Gravity Probe B Drags On

By Davide Castelvecchi

Gravity Probe B, the NASA mission that took 40 years to design and that was to be ready by the turn of the millennium, is facing yet another delay after pre-launch tests at Vandenberg Air Force Base found faulty on-board electronics, just two weeks before the scheduled Dec. 6 liftoff.

GP-B is a spacecraft conceived at Stanford University to fill in a gap in the evidence supporting general relativity, Albert Einstein’s greatest achievement.

Once it is in orbit 400 miles above Earth, the main goal of GP-B will be to measure a subtle effect predicted by Einstein’s theory, with a precision that has been compared to seeing the width of a human hair from 10 miles away. The effect, called frame dragging, is a slight distortion that the Earth’s rotation creates in the space around it, and has never before been directly observed.

After $250 million in budget overruns, this latest setback will cost an estimated $40 million, bringing the total tag close to $700 million.

“There was a design error that was quite subtle that wasn’t discovered until we were into our final testing,” said Stanford physics professor Francis Everitt, who is the project’s principal investigator. Everitt said that a component, designed by Lockheed Martin engineers working for the Stanford-run project, was not properly grounded – meaning not connected to the main body of the spacecraft. “Since Stanford is the prime contractor, we have to accept responsibility if our subcontractor Lockheed made an error,” Everitt said.

Clifford Will, a theoretical physicist at Washington University in St. Louis, Mo., who sat in two GP-B independent review panels, said that in spite of the setbacks there is a general consensus that the basic approach of the mission is sound. “I don’t think that GP-B has been plagued with more problems than any other NASA projects,” he said in a phone interview from Paris last week.

But no NASA program except the Space Shuttle has had a such a troubled history.

It was only last March that the spacecraft flunked a so-called thermal vacuum test, designed to simulate the sudden, harsh temperature changes that happen in space when a satellite goes in and out of the Earth’s shadow. NASA space science administrator Ed Weiler ordered an independent review of the project – the seventh since it began in 1962 – asking “if the time has come to put an end to GP-B,” Science magazine reported.

Weiler has pointed out that GP-B’s cost overruns threaten to hold up other science missions, including the Laser Interferometry Space Antenna, another ambitious relativity experiment now in its design phase.

Others have complained about the amount of money that has gone into what is essentially a single experiment, whose outcome – confirming Einstein – is taken for granted by much of the physics community.

The scientific review panel appointed by NASA in March, which included Will, recommended to go ahead but found “some erosion” in the scientific value of the mission. The panel cited indirect evidence of frame dragging coming from recent research.

GP-B’s experiment, however, would provide a rare direct experimental test for Einstein’s theory, in contrast with the astronomical observations that relativists have traditionally relied on. And the extent to which the project has withstood external reviews, supporters argue, is an attestation of its technical prowess and its sheer audacity.

When Stanford physicists William Fairbank, Robert Cannon and Leonard Schiff first came up with the idea for GP-B in 1960, the technology just wasn’t there to implement it: “It was beyond the cutting edge of what could have been done when the project started,” said Will.

Beyond the cutting edge

General relativity says that matter and energy deform space and time, so that the apples we see falling on Earth – and the satellites we see circling around it – are in reality following paths of least resistance in a curved universe.

Since the inception of general relativity in 1915, the evidence found in its favor has made it a pretty solid foundation stone for physics, replacing Isaac Newton’s classical view of the world.

The effects of Einstein’s gravity, however, are only imperceptibly different from those of Newton’s, unless one happens to be flying by a star much more massive than our sun. For phenomena such as frame dragging, land-based lab experiments are unable to distinguish between the two theories.

The concept that became the basis for GP-B was that frame dragging should be detectable using a gyroscope, a simple device that allows a spinning object at its core to rotate unperturbed.

Fairbank, Cannon and Schiff figured that if a gyroscope were to be left to orbit around Earth, frame dragging should slowly change its axis of rotation – something that would not happen in a Newtonian universe. The deviation, though, was expected to be so small that the gyroscope’s axis would take more than a million years to turn around in a full circle. Observing changes over a reasonable time scale – say, a year or two – would require exceptionally sensitive instruments and exceptionally effective shielding from disturbances.

At the heart of GP-B are four ping-pong ball-size spheres made of fused quartz – four gyroscopes – and a high-precision telescope. The spheres are the most perfectly round ones ever built and are kept in a housing with less than a thousandth of an inch of clearance around them, in a near-perfect vacuum.

Once in orbit, the gyroscopes will float in zero gravity, meaning that they should follow a near-perfect orbit. Cosmic rays, solar wind, impacts with micrometeorites and the friction of the atmosphere – which at 400 miles of altitude is rarefied, but not negligible – must be taken into account. The action of precise thrusters will compensate for any external forces that threaten to perturb the probe’s trajectory, so that no appreciable disturbances can reach the gyroscopes.

The telescope is for reference. Throughout the 18-month-long mission, it will keep the probe pointing at a star known as IM Pegasus, in order to measure the gyroscopes’ deviations.

The construction of the telescope itself was one of the toughest challenges. GP-B will not just point at IM Pegasus. In the telescope’s eye, the star’s image will be a thousandth of an inch wide, and GP-B will need to point at its center with a precision of one part in ten thousand.

On days 32 and 33 of the mission, the gyroscopes will be set into rotation, one by one. They will be spun slowly at first, then faster and faster, until they reach 10,000 rotations per minute. That will be the moment of truth, revealing if any damage resulted from the violent vibrations of launch.

“If you look through all the risks of setting this mission up and said, ‘What is the riskiest thing?’ The riskiest thing is spinning the gyroscopes,” Everitt said. Once in motion, they are expected to keep spinning for the duration of the mission.

Since he left Britain to join Stanford in 1962, Everitt has dedicated most of his professional life to designing, directing and fighting for the GP-B project. He has often gone as far as lobbying individual members of Congress to keep his brainchild alive.

Virtually anything imaginable that could go wrong during the mission has been considered in the design phases of GP-B, which is one of the reasons – along with Everitt’s lobbying skills – why the program has survived against all odds.

Still, Everitt wants to avoid complacency. “One of the things we will probably find when we get up into orbit,” he said, “is that all of the things that really happen are the things we hadn’t thought about.”

Note: This was one of two final-exam papers I wrote for the newspaper news writing class class I took in the autumn 2003, as part of the UCSC Science Communication program.

Update: After extensive testing, NASA went ahead and successfully launched GP-B on April 20, 2004 (see my article A 1960’s Dream Comes True, From June 4, 2004.) GP-B has now completed the science phase of its mission, and Everitt’s team is analyzing its data. Publication of results is expected in early 2007.

 

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