Gravity Probe B

From the GP-B Update (31-01-2006):


"Our GP-B team has now completed all planned, as well as some extended post-mission analyses on our spacecraft and its component systems. We are in the process of stabilizing the spacecraft in its current orbit normal orientation, and the spacecraft is now ready and available for use by other scientists to perform various types of experiments. The GP-B spacecraft is a state-of-the-art orbiting laboratory, and it has performed extraordinarily well throughout the GP-B mission and beyond. While it no longer maintains the cryogenic environment necessary for testing the geodetic and frame-dragging effects of general relativity, the on-board star trackers, magnetometers, and science gyros are still functioning perfectly, and they can be used individually or in combination for a number of other types of experiments.

Thus, we are actively seeking scientific partners around the world who would be interested in using this space borne laboratory to perform additional post-mission experiments. To this end, for the next few weeks we will be collecting sample data to demonstrate and validate the spacecraft's post-mission experimental capabilities in the following five areas.

1. Occlusion of Stars by Planets
Within the next few days, we will begin the process of slowing down the spacecraft's roll rate to 0.04 rpm (25 minutes per revolution). At this very slow roll rate, the on-board star trackers can be switched into a more sensitive tracking mode, in which the light from stars can be integrated over much longer time periods. By observing brightness variations of various star systems, the star trackers can detect the presence of orbiting planets in those systems.

2. Measuring Residual Drag on the Spacecraft
The four GP-B science gyroscopes can be used as 3-axis accelerometers that are capable of measuring solar pressure and upper atmospheric drag on the spacecraft to an accuracy of 5 x 10-12 g.

3. GPS Satellite Accuracy Measurements
Because the orbit plane of our GP-B spacecraft is very well established, we can use the four science gyroscopes as 3-axis accelerometers to determine the spacecraft's precise inertial position without GPS reckoning. We can then compare this internally-calculated spacecraft position information with corresponding position information generated by various GPS satellites to determine their level of accuracy.

4. Subtle Aurora Borealis Effects
Using a combination of the on-board magnetometers, the proton monitor, and the four science gyros as accelerometers, we can investigate what happens when the upper atmosphere heats up as a result of bombardment by charged particles. We can also measure the buffeting effects of the upper atmosphere as the spacecraft passes through a region containing charged particles.

5. Latitude Axis Gravity Gradient
As a complement to the GRACE mission that measured gravity gradients along a longitudinal axis between a pair of orbiting spacecraft, the GP-B science gyros can be used as 3-axis accelerometers to measure latitude-axis (cross-orbit) gravity gradients.

As noted earlier, for the next few weeks, we will be collecting sample data to illustrate the feasibility and limits of performing all of the above experiments with the GP-B spacecraft. Because the spacecraft is already in orbit and functioning, funding requirements for such "experiments of opportunity" will be minimal. The cost for using the spacecraft to perform any or all of these experiments amounts to the cost of a 4-5 person mission operations staff over the period of the experiments.


In summary, the GP-B spacecraft has performed exceedingly well to date. It has experienced no serious failures, and all the systems required for performing post-mission experiments are operational and ready for use. In a few weeks, after we have finished collecting the sample data, if there is no interest or funding for performing any of these experiments, we will place the spacecraft in a safe, hibernation configuration and reduce our maintenance monitoring of its health to once a week."


How about that?