| Date: | 2005-07-14 09:52:00 | |
| Sender: | Ken Galal/NASA AMES, Mike Pearlman/ILRS CB <Carey Noll <carey.noll@nasa.gov>> | |
| Subject: | [SLR-Mail] No. 1346: GP-B Update | |
| Author: | Ken Galal | |
| Content: | ******************************************************************************** SLR Electronic Mail 2005-07-14 09:52:00 UTC Message No. 1346 ******************************************************************************** Author: Ken Galal/NASA AMES, Mike Pearlman/ILRS CB Subject: GP-B Update Dear Colleagues: Here is a status report from Ken Galal on the GP-B Mission. Note that the superfluid helium should last another eight weeks and the mission is trying to decide how to use this time most effectively and what options are available there after. The project will keep us informed. In the meantime, this is a particularly important period, so your continued tracking support is crucial. Regards, Mike MISSION NEWS-WHAT WILL ULTIMATELY HAPPEN TO THE GP-B SPACECRAFT? The relativity measurements that Gravity Probe B is currently making require that the science gyros and SQUID magnetometer pick-up loops be maintained in a cryogenic environment, with all other sources of noise and disturbance either eliminated or minimized to insignificant levels. This environment had to be maintained from before launch through the Initialization and Orbit Checkout (IOC), science (data collection), and final instrument calibration phases of the mission. In other words, there needed to be enough liquid helium in the Dewar to last at least 14 months and preferably 16-20 months. This is the reason that the spacecraft needed a 650-gallon Dewar, the size of a cement mixer. In addition to maintaining the cryogenic environment inside the Dewar, the liquid helium also provides a constant source of propellant that enables the micro thrusters to both keep the spacecraft and telescope pointed at the guide star and also maintain the spacecraft in drag-free flight, using one of the science gyros (currently gyro #1), as the spacecraft´s center of mass or ”proof mass.” Towards the beginning of June, the spacecraft transitioned out of its two-week full sun season to once again being eclipsed from the Sun for part of each orbit. Furthermore, the position of the Earth relative to the Sun has been changing so that the Sun´s light is now moving towards the rear of the spacecraft, and this orientation will continue through August and September. During this period, the spacecraft´s Attitude and Translation Control (ATC) performance will be the most stable, giving us the ”cleanest” data of the entire mission. The pointing performance stabilizes during this period because the Sun is no longer heating up the spacecraft´s attitude reference platform (ARP), where the navigational rate gyros and star trackers are mounted, on the forward dome of the Dewar. Our measurements indicate that the superfluid helium in the Dewar will be exhausted in approximately eight weeks (around the beginning of September)--during the period of maximum ATC stabilization. This situation has raised some questions about how to proceed through the final two months of the mission. Our original plan was to stop collecting relativity data 3-5 weeks before the helium runs out and to spend those final weeks of the mission exclusively running calibration tests of the science instruments. Some of these tests involve placing torques (forces) on the gyros, and we cannot use the science data collected from a gyro while it is undergoing such a test. Another calibration test involves purposely moving the telescope´s pointing axis away from our guide star, IM Pegasi, to a different star and then back again. If we take this approach, we cannot collect any science data during this telescope pointing test. However, our plan for spending the final 3-5 weeks of the mission exclusively running calibration tests is not cast in stone, and we are currently at a point of making some trade-off decisions in this regard. For example, we have determined that it is possible to perform the gyro torquing calibration tests on an individual gyro, while the other gyros continue to collect science data. (We are performing such a calibration test on gyro #3 right now.) To address these issues and trade-offs, our GP-B management and science teams spent the entire day last week at an off-site meeting. The decisions resulting from this off-site meeting will determine our course of action for the remainder of the mission, and we will report on these decisions in the near future. When the helium runs out, the GP-B gyro rotors will continue to spin, but their niobium coatings will warm up and cease to be superconductive. Thus, the spinning gyro rotors will no longer generate magnetic moments around their axes of spin, and there will no longer be any signals for the SQUID magnetometers to readout. Moreover, there will no longer be any helium gas escaping from the Dewar to serve as propellant for the micro thrusters, so the spacecraft will cease to fly drag free in its orbit. Without drag-free flight and superconductivity in the gyros and SQUIDs, it will no longer be possible to conduct further relativistic measurements of the gyro´s spin axes. However, the spacecraft´s solar arrays will continue to provide power for many years, and the NiCAD batteries on-board are rated for a minimum of five years. While there would be no propellant for controlling the attitude of the spacecraft using the micro thrusters, the spacecraft has magnetic torquers, which can provide coarse attitude control (i.e., 5 degree). Furthermore, the Gyro Suspension System (GSS) electronics will continue to keep the gyros suspended, and if left untouched, the gyros would continue to spin for thousands of years. The GSS can also be used to measure torques or forces being exerted on the gyros. Thus, the GP-B spacecraft could be used to provide other kinds of scientific data, such as measurements of variations in the Earth´s shape. The decision as to whether or not to do this will ultimately be made by NASA, as a function of scientific interest, available funds, and agency priorities. If NASA chooses not to fund any further research uses of the GP-B spacecraft, then one of the final actions of the GP-B Mission Operations Team will be to spin down the gyros and then shut down all spacecraft systems, using a special double-safe shutoff procedure that ensures the spacecraft´s computers and telemetry system will never spontaneously re-start themselves (which once happened with another satellite). We will keep you updated as decisions are make. Meanwhile the project greatly appreciates your continued support. Ken Galal From: Carey Noll ******************************************************************************** |