Image: Gravity Probe B
Russ Underwood  /  Lockheed Martin
Gravity Probe B sits in a processing room at Vandenberg Air Force Base in California. The spacecraft's launch was postponed in November due to a technical snag.
By Senior Space Writer
updated 12/31/2003 3:26:28 PM ET 2003-12-31T20:26:28

Decades in the making, a space probe that delves into Albert Einstein's general theory of gravitation remains grounded on Earth, caught in its own space-time continuum of technical snags, costly delays and threats of cancellation. Scientists remain confident, however, that Gravity Probe B will provide unmatched measurements that can help reveal details about the nature of matter and the structure of the universe.

Gravity Probe B is an experiment developed by NASA and Stanford University to precisely test two predictions of Albert Einstein's general theory of relativity, measuring how space and time are warped by Earth's presence and how Earth's rotation drags space-time around with it.

The quest has not been easy.

Touted as one of the most unusual experiments ever attempted in the history of science, scientists and engineers confronted challenges in gyroscope technology, materials science, the handling of ultra-cold fluids and how best to appraise subtle gravitational effects.

With its liftoff already put on hold in the past, Gravity Probe B's launch date was again rescheduled this month due to a spacecraft hardware problem. The pioneering satellite is now targeted for takeoff in April 2004.

Meanwhile, Dr. Einstein will have to wait a little longer to revel in any "I told you so" scientific findings from Gravity Probe B.

Here’s the spin
When Gravity Probe B heads for space, it will ride atop a Delta 2 booster rocketing skyward from Vandenberg Air Force Base in California. The spacecraft is to reside in a 400-mile-high (644-kilometer-high) orbit that takes it directly over Earth's poles. The mission is expected to last some 16 months.

The experiment is to exactly measure minuscule changes in the direction of spin of four gyroscopes contained in the satellite. There are two key observations that scientists want to evaluate with Gravity Probe B:

  • Frame Dragging: A rotating massive body drags space and time around with it. A gyroscope orbiting Earth tends to tilt away from the plane of its orbit because the Earth is dragging it. GP-B is designed to sense the frame-dragging effect and will measure it to 1 percent precision.
  • Geodetic Effect: According to Einstein's General Theory of Relativity, space and time in the vicinity of a massive body is distorted. For a gyroscope orbiting near Earth, this distortion leads to a tilting of the gyroscope's spin axis in the plane of the orbit. This effect is predicted by general relativity theory to be 150 times larger than the frame dragging. Gravity Probe B will measure this effect to one part in 10,000.

The intention is that the spacecraft gyroscopes will be so free from interference that they will provide a nearly perfect system for measuring Earth's effects on them.

Einstein's theory calls for frame-dragging and geodetic effects that would gradually change the spin direction of a gyroscope orbiting Earth. Frame-dragging is expected to change the gyroscope's direction of spin by 42 milliarc-seconds in one year. In comparison, the geodetic effect is expected to cause a much larger change in spin direction -- 6,600 milliarc-seconds per year.

Extreme physics
To say that building Gravity Probe B to attain its scientific goals turned into a formidable challenge is an understatement. It all takes on the look of extreme physics merged with a bizarre kludge of space plumbing.

Here are a few bare-bone basics:

Stanford University, which originated the experiment, developed the science instrument assembly — a block of fused quartz holding four spinning ping-pong-ball-sized gyroscopes and a proof mass, all bonded to a quartz telescope. That telescope is to be trained on IM Pegasi — a steady guide star, at least steady enough to measure the minute effects of local space-time on spacecraft gyroscopes.

The science instrument package is placed within a giant thermos bottle built for space. Called a dewar, this vessel holds several hundred gallons of supercooled liquid helium. Because the precision-machined gyroscopes are constructed from quartz and coated with niobium, they become a superconductor at liquid helium temperatures. This permits the gyroscopes to be suspended electrically. Very sensitive magnetometers detect any changes in the gyroscope's spin axis, as they twirl at 10,000 revolutions a minute.

In addition to building the spacecraft that will carry the experiment, Lockheed Martin Space Systems developed the vacuum tube that houses the science instrument assembly, supporting electronics and the dewar that contains the delicate experimental apparatus at a temperature near absolute zero.

The Gravity Probe B project is managed for NASA by the space agency's Marshall Space Flight Center in Huntsville, Ala.

Scientific surprises
From a thought experiment to actual spacecraft hardware, the history of Gravity Probe B is lengthy, having its origins in the late 1950s, said Michael Salamon, Gravity Probe B Program Scientist at NASA Headquarters in Washington.

"The idea has been cooking for four decades now," Salamon told, noting that the present-day incarnation of Gravity Probe B was settled on in 1995, with a projected launch date set for five years later.

Salamon said that the Gravity Probe B experiment provides important insight into the frame-dragging effect that will be "extraordinarily precise" regarding this fundamental component of Einstein's theory of relativity.

As is the case for studying geodetic precession, Gravity Probe B measurements are so accurate that it might enable scientists for the first time to see deviations from Einstein's theory, Salamon added.

"We love surprises," Salamon said. "Scientific surprises are what we're here for."

Cautionary flags
"Gravity Probe B is one of the few space missions NASA has conducted with relevance to fundamental physics," states a 1995 "accelerated scientific review" of the mission undertaken by the Space Studies Board of the National Research Council. "If successful, it would assuredly join the ranks of the classical experiments of physics. By the same token, a confirmed result in disagreement with GR [General Relativity] would be revolutionary."

Nevertheless, the review also underscored the difficulty of carrying out ground tests of some critical systems at the necessary levels of accuracy, so much so that "significant risks" are introduced.

Several members of the review at the time voiced a minority skepticism about the large extrapolations required from ground testing to GP-B's performance in space. Furthermore, this same minority thought it likely that some "as yet unknown disturbance" may prevent the spacecraft from performing as required.

Lastly, the NRC task group waved one other cautionary flag.

In the event that Gravity Probe B does yield results different from those predicted by general relativity, "the scientific world would almost certainly not be prepared to accept them until confirmed by a repeat mission using GP-B backup hardware, or by a new mission using different technology."

Current problem
While scientific zingers are one thing, Gravity Probe B has also had its share of engineering surprises.

Rick Howard, Associate Director for NASA's Astronomy and Physics Division, said the project encountered significant technical woes in 1999. Those issues forced a complete relook at the entire system and the amount of testing needed, he said.

With fixes and extensive "good to go" ground checks behind them, Gravity Probe B moved steadily ahead over the last few years toward ready-to-launch status. After a booster glitch was corrected, the spacecraft was headed for a Dec. 6 liftoff this year.

However, results from an earlier test of onboard electronics came back to haunt the project. Following a couple of months of analyzing data, it was determined that an Experiment Control Unit aboard Gravity Probe B needed to be reworked.

"The problem has been isolated to one power converter," Howard told "This is a very subtle problem," he said, with the component producing unwanted signal noise that affects electrical wiring and other Gravity Probe B electronic devices.

The electronics problem with this unit is such that it mimics exactly the physics signal that the spacecraft is trying to measure.

"It was a very difficult problem to find," Howard said, adding that the trouble could have cropped up on any mission. "We have a very straightforward fix … and we've got to fix it. We can't go into orbit with a situation where we have this kind of potential problem."

"This current problem could happen to any program. It's not something that is a fatal flaw for the entire GP-B. We know the problem. We know the fix. It's just a matter of time it'll take to make that fix," Howard said.

Growing price tag
Gravity Probe B's launch is now set for April 20. The delay will add more dollars to the roughly $700 million project price tag — today's tally that includes cost of the launch vehicle and Gravity Probe B mission operations once in space.

Howard said that about $4 million a month is being spent on Gravity Probe B.

The funds to keep the project moving forward will not be easy to come by. "That money will have to come out of other programs in the [NASA] Office of Space Science. There is no reserve pot sitting here that we can dig into," Howard said.

There is some fear in the Gravity Probe B community that more delay or other technical snags could jeopardize the prospect that the satellite will ever get into space. Top NASA managers have repeatedly shown their dissatisfaction with the growing-in-price spacecraft mission at a time when other space projects were crying for financial help.

"Rather than either launching as is or thinking about terminating it … fix this problem and launch GP-B so it has the best chance of success for operating and carrying out its science objectives," Howard said. "It is the smart thing to do."

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