Gravity Probe B continues toward science operations
NASA STATUS REPORT
Posted: May 15, 2004
As of Day #24 of the mission, all spacecraft subsystems are functioning properly on Gravity Probe B, a NASA experiment to test two predictions of Albert Einstein's Theory of General Relativity. The orbit is stable and meets our requirements for next month's transition into the science phase of the mission, upon completion of the spacecraft initialization and orbit checkout. Furthermore, Gravity Probe B has successfully achieved several important milestones over the past week.
All four gyroscopes have now been digitally suspended for over a week. At launch, the gyros were unsuspended. Once on orbit, each gyro was first suspended in analog mode, which provides coarse control of the gyro's suspended position within its housing. Analog mode is used primarily as a backup or safe mode for suspending the gyros. Each gyro was then suspended digitally. The digital suspension mode is computer-controlled; it puts less torque on the gyros than analog mode and enables their position to be controlled with extremely high precision.
At the end of last week, the Gravity Probe B team practiced Low Temperature Bakeout (LTB), in which discs of sintered titanium (very tiny titanium balls, smaller than cake sprinkles) are "warmed up" a few Kelvin, thereby attracting helium molecules to them. This process will remove any remaining helium from the gyro housings after full gyro spin-up. Last week's practice LTB procedure had the added benefit of imparting a very small amount of spin-up helium gas to the gyros. Following the practice LTB, the SQUID gyro read-out data revealed that gyro #1, gyro #3, and gyro #4 were slowly spinning at 0.001, 0.002, and 0.010 Hz, respectively (1 Hz = 60 rpm). Amazingly, the Gyro Suspension Systems (GSS) were able to measure gas spin-up forces at the level of approximately 10 nano-newton (10-8 N). This means that the GP-B science team is able to interpret data from gyro spin rates four to five orders of magnitude smaller than what was planned for the GP-B science experiment.
Earlier this week, the GP-B spacecraft flew "drag free" around gyro #1, maintaining translation control of the spacecraft to less than 500 nanometers. The term, "drag-free" means that the entire spacecraft literally floats in its orbit -- without any friction or drag -- around one the gyros. Pairs of proportional micro thrusters put out a steady and finely controlled stream of helium gas, supplied by the Dewar, through its porous plug. Signals from the Gyro Suspension System (GSS) control the output of the micro thrusters, balancing the spacecraft around the selected gyro. The initial Drag Free Control (DFC) checkout lasted 20 minutes, as planned. Then, a two-hour DFC session was tested, during which the spacecraft roll rate was increased and then returned to its initial rate, maintaining drag-free status throughout the test. Achieving DFC indicates that we are on track to meet the science mission control requirements.
Last, but not least, early this week, the Attitude & Translation Control system (ATC) successfully used data from the on-board star sensors to point the spacecraft towards the guide star, IM Pegasi. This was the final step before initiating the dwell scan process, a series of increasingly accurate scans with the on-board telescope that enable the ATC to lock onto the guide star. Two days ago, the telescope's shutter was opened, and a first dwell scan was completed. We are now in the final stages of repeating the dwell scan to home in on the guide star and lock onto it.
The Initialization & Orbit Checkout phase of the Gravity Probe B mission remains on track for completion within 60 days after launch, at which time the 13-month science data collection will begin. This will be followed by a two-month final calibration of the science instrument assembly.
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.
Launch review