2 Experimental setup

The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi [3]), 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 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 Kelvin (-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.

3 Mission progress

• April 20
  • Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
• April 28
  • Currently in "Initialization and Orbit Checkout" phase (IOC) expected to last 40-60 days. At this point all gyros are spun up and the SQUID detectors are being checked. All other spacecraft subsystems perform well, including solar power and the attitude control system.
• May 1
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • GP-B officialy enters its science phase with all four gyros aligned.
• September 23
  • 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
  • The mission went back into science mode.
• October 19
  • 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
  • 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.

4 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.

5 See also

• Timeline of Gravitational Physics
• Gravity
• Frame-dragging

6 External links

• Graphic explaination of how Gravity Probe B works
• Gravity Probe B Web site at Stanford
• NASA GP-B launch site
• VAFB launch schedule
• Frame Dragging
• General Relativistic Frame Dragging
• Layman's article on the project progress
• Latest LAGEOS results reported at BBC news

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