Gravity Probe B

Gravity Probe B (GP-B) is a satellite-based mission to measure the stress-energy tensor (the distribution, and especially the motion, of matter) in and near Earth, and thus to test related models; in application of Einstein's general theory of relativity. Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976.

Overview

Gravity Probe B is a relativity gyroscope experiment funded by NASA. Efforts are being headed up by the Physics department at Stanford University with Lockheed Martin as the primary subcontractor. According to mission plans, it will test two unverified predictions of that theory:

The experiment plans to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 400 nautical miles (741 km) altitude and crossing directly over the poles. So free are the gyroscopes from disturbance that they should provide an almost perfect space-time reference system. They are intended to measure how space and time are "warped" by the presence of the Earth, and, more profoundly, if and how much the Earth's rotation "drags" space-time around with it; the so-called frame-dragging effect. Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20 percent and 10 percent respectively ([1] (http://xxx.lanl.gov/abs/gr-qc/9704065), [2] (http://www.nature.com/news/2004/041018/full/041018-11.html)).

When the mission is successfully completed, GP-B will be the most precise attempt at verification of any effect predicted by general relativity.

The launch was planned for April 19, 2004 at Vandenberg Air Force Base but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On April 20 at 9:57:23 AM PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 AM (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission is planned to last 16 months.

Experimental setup

The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi [3] (http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HR+8703)), a binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects come out at right angles, each gyroscope measuring both.

The gyroscopes are the most spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms. They are composed of fused quartz and coated with niobium. The gyros' spin axes are sensed with SQUIDs.

The gyroscopes are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (−271 degrees Celsius, −456 degrees Fahrenheit). Near absolute zero temperatures are required in order to minimize molecular interference.

This particular star was chosen for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions at the equator of the sky coordinates. Also important was a well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. As a preparation for the setup of this mission, astronomers analyzed the radio based position measurements taken over the last few years to understand its motion as precisely as needed.

Mission progress

• April 20, 2004
  • Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
• April 28, 2004
  • Mission controllers started the "Initialization and Orbit Checkout" phase (IOC), which was expected to last 40-60 days. At this point all gyros were spun up and the SQUID detectors were being checked. All other spacecraft subsystems performed well, including solar power and the attitude control system.
• May 1, 2004
  • During the IOC the primary computer of the spacecraft received too much radiation to cope with the built-in error correction mechanism. GP-B switched over to the backup computer as designed. Since the spacecraft crosses over the polar areas of the Earth with their high radiation, this was anticipated by the designers. The primary computer was repaired and put back into service. All science instruments on board were working perfectly throughout this incident.
• May 14, 2004
  • The spacecraft went into "safemode" for a short period when some of the helium micro-thrusters behaved in an unstable way. This problem was addressed quickly and GP-B went back into IOC mode. The cause of this incident was a high-pressure condition in the dewar, which was reached due to warm (10 K) helium being used to remove magnetic flux from the gyroscopes. Mission members believe that the IOC phase will still be completed on time after a total 60 mission days.
• July 13, 2004
  • The preparations for the science phase of the mission reached a major milestone: One of the gyros (No. 4) reached the science ready speed of 6,348 rpm (105.8 Hz) during a short test.
• July 16, 2004
  • An unexpectedly large slowdown of gyro 4 was detected during the full-speed spin-up of gyro 2. Although some "leakage" effect was expected, the amount seen led mission planners to search for ways to diminish the effect for this final step towards the science phase. This investigation took close to a week and delayed the planned spin-up of gyro 1 and 3.
  • Ground tests had indicated that a good signal-to-noise ratio for science data is reached, once the gyro spin rate exceeds 80 Hz. However, mission managers stress that a slightly lower number will also be sufficient for entering the science phase of GP-B.
• August 27, 2004
  • Mission manangers announced that GP-B entered its science phase, today. On mission day 129 all systems were configured to be ready for data collection, with the only exception being gyro 4, which needs further spin axis alignment.
  • After weeks of testing it was decided to use the "backup-up drag-free" mode, which balances the spacecraft around gyro 3. Also, the rotation period of GP-B was adjusted to 0.7742 rpm in order to avoid harmonic interferences with the sample rate during data taking.
  • They also report that it was planned to continue tuning the drag-free performance of the Attitude and Translation Control (ATC) system in the early portion of the Science Phase to correct for an unknown force, which is causing excess helium flow from the Dewar through the micro thrusters.
• September 7, 2004
  • The main computer suffered a "double-bit" error in its memory. The location of this error was non-critical to the mission and the function of the spacecraft. A correction that fixed the problem was successfully uploaded. All other subsystems are reported to continue to perform well.
• September 16, 2004
  • GP-B officially enters its science phase with all four gyros aligned.
• September 23, 2004
  • Due to problems with gyro 3, GP-B went into "safe mode". The mission team was able to ensure minimal impact to the science, safe the spacecraft, and switch the control system setup. It is now maintaining the drag-free orbit around gyro 1.
• September 24, 2004
  • The mission went back into science mode.
• October 19, 2004
  • Gyro 1 showed the same behavior as gyro 3 earlier, which prompted mission members to switch back to a drag-free orbit around gyro 3. Adjustments were made to both gyro suspension systems (GSS) to avoid future problems. All this was done in a span of three hours, and science data collection was interrupted only briefly.
• November 10, 2004
  • When passing over the South Atlantic Anomaly during a strong solar storm, a memory error in a critical region put GP-B into safe mode. This incident caused a computer to reboot and put the gyros into "analog mode." After about two days all memory problems were fixed and science data became available again. At first, it was assumed a proton hit from the storm was the cause, but later analysis showed that this was not the case. Instead, an earlier error at a presumed non-critical memory position was causing the "safe mode", when the memory was accessed during routine maintenance.
• January, 2005
  • Missing image
    Solar_activity_012005.JPG

    Proton flux due to Solar flares, January 2005
    A series of strong solar flares disrupted data taking for several days. On January 17 a very powerful radiation storm created multi-bit errors in the onboard computer memory, and saturated the telescope detectors so that GP-B lost track of the guide star. The science team, however, is confident that the temporary loss of science data will have no significant effect on the results. On January 20 the high level of proton flux is still generating "single bit errors" in GP-B memory, but the telescope is locked on the guide star again, and the gyroscope electronics seem to perform nominally.
• March 14, 2005
  • The onboard backup computer (B-side) rebooted after a safemode event, which came two weeks after the switch-over from the nominal computer (A-side). Both events were triggered by the occurance of Multi-Bit Errors (MBEs) in the memory of each computer. It took mission members about 29 hours to recover and transfer back to the nominal state, with the guide star locked in.
• May 6, 2005
  • Mission members deduce from a "heat pulse test" that there is enough liquid helium on board the space craft to cool the experiment until sometime between late August and early September of 2005. They are preparing to start the calibration procedures, and thus end the science phase, in early August.

History

The conceptual design for this mission was first proposed by Leonard Schiff (Stanford) and Edward E. Pugh (U.S. Department of Defense) (independently of each other) in 1959. It was proposed to NASA in 1961, and it supported the project with funds in 1964. This grant ended in 1977 after a long phase of engineering research into the basic requirements and tools for the satellite.

In 1986 NASA changed plans for the shuttle, which forced the mission team to switch from a shuttle-based launch design to one that is based on the Delta 2, and in 1995 tests planned of a prototype on a shuttle flight were cancelled as well.

Gravity Probe B marks the first time in history that a university has been in control of the development and operations of a space satellite funded by NASA.

See also

• Timeline of Gravitational Physics
• Gravity
• Frame-dragging

External links

• Graphic explaination of how Gravity Probe B works (http://news.bbc.co.uk/2/hi/science/nature/3639193.stm#graphic)
• Gravity Probe B Web site at Stanford (http://einstein.stanford.edu)
• NASA GP-B launch site (http://www.ksc.nasa.gov/elvnew/gpb)
• VAFB launch schedule (http://mocc.vandenberg.af.mil/launchsched.asp)
• Frame Dragging (http://www.rdrop.com/users/green/school/framdrag.htm)
• General Relativistic Frame Dragging (http://www.phy.duke.edu/~kolena/framedrag.html)
• Layman's article on the project progress (http://www.washingtonpost.com/ac2/wp-dyn/A43049-2004Jul11?language=printer)
• Latest LAGEOS results reported at BBC news (http://news.bbc.co.uk/2/hi/science/nature/3762852.stm)de:Gravity Probe