.34’32”

Magnitude max. - 5.85

Magnitude min. - 5.6


VLBI. Radio telescopes from New Mexico toAustralia to Germany, acting as a single radiotelescope the size of the Earth, focus on IMPegasi and map its movements. The results areimages of IM Pegasi and extremely accuratemeasurements of its motions with respect to areference quasar. With these measurements themotions of the gyroscope spin axes can berelated to the distant universe.

Splitting the Guide Star ImageDiffraction, the light-scattering phenomenonthat produces rings around the moon, spreadsIM Pegasi’s image to a diameter of 1,400milliarcseconds, corresponding to a focusedimage 0.001 inch across. Locating the star’scenter to 0.1 milliarcsecond means finding theimage’s optical center to one ten-millionth ofan inch—a formidable task.

GP-B accomplishes this task by focusing thestarlight in the “lightbox” at the telescope’sfront end, and passing it through a beam-splitter (a half-silvered mirror). The beam-splitter forms two separate images, each ofwhich falls on a roof-prism (a prism shaped likea peaked rooftop). The prism slices the star’simage into two half-disks, which are directed tohit opposite ends of a tiny sensor.

On the sensor, the light signals of each half-disk are converted to electrical signals and thencompared. If the signals are not precisely equal,this means that the roof-prism is not splittingthe image precisely in half. The entire space

vehicle is then adjusted until the signals areequal and the image is split right down themiddle. When this is accomplished in bothsensors for each axis (x- and y-axes), then thetelescope is focused on the exact center of theguide star.

Seven Near ZeroesDesigning an experiment involves a basicchoice: maximize the effect to be measured, orminimize the “noise” that obscures it. For theGravity Probe-B experiments, however, thatchoice was moot because Einstein’s relativisticeffects, that literally “roar” near black holesand neutron stars, barely whisper here onearth. The Gravity Probe-B has to turn thevolume of the extraneous babble down to zeroin order to hear the whisper. It’s like askingeveryone in a football stadium to sit quietly tohear a bird sing.

The noise affecting near-earth relativistic effectscomes from virtually anything that mightdistort the results. From the slightest amount ofheat or pressure, the influence of any magneticfield, any kind of gravitational acceleration, orthe tiniest amount of atmospheric turbulence orsolar radiation, to the smallest imperfections inthe instruments themselves, the tolerances mustbe at or near zero.

Without seeing them up close it’s hard to imagethe precision of the instruments. Gravity ProbeB’s four gyroscopes, housings, and telescopesare all carved out of a single purified quartzblock, grown in Brazil, refined in Germany, anddelivered to GP-B as a trash-can-size crystal.From side to side, the crystalline structure of thequartz is identical and homogenous to withinthree parts per million. The gyroscope spheresare less than 40 atomic layers from a perfectsphere, the most perfectly round objects evermade by man, so they can spin for over a yearwithout any significant drift. To put that inperspective, if the gyroscopes were enlarged tothe size of the Earth, the highest mountain orthe deepest ocean trench would be only 8 feet.The electrical charge at the gyroscopes’ surface,known as the electric dipole moment, arelikewise vanishing small, held to less than fiveparts in ten million.

The isolation of the instruments from theirsurroundings is equally impressive. The entireset of instruments is in a near-perfect vacuum,


surrounded by 645 gallons of liquid heliumcooled to a temperature of 1.8K. The helium’stemperature is held constant, just aboveabsolute zero, by insulating the Thermos jug-like Dewar and releasing any evaporatinghelium through the one-of-a-kind porous plug.

To zero out unwanted magnetic fields, theinstruments were built within a cascadingsuccession of four lead bags within bags. Aftereach new bag was inserted, the outside bag wasremoved, to virtually eliminate any residualmagnetic fields. Additionally, while in orbit, theon-board electronics will monitor the positionof the spinning gyroscopes by sensing the weakmagnetic field produced by each one, andmake the necessary corrections. The net result isthe absence of any magnetic fields, even fromthe earth itself, that might affect the results.

Finally, and perhaps most impressive of all, isthat the GP-B space vehicle will orbit the Earthin a near-perfect circle, almost completely freefrom any acceleration due to gravity. Eightopposing pairs of proportional micro thrustersenable the space vehicle to automaticallycontrol its attitude or orientation with extreme

precision. In addition, the satellite spinscontinuously to average out various disturbingeffects. Floating serenely in orbit, the satelliteexperiences less than one ten-billionth of thegravity felt on the Earth’s surface, and thegyroscopes spin suspended in space touchingnothing solid for their entire lifetimes.

Gravity Probe B promises to be the near-perfectinstrument placed in a near-zero environment.

GP-B Sever Near Zeroes

Temperature: 1.8 Kelvin (-271.4° Celsiusor –456.5 Farenheit)

GravitationalAcceleration

Less than 10-10 g

Magnetic Field Less than 10-6 gauss

Pressure Less than 10-11 torr

MaterialHomogeneity

Less than 3 parts per million

MechanicalSphericity

Less than 3 ten millionths of aninch

ElectricalSphericity

Less than 5 parts per ten million

Gravity Probe B on the Web

Following are the URLs of several Web siteswhere you can find more information aboutGravity Probe B:

http://einstein.stanford.edu

http://www.gravityprobeb.com

http://www.ksc.nasa.gov/elvnew/gpb/vlcc.htm

http://www.nasa.gov/missions/highlights/launc h_update_gpb.html

http://science.nasa.gov/headlines/y2004/26apr _gpbtech.htm?list80268

http://www.spaceflightnow.com/delta/d304/stat us.html

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