April 16, 2007
Beyond theory, space mission pushes frontiers of human possibility, engineering
By Annie Jia
When Gravity Probe B (GP-B) was conceived in 1959, much of the technology needed to conduct the experiment did not exist. An unprecedented level of technological precision was needed to measure the curvature of space, and it took more than three decades for scientists and engineers to create more than a dozen technologies needed to make their vision a reality.
"The number of new technologies that has spawned spinoffs on this experiment is astounding, but probably the most important spinoff is the over 90 PhD theses that have been sponsored by this experiment, because that represents education," said Brad Parkinson, the mission's co-principal investigator and co-inventor of the Global Positioning System (GPS).
Following are just a few examples of the technologies GP-B research has spawned.
Gyroscopes steadier than an owl's eyes
To create wobble-free gyroscopes, scientists produced the world's most perfect spheres. Enlarged to the size of the Earth, the spheres would have mountains no more than 8 feet high. The gyroscopes are now recorded in the Guinness Database of World Records as the roundest manmade objects. They are surpassed in roundness by only one type of object in the entire universe: dense neutron stars.
World's best protractor
The width of a human hair viewed from a quarter of a mile awaythat is the tiny angle by which space-time around Earth was predicted to tilt GP-B's gyroscopes. To measure these minuscule angles, engineers had to develop sensors of astounding precision. The Superconducting QUantum Interference Device (SQUID) magnetometers use a superconducting niobium loop to measure tiny changes in magnetic field that result from the gyroscopes' shift. The devices are so sensitive that they can detect a magnetic field 10 trillion times smaller than Earth's.
A minivan-sized dewar, or thermos, filled with liquid helium surrounds the gyroscope assembly and keeps it at a cryogenic temperature near absolute zero. To maintain the experimental apparatus at a temperature of 1.8 Kelvin for 16 months in space, the dewar employed advanced technology to insulate, block space radiation and cool by evaporation.
Despite the dewar's incredible insulating abilities, a small amount of heat seeps in and turns some liquid helium into gas. A porous plug allows this "warm" gas to escape while keeping the liquid helium inside to cool the assembly through evaporation, just as sweating cools a person's skin. What's more, the porous plug helps fine-tune the position of the spacecraft. Escaping helium gas is directed through micro-thrusters that puff out amounts one-fiftieth the size of a person's breath to minutely shift and turn the spacecraft to achieve perfect alignment with its guide star. The porous plug has since been used in other NASA flights, including this year's Nobel Prize-winning COBE (for COsmic Background Explorer) mission.
World's best star tracker
Scientists chose a distant star as the fixed reference point for measuring the small angles of deflection of GP-B's gyroscopes. Because of the minuteness of the angles, the telescope's focus on the guide star had to be precise to one 10-millionth of an inch. A telescope lens alone would provide an image 10,000 times too rough, so GP-B engineers created a device to split the incoming telescope image of the star into two separate images, one for the horizontal alignment and one for the vertical. Each image was again divided in two. Then delicate sensors measured the amount of light in each half image, and the spacecraft's orientation was adjusted until the two halves were perfectly balanced, indicating that the star's perfect center had been found.
Gyro suspension system
With the gyroscopes spinning at 4,000 rpm, their suspension system used electrostatic fields to hold each rotor in a vacuum a mere paper's width from the walls of its housing. Any contact of the rotors with the walls would destroy the equipment. The gyro suspension system adjusts the rotors' positions 220 times a second to continually ensure perfect suspension.
It is customary to track the path of an orbiting spacecraft, not the path of the equipment inside it. GP-B's drag-free technology flips that paradigm and instead tracks the path of the equipment inside the spacecraft. Like gnats swarming around a person, the GP-B spacecraft "chases" one of four gyroscopes, which serves as the experiment's central mass. The spacecraft's body shields the gyroscope from outside disturbances, such as friction and magnetic fields, so that the gyroscope is affected only by gravity and orbits the Earth in perfect free-fall. The spacecraft itself, constantly disturbed by the harsh forces of space, adjusts its own position 10 times a second, based on information from the gyro suspension system, to remain perfectly centered around the drag-free gyroscope. Stanford's Aeronautics and Astronautics Department pioneered drag-free satellite technology in the early 1960s.
Scientists needed an accurate method to map GP-B's orbit because the distortion of space-time varies with location. Hence they adapted conventional GPS technology for GP-B's high speeds and rotating motion. In the process they discovered a way to enhance the standard GPS receiver to measure position down to the centimeter level. Precision GPS has since been adopted in automated tractors, aircraft landing systems and vehicles used in mining and building roads. "[Robotic farm tractors have] launched a market now valued at well over $100 million and growing," Parkinson said. "The productivity enhancements are startling, and clearly benefit many with less expensive food."
Annie Jia is a science-writing intern at the Stanford News Service.
This is a sidebar to a story about the presentation of Gravity Probe B's first results. Science-writing intern Annie Jia wrote this release. High-resolution images are posted at http://einstein.stanford.edu/pix/hires_graphics/.