For release: 06/21/04
Status report #: 04-168
Gravity Probe B — a NASA experiment to test two predictions of Albert Einstein's Theory of General Relativity — continues to perform well. The spacecraft is in good health, and as planned, the science telescope has locked onto the guide star, IM Pegasi. Gravity Probe B is managed by the Marshall Center.Photo: Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. (NASA/MSFC)
Two months into the mission, the spacecraft is in good health, and all subsystems are performing well. The spacecraft's orbit continues to be stable, meeting our requirements for transition into the science phase of the mission. All four gyros remain digitally suspended, and the GP-B team is completing a planned series of calibration tests at very low gyro spin rates. The science telescope has locked onto the guide star, IM Pegasi, and the team is beginning the process of distributing and balancing the mass of the spacecraft at increased roll rates, as required for the science mission. For reasons discussed below, the Initialization and Orbit Checkout (IOC) phase of the mission has been extended to 90 days.
This past week, the team increased the roll rate of the spacecraft from 0.1 rpm to 0.3 rpm, in preparation for "mass trim" and "bubble wrap." These procedures are used to bring the entire spacecraft into balance so that it rolls smoothly about its main axis, while continuing to focus on the guide star through the telescope. The mass trim operation is similar to dynamically spin balancing a tire, using movable weights on the spacecraft frame under computer control to adjust the spacecraft's center of mass. Bubble wrap is the process of uniformly distributing the liquid helium around the Dewar's outer shell. This is accomplished by increasing the roll rate of the spacecraft in steps, from 0.3 rpm to 0.6 rpm.
Also during this past week, the spacecraft/telescope re-visited guide star neighbor HD 216635 (SAO 108242) as well as guide star neighbor HR Peg (HR 8714), for further testing and brightness calibration.
Another important event this past week was using the results of prior gyro calibration tests to fine-tune the Gyro Suspension System (GSS) for each gyro. This significantly improved the suspension performance of all the gyros, especially gyro #2. Parameters are now in place for spinning up the gyros to 5 Hz (300 rpm).
Two difficulties — now both overcome — have made the task of locking onto the guide star take longer than anticipated, and as a result, the IOC phase of the mission has been extended from 60 to 90 days. First, the side-facing star trackers on the spacecraft required an extended search period to properly identify the known field of stars. Feedback from these star trackers is used to orient the spacecraft in the vicinity of the guide star, similar to using a spotting telescope to position a high-powered telescope on a particular part of the sky. The second difficulty is the malfunction of two of the spacecraft's 16 ultra-sensitive micro thrusters. Redundancy built into the system enables the spacecraft to fly without the two problematic thrusters, but to optimize performance with 14 instead of 16 thrusters, it was necessary to revise the thruster-control software. This software change has now been implemented, and after a Flight Readiness Review on June 25, 2004, it will be uploaded to the spacecraft. At a later stage, the team will explore partial re-activation of the two problematic thrusters.
NASA's Gravity Probe B mission, also known as GP-B, will use four ultra-precise gyroscopes to test Einstein's theory that space and time are distorted by the presence of massive objects. To accomplish this, the mission will measure two factors — how space and time are warped by the presence of the Earth, and how the Earth's rotation drags space-time around with it.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Gravity Probe B program for NASA's Office of Space Science. Stanford University in Stanford, Calif., developed and built the science experiment hardware and operates the science mission for NASA. Lockheed Martin of Palo Alto, Calif., developed and built the GP-B spacecraft.