GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Based on data from the on-board telescope and the gyroscope that was used as a proof mass for maintaining adrag-free orbit, the flow of the escaping helium gas was carefully metered through these thrusters in order toprecisely control the spacecraft's position. In fact, the position of the entire spacecraft was balanced around theproof mass gyroscope by increasing or decreasing the flow of helium through opposing thrusters, creating adrag-free orbit. Thus, the Dewar and liquid helium served two critically important functions in the mission:1. Maintaining a supercold temperature around the science instrument.2. Providing a constant stream of gas propellant for precisely controlling the position and attitude of theentire satellite.3.2.1.4 The Realization of Drag-Free TechnologyGravity Probe B is the first spacecraft ever launched requiring six degrees of freedom in active attitude control—three degrees of freedom in pointing control to maintain its guide star pointing orientation and constant rollrate (pitch, yaw, and roll) and three degrees of freedom in translational control (up-down, front-to-back, andside-to-side), to maintain a drag-free orbit throughout the 17-month flight mission.The term “drag-free” refers to a body that is moving without any friction or drag, and thus its motion is affectedonly by gravity. Here on Earth, and even 642 km (400 miles) above the Earth where the GP-B satellite is in orbit,there is always some drag caused by the Earth's atmosphere, solar pressure, and other forces. However, a bodycan achieve drag-free motion if it is shielded by another body. A drag-free satellite, therefore, refers to afeedback system consisting of a satellite within a satellite. The inner satellite, often called a proof-mass, istypically a small homogeneous object, such as a spherical GP-B gyroscope. The position of the outer shieldingsatellite must be tightly controlled to prevent it from ever touching the proof mass. This is accomplished byequipping the outer satellite with sensors that precisely measure its position relative to the proof mass and a setof thrusters that automatically control its position, based on feedback from the sensors. Through this feedbacksystem, the satellite continually “chases” the proof-mass, always adjusting its position so that the satelliteremains centered about the proof mass, which is orbiting the Earth in a constant state of free fall. The drag-freefeature of the GP-B spacecraft is critically important to the experiment because orbital drag on the spacecraftwould cause an acceleration that might obscure the minuscule relativistic gyro precessions being measured.Figure 3-6. Proportional micro thrusters used for GP-B spacecraft attitude controlGP-B needs extremely sensitive thrusters to re-orient the satellite and keep it on its proper path. Here's wherethe escaping helium gas that slowly boils off from the liquid helium comes in handy. Minute amounts of gas,1/10th of a human breath or a few millinewtons of force, provide just the right amount of thrust necessary toadjust the satellite's position.74 March 2007 Chapter 3 — Accomplishments & Technology Innovations