2007 Status Updates

14 April 2007

WAS EINSTEIN RIGHT? SCIENTISTS PROVIDE FIRST PUBLIC PEEK AT GRAVITY PROBE B RESULTS.

For the past three years a satellite has circled the Earth, collecting data to determine whether two predictions of Albert Einstein's general theory of relativity are correct. Today, at the American Physical Society (APS) meeting in Jacksonville, Fla., Professor Francis Everitt, a Stanford University physicist and principal investigator of the Gravity Probe B (GP-B) Relativity Mission, a collaboration of Stanford, NASA and Lockheed Martin, will provide the first public peek at data that will reveal whether Einstein's theory has been confirmed by the most sophisticated orbiting laboratory ever created.

"Gravity Probe B has been a great scientific adventure for all of us, and we are grateful to NASA for its long history of support," said Everitt. "My colleagues and I will be presenting the first results today and tomorrow. It's fascinating to be able to watch the Einstein warping of spacetime directly in the tilting of these GP-B gyroscopes —more than a million times better than the best inertial navigation gyroscopes."

The GP-B satellite was launched in April 2004. It collected over a year's worth of data that the Stanford GP-B science team has been poring over for the past 18 months. The satellite was designed as a pristine, space-borne laboratory, whose sole task was to use four ultra-precise gyroscopes to measure directly two effects predicted by general relativity. One is the geodetic effect-the amount by which the mass of the Earth warps the local space-time in which it resides. The other effect, called frame-dragging, is the amount by which the rotating Earth drags local space-time around with it. According to Einstein's theory, over the course of a year, the geodetic warping of Earth's local space-time causes the spin axes of each gyroscope to shift from its initial alignment by a minuscule angle of 6.606 arc-seconds (0.0018 degrees) in the plane of the spacecraft's orbit. Likewise, the twisting of Earth's local space-time causes the spin axis to shift by an even smaller angle of 0.039 arc-seconds (0.000011 degrees) — about the width of a human hair viewed from a quarter mile away — in the plane of the Earth's equator.

GP-B Scientists expect to announce the final results of the experiment in December 2007, following eight months of further data analysis and refinement. Today, Everitt and his team are poised to share what they have found so far — namely that the data from the GP-B gyroscopes clearly confirm Einstein's predicted geodetic effect to a precision of better than 1 percent. However, the frame-dragging effect is 170 times smaller than the geodetic effect, and Stanford scientists are still extracting its signature from the spacecraft data. The GP-B instrument has ample resolution to measure the frame-dragging effect precisely, but the team has discovered small torque and sensor effects that must be accurately modeled and removed from the result.

"We anticipate that it will take about 8 more months of detailed data analysis to realize the full accuracy of the instrument and to reduce the measurement uncertainty from the 0.1 to 0.05 arc-seconds per year that we've achieved to date down to the expected final accuracy of better than 0.005 arc-seconds per year," says William Bencze, GP-B Program Manager. "Understanding the details of this science data is a bit like an archeological dig: a scientist starts with a bulldozer, follows with a shovel, and then he finally uses dental picks and toothbrushes to clear the dust away from the treasure. We are passing out the toothbrushes now."

Beyond theory, space mission pushes frontiers of human possibility, engineering

By Annie Jia

When Gravity Probe B (GP-B) was conceived in 1959, much of the technology needed to conduct the experiment did not exist. An unprecedented level of technological precision was needed to measure the curvature of space, and it took more than three decades for scientists and engineers to create more than a dozen technologies needed to make their vision a reality. "The number of new technologies that has spawned spin-offs on this experiment is astounding, but probably the most important spin-off is the over 90 Ph.D. theses that have been sponsored by this experiment, because that represents education," said Brad Parkinson, the mission's co-principal investigator and co-inventor of the Global Positioning System (GPS). Following are just a few examples of the technologies GP-B research has spawned.

Gyroscopes steadier than an owl's eyes

To create wobble-free gyroscopes, scientists produced the world's most perfect spheres. Enlarged to the size of the Earth, the spheres would have mountains no more than 8 feet high. The gyroscopes are now recorded in the Guinness Database of World Records as the roundest manmade objects. They are surpassed in roundness by only one type of object in the entire universe: dense neutron stars.

World's best protractor

The width of a human hair viewed from a quarter of a mile away-that is the tiny angle by which space-time around Earth was predicted to tilt GP-B's gyroscopes. To measure these miniscule angles, engineers had to develop sensors of astounding precision. The Superconducting QUantum Interference Device (SQUID) magnetometers use a superconducting niobium loop to measure tiny changes in magnetic field that result from the gyroscopes' shift. The devices are so sensitive that they can detect a magnetic field 10 trillion times smaller than the Earth's.

Giant thermos

A minivan-sized dewar, or thermos, filled with liquid helium surrounds the gyroscope assembly, keeping it at a cryogenic temperature near absolute zero. To maintain the experimental apparatus at a temperature of 1.8 Kelvin for 16 months in space, the dewar employed advanced technology to insulate, block space radiation and cool by evaporation.

Porous plug

Despite the dewar's incredible insulating abilities, a small amount of heat seeps in and turns some liquid helium into gas. A porous plug allows this "warm" gas to escape while keeping the liquid helium inside to cool the assembly through evaporation, just as sweating cools a person's skin. What's more, the porous plug helps fine-tune the position of the spacecraft. Escaping helium gas is directed through micro-thrusters that puff out amounts one-fiftieth the size of a person's breath to minutely shift and turn the spacecraft to achieve perfect alignment with its guide star. The porous plug has since been used in other NASA flights, including this year's Nobel prize-winning COBE (for COsmic Background Explorer) mission.

World's best star tracker

Scientists chose a distant star as the fixed reference point for measuring the small angles of deflection of GP-B's gyroscopes. Because of the minuteness of the angles, the telescope's focus on the guide star had to be precise to one 10-millionth of an inch. A telescope lens alone would provide an image 10,000 times too rough, so GP-B engineers created a device to split the incoming telescope image of the star into two separate images, one for the horizontal alignment and one for the vertical. Each image was again divided in two. Then delicate sensors measured the amount of light in each half image, and the spacecraft's orientation was adjusted until the two halves were perfectly balanced, indicating that the star's prefect center had been found.

Gyro suspension system

With the gyroscopes spinning at 4,000 rpm, their suspension system used electrostatic fields to hold each rotor in a vacuum a mere paper's width from the walls of its housing. Any contact of the rotors with the walls would destroy the equipment. The gyro suspension system adjusts the rotors' positions 220 times a second, continually ensuring perfect suspension.

Drag-free orbit

It is customary to track the path of an orbiting spacecraft, not the path of the equipment inside it. GP-B's drag-free technology flips that paradigm and instead tracks the path of the equipment inside the spacecraft. Like gnats swarming around a person, the GP-B spacecraft "chases" one of four gyroscopes, which serves as the experiment's central mass. The spacecraft's body shields the gyroscope from outside disturbances, such as friction and magnetic fields, so that the gyroscope is affected only by gravity and orbits the Earth in perfect free-fall. The spacecraft itself, constantly disturbed by the harsh forces of space, adjusts its own position 10 times a second, based on information from the gyro suspension system, to remain perfectly centered around the drag-free gyroscope. Stanford's Aeronautics and Astronautics Department pioneered drag-free satellite technology in the early 1960s.

Precision GPS

Scientists needed an accurate method to map GP-B's orbit because the distortion of space-time varies with location. Hence they adapted conventional GPS technology for GP-B's high speeds and rotating motion. In the process they discovered a way to enhance the standard GPS receiver to measure position down to the centimeter level. Precision GPS has since been adopted in automated tractors, aircraft landing systems and vehicles used in mining and building roads. "[Robotic farm tractors have] launched a market now valued at well over $100 million and growing," Parkinson said. "The productivity enhancements are startling, and clearly benefit many with less expensive food."

Annie Jia is a science-writing intern at the Stanford News Service.

The two discoveries

Two important discoveries were made while analyzing the gyroscope data from the spacecraft: 1) the "polhode" motion of the gyroscopes damps out over time, and 2) the spin axes of the gyroscopes were affected by small classical torques. Both of these discoveries are symptoms of a single underlying cause: electrostatic patches on the surface of the rotor and housing. Patch effects in metal surfaces are well known in physics, and were carefully studied by the GP-B team during the design of the experiment to limit their effects. Though previously understood to be microscopic surface phenomena that would average to zero, the GP-B rotors show patches of sufficient size to measurably affect the gyroscopes' spins.

The gyroscope's polhode motion is akin to the common "wobble" seen on a poorly thrown (American) football, though it shows up in a much different form for the ultra-spherical GP-B gyroscopes. While it was expected that this wobble would exhibit a constant pattern over the mission, it was found to slowly change due to minute energy dissipation from interactions of the rotor and housing electrostatic patches. The polhode wobble complicates the measurement of the relativity effects by putting a time-varying wobble signal into the data.

The electrostatic patches also cause small torques on the gyroscopes, particularly when the space vehicle axis of symmetry is not aligned with the gyroscope spin axes. Torques cause the spin axis of the gyroscopes to change orientation, and in certain circumstances, this effect can look like the relativity signal GP-B measures. Fortunately, the drifts due to these torques has a precise geometrical relationship to the misalignment of the gyro spin/vehicle symmetry axis and can be removed from the data without directly affecting the relativity measurement.

Both of these discoveries first had to be investigated, be precisely modeled and then be carefully checked against the experimental data before they are removed as sources of error. These additional investigations have added more than a year to the data analysis, and this work is still in process. To date, the team has made very good progress in this regard, according to its independent Science Advisory Committee, chaired by relativistic physicist Clifford Will of Washington University in St. Louis, Mo., that has been monitoring every aspect of GP-B for the past decade.

In addition to providing a first peek at the experimental results at the APS meeting, the GP-B team has released an archive of the raw experimental data. The data will be available through the National Space Sciences Data Center at the NASA Goddard Space Flight Center beginning in June 2007.

Conceived by Stanford Professors Leonard Schiff, William Fairbank and Robert Cannon in 1959 and funded by NASA in 1964, GP-B is the longest running, continuous physics research program at both Stanford and NASA. While the experiment is simple in concept — it utilizes a star, a telescope and a spinning sphere — it took more than four decades and $760 million to design and produce all the cutting-edge technologies necessary to bring the GP-B satellite to the launch pad, carry out this "simple" experiment and analyze the data. On April 20, 2004, GP-B made history with a perfect launch from Vandenberg Air Force Base in California. After a four-month initialization and on-orbit check-out period, during which the four gyroscopes were spun up to an average of 4,000 rpm and the spacecraft and gyro spin axes were aligned with the guide star, IM Pegasi, the experiment commenced. For 50 weeks, from August 2004 to August 2005, the spacecraft transmitted more than a terabyte of experimental data to the GP-B Mission Operations Center at Stanford. One of the most sophisticated satellites ever launched, the GP-B spacecraft performed magnificently throughout this period, as did the GP-B Mission Operations team, comprised of scientists and engineers from Stanford, NASA and Lockheed Martin, said Stanford Professor Emeritus Bradford Parkinson, a co-principal investigator with John Turneaure and Daniel DeBra, also emeritus professors at Stanford. The data collection ended on Sept. 29, 2005, when the helium in spacecraft's dewar was finally exhausted. At that time, the GP-B team transitioned from mission operations to data analysis.

Over its 47-year lifetime, GP-B has advanced the frontiers of knowledge, provided a training ground for 79 doctoral students at Stanford (and 13 at other universities), 15 masters degrees, hundreds of undergraduates and dozens of high school students who worked on the project. In addition, GP-B spawned over a dozen new technologies, including the record-setting gyroscopes and gyro suspension system, the SQUID (for Superconducting QUantum Interference Device) gyro readout system, the ultra-precise star-pointing telescope, the cryogenic dewar and porous plug, the micro-thrusters and drag-free technology and the Global Positioning System-based orbit determination system. All of these technologies were essential for carrying out the experiment, but none existed in 1959 when the experiment was conceived. Furthermore, some technologies which were designed at Stanford for use in GP-B, such as the porous plug that controlled the escape of helium gas from the dewar, enabled and were used in other NASA experiments such as COBE (the COsmic Background Explorer, which won this year's Nobel prize) WMAP (for Wilkinson Microwave Anisotropy Probe) and the Spitzer Space Telescope.

The experiment's final result is expected on completion of the data analysis in December of this year. Asked for his final comment, Francis Everitt said: "Always be suspicious of the news you want to hear."

NASA's Marshall Space Flight Center manages the GP-B program and contributed significantly to its technical development. NASA's prime contractor for the mission, Stanford University, conceived the experiment and is responsible for the design and integration of the science instrument, as well as for mission operations and data analysis. Lockheed Martin, Stanford's major subcontractor, designed, integrated and tested the spacecraft and built some of its major payload components, including the dewar and probe that houses the science instrument. NASA's Kennedy Space Center, Fla., and Boeing Expendable Launch Systems, Huntington Beach, Calif., was responsible for the launch of the Delta II.

Bob Kahn, author of this press release, is the public affairs coordinator for Gravity Probe B at Stanford.

GP-B APS CONFERENCE SCHEDULE

The GP-B Results Announcement & Presentations at the APS Meeting in April

aps logo GP-B will have a strong presence at the American Physical Society (APS) meeting in Jacksonville, Florida, on 14-17 April 2007. During this meeting, we will emphasize three main themes:

Successful completion of most challenging space-based experiment in NASA's history
First scientific results from this historic mission
Public release of Level2 science data (via NSSDC)

Four members of the GP-B team have been invited to speak at the APS meeting, beginning on Saturday morning, April 14th, with GP-B Principal Investigator, Francis Everitt, giving the plenary conference talk, entitled First Results from Gravity Probe B.

To listen to the MP3 audio while viewing the presentation slides, click the presentation in the table below, then click on your audio choice.

First Results from Gravity Probe B, Dr. Francis Everitt speaking
(Presentation slides and audio from the other APS talks will be available soon.)

In addition, on Saturday afternoon, two papers related to GP-B will be delivered in Session C12: Experimental Tests of Gravity.

C12.00004: " Lessons Learned from Gravity Probe B for STEP, LISA and other experiments" by GP-B team members Paul Worden and Sasha Buchman
C12.00005: "Proper Motion of the GP-B Guide Star" by the Harvard-Smithsonian Center for Astrophysics GP-B guide star tracking team: Irwin Shapiro, Daniel Lebach, Michael Ratner, Norbert Bartel, Ryan Ransom, Michael Bietenholz, Jerusha Lederman, and Jean-Francois Lestrade

On Sunday morning, April 15th, three members of the GP-B team have been invited to give special talks on three aspects of the GP-B program:

H7.00001: "The Gravity Probe B Science Instrument," by GP-B Co-Principal Investigator, John Turneaure
H7.00002: "The Development Challenges of Gravity Probe-B—an ongoing partnership between Physics and Engineering" by GP-B Co-Prinipal Investigator, Bradford Parkinson
H7.00003: "Gravity Probe B Data Analysis Challenges, Insights, and Results" by GP-B Co-Investigator and Chief Scientist, George (Mac) Keiser

Finally, on Sunday afternoon, April 15th, a large part of the GP-B team and associated scientists and engineers will present 22 poster sessions on a host of scientific and technology topics, as listed below.

Click on any of the titles below to view or download the associated poster.

Session L1: Poster Session II L1.00011: GRAVITATION

L1.00012: "Radio Imaging of the Gravity Probe B Guide Star IM Pegasi" by Michael Bietenholz, Ryan Ransom, Norbert Bartel, Daniel Lebach, Michael Ratner, Irwin Shapiro, Jean-Francois Lestrade
L1.00013: "The 'Core' of the Quasar 3C454.3 as the Extragalactic Reference for the Proper Motion of the Gravity Probe B Guide Star" by Norbert Bartel, Ryan Ransom, Michael Bietenholz, Jerusha Lederman, Daniel Lebach, Michael Ratner, Irwin Shapiro, Leonid Petrov
L1.00014: "Performance of the Gravity Probe B Inertial Reference Telescope" by Suwen Wang, John Goebel, John Lipa John Turneaure
L1.00015: "Gravity Probe B Timing System and Roll Phase Determination" by Jie Li , Jeffery Kolodziejczak
L1.00016: "The Gravity Probe B SQUID Readout Detector" by Barry Muhlfelder, Bruce Clarke, Gregory Gutt, James Lockhart, Ming Luo
L1.00017: "SQUID Control, Temperature Regulation, and Signal Processing Electronics for Gravity Probe B" by James Lockhart, Barry Muhlfelder, Jie Li, Bruce Clarke, Terry McGinnis, Peter Boretsky, Gregory Gutt
L1.00018: "Gravity Probe B Science Instrument Assembly (SIA)" by Saps Buchman, Barry Muhlfelder, John Turneaure
L1.00019: "Polhode Motion of the Gravity Probe-B Gyroscopes" by Michael Dolphin, Alex Silbergleit, Michael Salomon, Paul Worden, Daniel DeBra
L1.00020: "Evidence for Patch Effect Forces on the Gravity Probe B Gyroscopes" by Dale Gill, Saps Buchman
L1.00021: "Gravity Probe B Orbit Determination" by Paul Shestople , Huntington Small
L1.00022: "Simulator Technology of the Gravity Probe-B Mission" by David Hipkins , Robert Brumley , Yoshimi Ohshima , Thomas Holmes
L1.00023: "Achievement of the Magnetic Environment Requirements for Gravity Probe B" by John Mester, James Lockhart, Michael Taber
L1.00024: "The Gravity Probe B Gyroscopes" by Saps Buchman, Bruce Clarke, Mac Keiser, Ping Zhou, Dale Gill, Frane Marcelja, Robert Brumley
L1.00025: "Gravity Probe B Gyroscope Electrostatic Suspension System (GSS)" by William Bencze, David Hipkins, Tom Holmes, Sasha Buchman, Robert Brumley
L1.00026: "The Gravity Probe B Relativity Mission (GP-B)" by C.W. Francis Everitt
L1.00027: "Gravity Probe B Experiment Error" by Barry Muhlfelder, G. Mac Keiser, John Turneaure
L1.00028: "Gravity Probe B Science Data Analysis: Filtering Strategy" by Michael Heifetz, Thomas Holmes, David Hipkins, Alex Silbergleit, Vladimir Solomonik
L1.00029: "Performance of the Gravity Probe B Cryogenic Sub-System" by Michael Taber, David Murray
L1.00030: "The Gravity Probe B Drag-free and Attitude Control System" by Michael Adams, Daniel DeBra
L1.00031: "Features of the Gravity Probe B Space Vehicle" by William Reeve, Gaylord Green
L1.00032: "Classical Torques on Gravity Probe B Gyroscopes" by Alex Silbergleit, G. Mac Keiser, Yoshimi Ohshima
L1.00033: "Trapped Flux Mapping for the Gravity Probe B Gyroscopes" by Michael Salomon, John Conklin, Michael Dolphin, G. Mac Keiser, Alex Silbergleit, Paul Worden

Gravity Probe B

Testing Einstein's Universe