At five weeks past launch, the Gravity Probe B mission is now about half way through the Initialization and Orbit Checkout (IOC) phase of the mission. Thus far, the team has successfully transmitted over 5,000 commands to the spacecraft, which remains healthy on orbit. All spacecraft subsystems are continuing to perform well. The spacecraft's orbit is stable, meeting our requirements for next month's transition into the science phase of the mission. All four gyros are now digitally suspended, and the team is in the process of locking the on-board telescope onto the guide star, IM Pegasi.
This past week, the team began spinning up the gyros. During this complex and delicate process, which spans the entire second half of IOC, each of the four gyros undergoes a series of spin-up and testing sequences, the first of which gets the gyros spinning--but at a very slow speed. Because the correct performance of the gyros is critical to the success of the experiment, the team is proceeding through the spin-up process slowly, painstakingly monitoring and checking the performance of the gyros throughout the process.
In preparation for spin-up, the digital suspension system for each gyro was first tested. This was accomplished by suspending each gyro in the center of its housing, electrically "nudging" it slightly off center in one of eight directions (the corners of a cube), and monitoring its automatic re-centering. This checkout was first performed under low voltage conditions (fine control) and then under high voltage conditions (secure hold).
To begin the actual spin-up process, ultra-pure helium gas was flowed over gyros #1 and #4 for 15 seconds, which started them spinning at approximately 0.125 Hz (7.5 rpm). While these gyros were slowly spinning, the suspension test was repeated under high voltage conditions on gyros #2 and #3. During this high voltage suspension test on gyro #3, the team discovered an error in its command template, which turned off the high voltage amplifier to gyro #1 and caused it to lose suspension. There was no damage to gyro#1.
More than 1,000 commands have now been sent to the Gyro Suspension System (GSS), and this was the first error found. Discovering an error in these numerous, intricate command templates was exactly the kind of situation that the painstaking gyro spin-up process was designed to identify; it enabled the team to correct the command template for gyro #2 without serious consequences. Also, as a further precaution, the team has thoroughly reviewed the command templates for the remaining three gyros. Gyros #2 and #3 have now been spun-up to 0.26 Hz (15 rpm) and 0.125 Hz (7.5 rpm) respectively without incident, and gyro #1 is currently being spun-up, as well.
Over the past week, much progress has been made towards the goal of locking the spacecraft's on-board telescope onto the guide star. The pointing error of the spacecraft has been reduced to within 385 arc-seconds (0.11 degrees). This allows the on-board telescope to see the guide star over a portion of a spacecraft roll cycle. The team is in the process of fine-tuning the spacecraft's attitude by adjusting the navigational gyroscope in the Attitude and Translation Control (ATC) system, so that the telescope will be able to see the guide star throughout the entire spacecraft roll cycle, and the team hopes to lock onto the guide star by the beginning of next week.
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.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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