Is Einstein's general theory of relativity correct? Testing two extraordinary predictions of that theory is the goal of Gravity Probe B, or GP-B, a satellite that just marked its first anniversary in space. Published in 1916 and hailed as one of the most brilliant creations of the human mind, Albert Einstein's general theory of relativity weaves together space and time into a four-dimensional "fabric," called spacetime, and defines gravity as the warping and twisting of spacetime by massive objects.
With this theory, Einstein forever changed our notions of space, time, and gravity. And although it has become one of the cornerstones of modern physics, general relativity is the least tested of Einstein's theories. While the warping and twisting of spacetime is calculated to be enormous in the neighborhood of ultra-massive black holes, these effects are minuscule and difficult to measure in the vicinity of a tiny celestial object such as our Earth.
The GP-B spacecraft uses four ultra-precise, spherical gyroscopes to experimentally test, with unprecedented precision, two specific effects predicted by Einstein's theory: 1) the geodetic effect--the amount by which the Earth warps local spacetime in which it resides, and 2) the frame-dragging effect--the amount by which the rotating Earth drags local spacetime around with it. At the one-year mark the spacecraft continues to perform very well. The four on board gyroscopes have experienced and measured relativistic effects for almost eight months. After completing the mission and thoroughly analyzing the data the team plans to release the final results in the summer of 2006.
One year ago, on April 20, 2004, GP-B was launched into a nearly perfect polar orbit from Vandenberg Air Force Base, Calif. atop a Boeing Delta II launch vehicle. During a four-month initialization phase, the spacecraft underwent a complete checkout and optimization of all systems. Over 10,000 commands were successfully executed by on-board computers. The four gyroscopes were spun up to their final speeds, averaging 72 Hz, or 4,300 rpm, and their spin axes were aligned with the GP-B guide star -- IM Pegasi/HR 8703. The GP-B team began the science phase of the mission, collecting data on the changing spin axis orientation of the four gyros that will ultimately confirm or disprove the geodetic and frame-dragging predictions of general relativity.
"Developing GP-B was a supreme challenge, requiring the skillful integration of an extraordinary range of new technologies. It could never have been done without the kind of unique long-term collaboration we have had between Stanford, NASA, and Lockheed Martin," said GP-B Principal Investigator, Francis Everitt, of Stanford University. "It is wonderful to be at the stage we are, with the experiment working so well and proceeding steadily with the analysis of the science data."
It is fitting that the completion of the Gravity Probe B experiment, the most rigorous test to date of general relativity, will come to fruition this year--the 100th anniversary of Einstein's "miracle" year, in which he published four seminal papers, including the special theory of relativity and his paper on the production and transformation of light, for which he was awarded the Nobel Prize in 1921. "The GP-B team has shown that through hard work and sustained effort, great things can be accomplished", said Tony Lyons, NASA's GP-B program manager at the Marshall Space Flight Center in Huntsville, Ala. "The spacecraft keeps getting better as we get farther into the mission, and that's a tribute to the hard work of our excellent team."
The GP-B team is in the process of measuring the amount of liquid helium remaining in the spacecraft's dewar. Shortly before the helium runs out, the team plans to perform a series of instrument calibrations. "The purpose of the calibration phase is to ensure data accuracy and analysis integrity prior to releasing results," says Mac Keiser, project chief scientist.
Was Einstein Correct? Mac Keiser is not saying. Project policy maintains that the program will not release scientific results obtained during the mission until after the data analysis is completed next year.
NASA's Marshall Space Flight Center manages the GP-B program. NASA's prime contractor for the mission, Stanford University, conceived the experiment and is responsible for the design and integration of the science instrument, as well as for mission operations and data analysis. Lockheed Martin, a major subcontractor, designed, integrated and tested the space vehicle and built some of its major payload components. NASA's Kennedy Space Center, Fla., and Boeing Expendable Launch Systems, Huntington Beach, Calif., was responsible for the launch of the Delta II.
More information about the Gravity Probe B mission is available at: