In January 1961, Fairbank and Schiff kicked off the experiment officially with a proposal to NASA for an orbiting gyroscope experiment. Fairbank also recruited a young low-temperature physicist named C. W. Francis Everitt to join the Stanford project. Born in England in 1934, Everitt had received a Ph.D. from Imperial College in London in 1959, followed by a post-doctoral stint at the University of Pennsylvania, working on liquid helium. In late 1963, NASA began funding the initial research and development work at Stanford to identify the new technologies that would be needed to make such a difficult measurement possible. In 1971, NASA selected its Marshall Space Flight Center in Huntsville Alabama as program manager, both for the Stanford experiment and for the rocket redshift experiment that was being developed by Robert Vessot at Harvard. NASA headquarters designated Vessot’s experiment as Gravity Probe-A, planned for a 1976 launch, to be followed soon thereafter by the gyroscope experiment, designated Gravity Probe-B or GP-B. While the redshift experiment went off as planned (see Chapter 2), things turned out rather differently for GP-B. It is not known if serious plans were ever made for a Gravity Probe-C, Gravity Probe-D, and so on.
Stanford’s modest research and development effort lasted until about 1981, when Everitt became Principal Investigator of the project, and soon it moved toward the mission design phase. At that time, plans called for a preliminary flight on board the Space Shuttle to test key technologies to be used in GP-B, followed by a launch of the full spacecraft from the Shuttle a few years later. Unfortunately the 1986 Space Shuttle Challenger catastrophe forced a cancelation of the technology test, and a complete redesign of the spacecraft for a launch from a Delta rocket.
The goal of the experiment was to measure both the geodetic effect and the frame dragging effect to an accuracy of better than a milliarcsecond per year. Because the smaller frame dragging effect is only about 40 milliarcseconds per year for the orbit being planned (a polar orbit at an altitude of about 642 kilometers), this meant that a one or two percent measurement of this effect would be possible. The task of building an orbiting gyroscope laboratory that could measure such tiny effects put the Stanford scientists at or beyond the frontiers of experimental physics and precision fabrication techniques, presenting them with apparently insuperable problems. Miraculously, they managed to overcome each one.
A brief description of the experiment will illustrate the things that had to be done. The gyroscopes (for redundancy, there were four) were spherical rotors of fused silica, about 4 centimeters in diameter, housed in a chamber (see Figure 4.5). The rotors had to be uniform in density and perfectly spherical in shape to better than one part in a million. This is like imagining an Earth where the tallest mountain and deepest valley reach 1 meter! The reason for this requirement is that gravitational forces from the Earth and Moon and from the spacecraft itself would interact with any mass irregularities in the rotor and cause spurious precessions. Similar effects caused by gravitational forces from the Sun and Moon acting on the Earth’s equatorial bulge make the Earth’s rotation axis precess with a period of about 26,000 years, causing the North Star to appear to wander from true north. Overcoming the problems of making a perfect sphere and then testing how spherical it is to the above precision required the invention of completely new fabrication and testing procedures.
Figure 4.5 GP-B rotor and its enclosing chamber. In space, the rotor is levitated inside the spherical chamber. Six electrodes spaced around the wall of the chamber sense if the rotor gets too close and send signals to the spacecraft to nudge it in the proper direction. The “pick-up loop” is a superconducting wire embedded in the wall to measure any changes in direction of the magnetic field of the rotor caused by precession of its spin. The “spin-up channel” lets helium gas from the surrounding dewar of liquid helium flow past the rotor, spinning it up to around 4,000 revolutions per minute, after which the gas is vented to space. Credit: NASA and Stanford University.
The main reason for going into space was to avoid having to support the gyroscopes against the force of gravity, because those support forces can generate spurious precessions. Unfortunately, while the gyroscopes move in complete free fall inside the spacecraft, the spacecraft itself is being pushed around by the residual atmosphere of the Earth, by the solar wind and by periodic attitude control forces required to orient the spacecraft. How do you avoid having the gyroscopes collide with the walls of the spherical chambers inside which they are spinning? The answer is a technique called “drag-free control.” Six circular electrodes are installed in the walls of the spherical chambers in which the rotors reside, so that if a rotor gets too close to an electrode a signal is sent to thrusters that nudge the spacecraft a little bit to keep the separation at a pre-selected value. This was a delicate and critical technology, because the average gap between each rotor and the wall of its spherical chamber was about one-thirtieth of a millimeter. We will return to drag-free control in Chapter 9, when we discuss LISA, the planned gravitational wave detector in space.
If a rotor is perfectly spherical, how do you determine the direction of its spin? You can’t just attach a stick to the rotor at one of the poles, because the stray gravitational forces acting on the mass of the stick would cause enormous precessions that would swamp the relativity effects. The solution was to coat each rotor with a thin, perfectly uniform layer of the element niobium. When the ball is spinning at low temperatures, near absolute zero, the niobium becomes a superconductor, its electrical resistance vanishes, and it develops a magnetic field whose north and south poles are exactly aligned with the rotation axis of the rotor. A tiny superconducting wire, called a “pick-up loop,” is embedded in the wall of the chamber that houses the rotor. If the axis of the rotor changes direction, the change in the magnetic field induces currents in the pick-up loop that are measured by very precise devices known as superconducting quantum interference devices, or SQUIDs, also operating at near absolute zero. This required new techniques for working near absolute zero using liquid helium, and adapting those techniques to a space environment. The spacecraft itself contained a very special “thermos bottle” or dewar to hold the 2,400 liters (over 600 gallons) of liquid helium and to maintain it at 1.8 degrees above absolute zero.
If the balls are perfectly spherical, how were they to be set spinning? The solution to this problem was to incorporate into the wall of the chamber that housed each rotor a small “spin-up channel” that forces helium gas past the sphere, using friction to get it spinning. The helium gas came from natural “boil off” from the liquid helium used to cool the apparatus (no thermos can keep liquid helium cold enough to not boil some of it). At the start of the space flight the four rotors were spun up to around four thousand revolutions per minute, after which venting holes in the housing chamber allowed the helium gas to escape to the vacuum of space.
As we described previously, the gyroscopes precess relative to the distant stars, so a very accurate telescope had to be designed and built into the spacecraft package to determine a reference direction accurate to the milliarcsecond level per year. The spacecraft was controlled so that the telescope always pointed toward a selected star, called IM Pegasi. This star lies at a distance of about 300 light years from Earth, about 17 degrees north of the equator. In addition to being optically bright and relatively isolated in the sky, it was also bright in the radio band. This was important because, being in the environment of the Milky Way, it moves, and thus its own motion relative to truly distant objects, i.e. the quasars, needed to be measured to the required precision using VLBI (see Chapter 3).
While simple to state in words, each of these problems was a major multi-year research and fabrication project, and integrating all the components into a functioning spacecraft was a major challenge. The cancelation of the Shuttle test mission in 1986 and the spacecraft redesign resulted in delays and cost overruns for the GP-B program. Similar delays and budget problems with the Hubble Space Telescope, combined with the discovery of its flawed main mirror following i
ts launch in 1990, caused considerable anxiety at NASA about budgets, and worries among astronomers and space scientists about funding for their own projects. One result was rising criticism of the GP-B program and calls for its cancelation. In fact, on more than one occasion NASA and the Office of Management and Budget, the fiscal oversight arm of the US Administration, would set the GP-B budget to zero for the subsequent fiscal year, effectively canceling the project, only to find that members of Congress would restore the budget following judicious lobbying by Francis Everitt and other supporters of GP-B.
In 1992, Daniel Goldin was appointed NASA Administrator by President George H. W. Bush, and he was determined to end the bickering over GP-B. He asked the National Academy of Sciences to conduct a thorough review of the project, promising to abide by their recommendation, whether it be thumbs up or thumbs down.1 In addition to investigating the technical challenges that remained to be overcome and estimating the remaining cost of the mission, the panel debated whether the scientific return of the mission was worth it. This debate was not trivial.
When GP-B was first conceived in the early 1960s, tests of general relativity were few and far between, and most were of limited precision. But by 1994 there had been enormous progress in experimental gravity in the solar system and in binary pulsars, as we have described in this book. Some panel members argued that the many experiments had so constrained the theoretical possibilities in favor of general relativity that GP-B would not give any improvement or new information. The counter-argument was that all the prior experiments involved phenomena entirely different from the precession of a gyroscope, and therefore that GP-B was testing something potentially new. Another issue was that, if GP-B were to give a result in disagreement with general relativity, it would very likely not be believed, and given the high cost of the experiment, the probability of repeating it was extremely small. In the end, while the panel was not unanimous, a majority did recommend going ahead with GP-B, and Goldin committed NASA to the project. NASA then engaged the aerospace company Lockheed Martin in Palo Alto to build, integrate and test the spacecraft in collaboration with Stanford and Marshall Space Flight Center.
The satellite finally was launched on 20 April 2004, and injected into an almost perfectly circular polar orbit at an altitude of 642 kilometers above the Earth’s surface, precisely as planned. Almost every aspect of the spacecraft, its subsystems and the science instrumentation performed extremely well, some far better than expected. The plan of the mission was to begin with a three-month period of testing, calibration and fine-tuning. This would be followed by 12 months of science data taking, and a final month of additional calibrations. Early on in the science phase, the data showed clearly the larger geodetic precession of all four gyroscopes, giving initial hope that all would go well. That hope was soon dashed when a nasty source of error reared its head. Each rotor appeared to be experiencing strange precessions of its spin axis, with no apparent pattern or commonality among them. As the sixteen-month mission period drew to a close in the fall of 2005, there was serious concern that the experiment would fail to detect the tiny frame dragging precession, the main goal of the mission.
What ensued during the data analysis phase following the mission was worthy of a detective novel. The critical clue came from the calibration tests carried out at the end of the mission. The four rotors had been set spinning with their spin axes initially parallel to the axis of the telescope directed to the guide star. The spacecraft also rolled slowly about this axis about once every 78 seconds. For one of the post-science tests, they deliberately forced the spacecraft to point away from the guide star by as much as 7 degrees. This was such an extreme maneuver that you would never try it at the start of the mission, because if anything went wrong the game would be over. After the data has been taken and safely stored away, it is worth the risk. As it happened, the rotors experienced unexpectedly large precessions of their spins during the maneuver, but there was a very specific pattern to the effects. Unraveling this pattern helped the GP-B team to determine that the extra precessions were being caused by interactions between random patches of electric potential fixed to the niobium surface of each rotor, and similar patches on the inner surface of its spherical chamber. Such patches were known to occur on superconducting niobium films, but pre-flight tests of the rotors had shown that the patches would be too weak to cause a problem. For some unknown reason, the rotors in the spacecraft developed stronger patches. This insight allowed the researchers to build a mathematical model for the “patch effects” and thereby to subtract the anomalous precessions from the data on each rotor. When this was done, all four rotors showed the same precession behavior, clearly revealing both the larger geodetic effect and the smaller frame dragging effect. The original goal of GP-B was to measure the frame dragging precession to about 1 percent, but the problems discovered over the course of the mission dashed the initial optimism that this was possible. Everitt and his team had to pay the price of the increase in measurement uncertainty that came from using a complex model to remove the anomalous precessions. The experiment uncertainty quoted in the final result was roughly 20 percent for the frame dragging effect, but the result agreed with general relativity.
This data analysis effort took five years. When the long-awaited results were finally announced at a NASA press conference on 4 May 2011, the feeling of many could be summed up by the opening line of the song by the great blues singer Etta James: “At laaaassst, my love has come along …” The half-century adventure started by three naked professors was over.
The story of Gravity Probe-B has all the ingredients of a case study in science politics, raising many thorny questions about how science, especially “big” science, is carried out. How do we balance the value of a scientific return against the cost of a project? What is the best way to make critical decisions, particularly concerning cancelation of projects? How do we weigh the value of different kinds of science, say fundamental physics versus astronomical discovery, in setting priorities or deciding among competing proposals or competing scientific constituencies?
For GP-B, these kinds of questions became even more relevant in 1986, when a young post-doctoral researcher at the University of Texas named Ignazio Ciufolini suggested a way to measure the frame dragging effect almost as accurately as the stated goal for GP-B, and at a tiny fraction of the cost. He pointed out that general relativity predicts that the tilted orbital plane of a body revolving around a rotating object such as the Earth will rotate by a small amount, in the same direction as the rotation, as a result of frame dragging. One consequence is that the point where the orbit crosses the equatorial plane of the Earth will rotate (see Figure 4.6). This was one of the effects that Lense and Thirring had pointed out in 1918. But in 1976 geophysicists had launched an Earth-orbiting satellite called LAGEOS, and Ciufolini realized that precision tracking of such satellites could potentially detect this frame dragging effect.
Figure 4.6 Frame dragging and LAGEOS. The rotation of the Earth causes the plane of an inclined LAGEOS orbit to rotate at about 30 milliarcsecond per year in the same sense as the rotation of the Earth (short arrow). The variations in the Newtonian gravity of the Earth caused by its flattening and by the uneven distribution of mass also cause the planes to rotate. The amount of rotation is as large as 126 degrees per year, but the direction depends on the inclination angle of the orbit. For LAGEOS I (solid arrow) and II (dashed arrow), the effects are in the opposite direction. Inset: The LAGEOS I satellite, showing the corner reflectors embedded on the surface. Credit: NASA
LAGEOS is an acronym for Laser Geodynamics Satellite, and it is about as simple a satellite as one could possibly imagine. It is a massive spherical ball of solid brass covered in aluminum weighing about 400 kilograms. The surface is studded with 426 fused silica glass mirrors, called retroreflectors, each in the shape of one corner of the interior of a cube (see Figure 4.6). A light ray that approaches any one of the “corner-cube” mirrors will bounce off one face t
hen off an opposite face and then return in exactly the same direction from which it came. By sending pulsed laser beams from Earth and measuring the round-trip travel time of the pulses, researchers can measure the distance between the laser and the satellite with sub-millimeter precision. This technique is called “laser ranging” and was developed during the late 1960s for precise ranging to the Moon (see page 102). The satellite orbits at an altitude of 5,990 kilometers (or 12,200 kilometers from the center of the Earth) on a nearly perfect circle. Because it is so massive and simple (for example, it has no large solar panels), the residual atmosphere at that altitude has almost no effect on it. It even contains a plaque designed by astronomer Carl Sagan providing information about the present Earth for future humanity (if any still exist) when the satellite reenters the atmosphere and falls to Earth in 8.4 million years! Because its orbit is so pure, geophysicists could use laser ranging to it to measure the shape of the Earth, and to study continental drift and other dynamics of the Earth’s crust.
But Ciufolini immediately recognized a difficulty. The Earth is not perfectly spherical, but instead is slightly flattened at the poles and bulges a bit at the equator, a result of its rotation. The deviation is only one part in two thousand, but it modifies the Newtonian gravitational field of the Earth in such a manner that those deviations cause the plane of LAGEOS to rotate in the same direction as the relativistic frame dragging effect. Ciufolini calculated that the relativistic effect would be about 30 milliarcseconds per year. However, the effect due to the Earth’s bulge is huge, 126 degrees per year, almost fifteen million times larger. There was no way to measure this tiny effect on top of such a large effect, and although the value of the Earth’s flattening had been measured to reasonable accuracy, it wasn’t good enough.
Is Einstein Still Right? Page 12