Gravity Probe B
Gravity Probe B (GP-B) is a satellite-based mission to test Einstein's general theory of relativity. Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976.
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2 Experimental setup 3 History 4 See also 5 External links |
Overview
Gravity Probe B is a relativity gyroscope experiment funded by NASA. Efforts are being headed up by the Physics department at Stanford University with Lockheed Martin as the primary subcontractor. According to mission plans, it will test two unverified predictions of that theory:
The experiment plans to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 400 nautical mile (741 km) altitude directly over the poles. So free are the gyroscopes from disturbance that they should provide an almost perfect space-time reference system. They are intended to measure how space and time are "warped" by the presence of the Earth, and, more profoundly, if and how much the Earth's rotation "drags" space-time around with it; the so-called frame-dragging effect. Previously, only one analysis (1) of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997, claimed to have found the frame-dragging effect with an accuracy of about 20 percent.
When the mission is successfully completed, GPB will be the most precise attempt at verification of any effect predicted by general relativity.
The launch date was scheduled for April 19, 2004 at Vandenberg Air Force Base but was scrubbed due to changing winds in the upper atmosphere. On April 20 at 9:57:23 AM PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 AM (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission is planned to last 16 months.
The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi), a binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects come out at right angles, each gyroscope measuring both.
The gyroscopes are the most spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms. They are composed of quartz and coated with niobium.
The gyroscopes are housed in a dewar at a temperature of under 2 Kelvin (-271 degrees Celsius, -456 degrees Fahrenheit). Near absolute zero temperatures are required in order to minimize molecular interference.
This particular star was chosen for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions at the equator of the sky coordinates. Also important was a well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. As a preparation for the setup of this mission, astronomers analyzed the radio based position measurements taken over the last few years to understand its motion as precisely as needed.
The conceptual design for this mission was first proposed by Leonard Schiff (Stanford) and Edward E. Pugh (U.S. Department of Defense) (independently of each other) in 1959. It was proposed to NASA in 1961, and it supported the project with funds in 1964. This grant ended in 1977 after a long phase of engineering research into the basic requirements and tools for the satellite.
In 1986 NASA changed plans for the shuttle, which forced the mission team to switch from a shuttle-based launch design to one that is based on the Delta 2, and in 1995 tests planned of a prototype on a shuttle flight were cancelled as well.
Gravity Probe B marks the first time in history that a university has been in control of the development and operations of a space satellite funded by NASA.Experimental setup
History