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

Each quartz rotor is coated with a sliver-thin layer of niobium, a superconducting metal. Inside each housing,six electrodes electrically suspend the gyroscope, allowing it to spin freely at up to 10,000 rpm. Channels are cutin the quartz housing to allow helium gas to start the rotor spinning. A wire loop, embedded in the housing, isconnected to a SQUID (Superconducting Quantum Interference Device) magnetometer to detect any change indirection of the gyroscope’s spin axis.Three physical characteristics of any gyroscope can cause its spin axis to drift, independently of the generalrelativity precession predicted by Einstein’s theory:1. An imbalance in mass or density distribution inside the gyroscope2. An uneven, asymmetrical surface on the outside of the gyroscope3. Friction between the bearings and axle of the gyroscope.This meant that a GP-B gyroscope rotor had to be perfectly balanced and homogenous inside, had to be freefrom any bearings or supports, and had to operate in a vacuum of only a few molecules.After years of work and the invention of new technologies and processes for polishing, measuring sphericity,and coating, the result was a homogenous 1.5-inch sphere of pure fused quartz, polished to within a few atomiclayers of perfectly smooth. In fact, the GP-B gyro rotors are now listed in the Guinness Database of WorldRecords as being the roundest objects ever manufactured, topped in sphericity only by neutron stars.The spherical rotors are the heart of each GP-B gyroscope.They were carved out of pure quartz blocks, grown inBrazil and then fused (baked) and refined in a laboratory in Germany. The interior composition of each gyrorotor is homogeneous to within two parts in a million.On its surface, each gyroscope rotor is less than three ten-millionths of an inch from perfect sphericity. Thismeans that every point on the surface of the rotor is the exact same distance from the center of the rotor towithin 3x10 -7 inches.Here are two ways to imagine how smooth this is. First, compare the GP-B gyro rotor’s smoothness withanother smooth object—a compact disk. CD's and DVD's both appear and feel incredibly smooth. The pits onthe compact disk's surface, which carry the digital information, are less than 4/100,000ths of an inch deep (onemillionth of a meter). However, compared to the GP-B gyroscope, the surface of a CD is like sandpaper. Thebumps and valleys on the surface of the GP-B gyroscope are 100 times smaller than those on a CD. Viewed atthe same magnification, one could barely see any imperfections on the gyroscope's surface.Alternatively, imagine a GP-B gyroscope enlarged to the size of the Earth. On Earth, the tallest mountains, likeMount Everest, are tens of thousands of feet high. Likewise, the deepest ocean trenches are tens of thousands offeet deep. By contrast, if a GP-B gyroscope were enlarged to the size of the Earth, its tallest mountain or deepestocean trench would be only eight feet!Finally, the gyroscope is freed from any mechanical bearings or supports by levitating the spherical rotor withina fused quartz housing. Six electrodes evenly spaced around the interior of the housing keep the rotor floating inthe center. A stream of pure helium gas spins the gyroscopes up to 4,000 rpm or faster. After that, and with allbut a few molecules of the helium spin-up gas evacuated from the housings, each gyroscope spins, a mere 32microns (0.001 inches) from its housing walls, free from any interfering supports.3.2.1.2 Spinning SuperconductivityEach Gravity Probe B gyroscope is nearly perfectly spherical and nearly perfectly homogenous. While thisensures that the gyroscope will spin with near-perfect stability, its “near-perfectness” creates a dauntingchallenge—we cannot mark the gyroscope to see exactly which direction its spin axis is pointing.70 March 2007 Chapter 3 — Accomplishments & Technology Innovations