Almost 90 years after Einstein postulated his general theory ofrelativity -- our current theory of gravity -- scientists have finallyfinished collecting the data that will put this theory to anexperimental test. For the past 17 months, NASA's Gravity Probe-B(GP-B) satellite has been orbiting the Earth using four ultra-precisegyroscopes, about a million times better than the finest navigationalgyroscopes, to generate the data required for this unprecedented test.As planned, the helium that cooled the experiment and powered itsmicro-thrusters has run out, ending the data-collection and finalinstrument calibration phase of the experiment. All the data—50 weeks'worth—has been downloaded from the spacecraft and relayed to computersin the GP-B Mission Operations Center at Stanford University, whereGP-B scientists have begun the final painstaking task of data analysisand validation. Was Einstein correct? They won't know for another 15months, when the analysis has been completed, but physicists around theworld are eagerly awaiting the results.

"This has been a tremendous mission for all of us," said Stanford'sFrancis Everitt, GP-B's principal investigator. "Gravity Probe Bpresented many challenges along the way and the team rose magnificentlyto every occasion. With all the data now gathered, we are nowproceeding very deliberately over the next 15 months to make sure thateverything is checked and re-checked in as many ways as possible. NASAand Stanford can be proud of what has been achieved so far."

This year, physicists celebrate the 100th anniversary of Einstein's"miraculous year," in which he received his doctorate in physics fromthe University of Zurich and published four seminal papers, includingthe special theory of relativity and a paper on light that garnered himthe Nobel Prize in 1921. But Einstein's crowning achievement came in1916, with his publication of the general theory of relativity, inwhich he expanded the special theory of relativity to include theelusive concept of gravity. With general relativity, Einstein foreverchanged our Newtonian view of gravity as a force, postulating ratherthat space and time are inextricably woven into a four-dimensionalfabric called spacetime, and that gravity is simply the warping andtwisting of the fabric of spacetime by massive celestial bodies. Eventhough it has become one of the cornerstones of modern physics, generalrelativity has remained the least tested of Einstein's theories. Thereason is, as Caltech physicist Kip Thorne once put it: "In the realmof black holes and the universe, the language of general relativity isspoken, and it is spoken loudly. But in our tiny solar system, theeffects of general relativity are but whispers." And so, anymeasurements of the relativistic effects of gravity around Earth mustbe carried out with utmost precision. Over the past 90 years, varioustests of the theory suggest that Einstein was on the right track. But,in most previous tests, the relativity signals had to be extracted froma significant level of background noise. The purpose of GP-B is to testEinstein's theory by carrying out the experiment in a pristine orbitinglaboratory, thereby reducing background noise to insignificant levelsand enabling the probe to examine general relativity in new ways.

Deceptively simple

Launched on April 20, 2004, from Vandenberg Air Force Base on theCalifornia coast, GP-B has been using four spherical gyroscopes tomeasure precisely two extraordinary effects predicted by Einstein'stheory. One is the geodetic effect—the amount by which the Earth warpsthe local spacetime in which it resides. The other effect, calledframe-dragging, is the amount by which the rotating Earth drags localspacetime around with it.

How does GP-B measure these effects? Conceptually, the experiment issimple: Place a gyroscope and a telescope in a satellite orbiting theEarth. (GP-B uses four gyroscopes for redundancy.) At the start of theexperiment, align both the telescope and the spin axis of the gyroscopewith a distant reference point—a guide star. Keep the telescope alignedwith the guide star for a year as the spacecraft orbits the Earth morethan 5,000 times. According to Einstein's theory, over the course of ayear, the geodetic warping of Earth's local spacetime should cause thespin axis of the gyroscope to drift away from its initial guide staralignment by a minuscule angle of 6.6 arcseconds (0.0018 degrees).Likewise, the twisting of Earth's local spacetime should cause the spinaxis to drift in a perpendicular direction by an even smaller angle of0.041 arcseconds (0.000011 degrees), about the width of a human hairviewed from 10 miles away.

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As the late Stanford physicist and GP-B co-founder William Fairbankonce put it: "No mission could be simpler than Gravity Probe B. It'sjust a star, a telescope and a spinning sphere." However, it took theexceptional collaboration of Stanford, NASA, Lockheed Martin and a hostof other physicists, engineers and space scientists almost 44 years todevelop the ultra-precise gyroscopes and the other cutting-edgetechnology necessary to carry out this deceptively "simple" experiment.The ping-pong-ball-sized gyroscope rotors, for example, had to be soperfectly spherical and homogeneous that it took more than 10 years anda whole new set of manufacturing techniques to produce them. They'renow listed in the Guinness Database of Records as the world's roundestobjects. Similarly, it took two years to make the flawless roof prismsin the GP-B science telescope that tracks the guide star. Somescientists have mused about how Einstein, himself once a patent clerk,would have enjoyed reviewing these extraordinary technologies.

Stanford's Bradford Parkinson, GP-B's co-principal investigator andwinner of the 2003 Draper Prize in Engineering, said: "Optimism wasrampant [in 1960, when GP-B began]. We didn't have any idea how hardthis was, and I would contend it was probably not until 30 years laterthat we brought [into existence] the technology to make perfectspheres, the coating technology, the readout technology, the cryogenictechnology, the [telescope] pointing technology. Â… None of this waspossible in 1960."

Running on empty

At launch, the Dewar, a giant Thermos bottle that comprises most ofthe body of the spacecraft, contained approximately 650 gallons ofhelium, cooled to a superfluid state just above absolute zero. Thehelium in the Dewar served two vital functions: First, it was thesuperfluid bath that kept the four gyroscopes at a superconductivetemperature, required for the readout of their spin axes. Second,helium gas that constantly evaporated from the bath was reused as thepropellant for the spacecraft's micro-thrusters to maintain both itsproper orientation and roll rate in orbit and to keep it pointed at theguide star. When designing the Dewar, the team carefully calculatedthat 650 gallons of helium would be adequate to sustain the GP-Bmission for at least 16 months, and that a Dewar large enough to holdthat amount would just barely fit in the nose of the Boeing Delta IIrocket that would launch the experiment. When the helium in the Dewarwas depleted on Sept. 29, it had outlived the team's initialcalculations by more than three weeks.

Mac Keiser, GP-B chief scientist who heads the data analysis team atStanford, said: "Getting 50 weeks of data from the satellite has beenparticularly important—not only because it will allow us to reduce ourstatistical errors but also because the Earth has made almost acomplete revolution around the sun. This complete cycle will allow usto take full advantage of one of our calibrating signals and eliminatepotential sources of systematic error."

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Next-to-last milestone

The completion of data collection marks the last milestone prior toannouncing and publishing the results of this historic 44-year program.It is a time of both triumph and emotion for the GP-B team. Some teammembers have been working together on the program for more than 15years. As the focus of the mission shifts from spacecraft operations todata analysis, it is time for many of the team's engineers and missionoperations specialists to move on, and this naturally brings a note ofsadness into the otherwise joyful spirit of accomplishment.

"It's a bit like sending your kid off to college," said GP-B ProgramManager Gaylord Green. "Our operations team became a familyaccomplishing this mission, and after a good job the members will bedeparting to the next phase of their lives."

Added Tony Lyons, NASA's GP-B program manager from Marshall SpaceFlight Center in Huntsville, Ala.: "The completion of the GP-B missionis the culmination of years of hard work, training and preparation bythe GP-B team. Every team member should feel proud of thisaccomplishment."

It will take the GP-B science team more than a year to complete thedata analysis, followed by up to six months of preparing and submittingpapers to major scientific journals detailing the experimental results.Following NASA protocols used for other missions with precisequantitative measurements, there will be no preliminary announcementsof results nor any speculation about the data before a formalannouncement and publication of results, expected early in 2007.