EIU Astro

NASA Gravity Probe B July 19, 2009

Amy Brown has her blog at: http://neogajrhscience.wordpress.com/

Gravity probe B is a NASA mission first proposed in 1959 that was launched into space April 20, 2004. The probe contained a very accurate tracking telescope, and 4 gyroscopes. Its purpose was to test Einstein’s general theory of relativity, by measuring the amount of warp Earth causes in its surrounding spacetime (the geodetic effect) and the amount that Earth drags its local spacetime along as it rotates (the frame dragging effect). Data collection from the probe was completed in August, 2005, and data analysis has continued to the current date.

History of the Gravity Probe B Mission

Albert Einstein proposed his theory of general relativity in 1916, which linked the concepts of geometry and time with gravity.  Gravity, as we understood it from Isaac Newton, was an attractive force between bodies due to their mass.  Einstein proposed that, instead, gravity was a manifestation of the warping of spacetime around a body, which is also related to the body’s mass.  To visualize this warping of spacetime, imagine a bowling ball placed in the center of the fabric of a trampoline.  The mass of the bowling ball will pull the fabric down, warping the fabric in three dimensions.  The bowling ball, of course, is compared to any object in space, and the more massive the object, the greater the warp.

General relativity has stood up to several types of tests.  One of these involves the observational evidence of the precession of the perihelion of mercury, which shifts at the rate of 43 arc seconds per century.  After all other influencing factors have been accounted for, this shift is attibutable to the effect of general relativity from the mass of the sun.  Another type of test shows that light from distant objects bends as it travels past massive objects, such as the sun.  This has been measured both with visible light, and more accurately with radio waves. Gravitaional redshift is another method that has supported general relativity.  This measures the energy and time difference in objects at different positions in relation to earth.  GPS satellites must account for the difference in 38 microseconds per day from the height they are orbiting to the surface of the earth.  While these and other tests have provided substantial evidence to support general relativity, the evidence is not as precise as physicists would like it to be.  Scientists were striving to devise a way to test general relativity on a precision basis.

In 1959, Stanford Physics Departmment Chair Leonard Schiff and MITphysicist George Pugh both independently proposed testing general relativity using gyroscopes.  Schiff went forward with the idea, bringing on board other Stanford professores William Little, William Fairbank, and Robert Cannon.  Schiff, Fairbank and Cannon continued to research the idea from different angles, and this research led to a proposal to NASA in 1962.  NASA adopted the Gravity Probe project  in 1964, and Stanford remained the primary project base.

The concept of the Gravity Probe B

The idea behind Gravity Probe B was to construct a space probe containing gyroscopes aligned to a distant space object.  The spacecraft would surroound the gyroscopes, allowing them to remain in freefall.  As the spacecraft orbits the earth, any warping effect of the spacetime around the earth would cause a measurable orientation shift of the spinning gyroscopes.  This was to be measured in regard to two effects:  the geodetic effect, which is the simple warping of spacetime due to the earth’s mass, and the frame shifting effect, which is the effect caused by earth dragging spacetime along as it rotates.

The idea of the probe was a simple one, but the technology required was not.  More than a dozen new technologies had to be developed to make the probe work, and this took over 30 years to accomplish.  The spheres that make up the four gyroscopes hold a guiness world record as the roundest objects ever made, and required the invention of new manufacturing techniques to complete them.  They are made of quartz, refined to be homogeneous to within two parts in a million, and the sphericity is accurate to within 3 ten millionths of an inch. The spheres are coated with superconducting niobium.

Rotors

The gyroscopes are housed within a suspension system that is only 32 microns larger in radius than each gyroscope. Also attached to the housing is a SQUID magnetometer, which measures the tilt of the gyroscope spinning within as its magnetic field interacts with the sensor.  The satellite itself contains a nine foot long dewar (a large thermos) to contain the superchilled helium necessary to maintain the correct temperature to have the superconductive gyroscopes work properly.

Dewar

In order to combat the small amount of heat that would enter the dewar, a special plug had to be designed to allow helium condensate to seep out into the outer layer.
In the late 1970’s and early 1980’s, the probe underwent a changeover form a research project to a flight mission project.  Lockheed Martin was brought in to help with the design.  It wasn’t until the late 1990’s, however, that the project was brought directly to NASA as a definite flight program.  It took nearly seven years to work out all the bugs.  Gravity Probe B was launched into orbit on April 20, 2004.

Launch of spacecraft

The Mission

In a nutshell, the spacecraft that took over thirty years to design and launch was going to test the general theory of relativity.  The spacecraft contains a tracking telescope.  This telescope is pointed at a distant star, IM Pegasi, as a guide star. A quasar would be the desirable tracking object, but the telescope would not be able to stay focused on one, so IM Pegasi was used, and its position would then be compared to a distant quasar during data analysis.  Once the telescope locks onto the position of the guide star, the gyroscopes are caused to sart spinning, and their alignment is matched to the alignmnet of the telescope. As the gyroscopes continue to spin, and the spacecraft orbits the earth, electrical signals between the gyroscopes and sensors in their housings are measured and sent back ot earth as raw data.

After the succesful launch, Gravity Probe B was in orbit 642km above the Earth.  Before the probe could begin collecting data, a four month period of initialization and check out was accomplished.  This period was supposed to be shorter, but several problems had to be corrected or accounted for before data collection could begin.  One problem was that the spacecraft had trouble tracking the starfield due to the roll of the craft. Another problem was the loss of two of the sixteen helium thrusters.  Setting the gyroscopes to spinning and aligning their spin axes with the guide star also caused some delay.  The gyroscopes were expected to spin at a faster rate than they actually were spinning, so many adjustments and calculations had to be made on the ground to achieve alignment.  One further delay during initialization occured whern the probe passed over the Earth’s south pole, and was bombarded by proton radiation from the sun.  The delay was caused by one of the spacecraft’s computers going down and having to be rebooted after the proton bombardment.  Because the initialization phase took quite a bit longer than anticipated, the decision was made to allow the data colection phase to be shortened.  The spacecraft continued to send data until August 15, 2005. The remaining six weeks until the helium was depleted and the mission was ended on September 29, 2005 were spent claibrating and testing the equipment on the spacecraft.

Data Analysis

Scientists associated with the Gravity Probe B mission have been analyzing the data since 2005.  In the ideal scenario, every instrument on the spacecraft would have performed without complication, and staightforward data  would have been provided.  Some of the systems on the probe functioned very well.  The dewar and the telescope performed exactly as expected.  Unfortunately, the gyroscopes did not.  The spheres themselves did spin extremely predictably, but the magnetic fields that they produced as they did so have been difficult to analyze.  The spin axes of the gyroscopes were effected by the torque of the spacecraft, and scientists have been trying to account for the data anamolies by identifying and quantifying them.  In terms of the two phases of data, the geodetic effect jumped out obviously, even from the raw data.  The measurement of the warping of space around earth was calculated by the data to be within 1% of the predicted 6606 milliarcseconds/year.   It is the measurement of the frame-shifting, however that is more effected by the data problems.  NASA has closed the project, but other funding sources are allowing the data analysis to continue.  Scientists with the project predict that with further analysis, they will be able to get the frame shifting data to within 3 to 5 percent of the expected 39 milliarcseconds per year.

The Legacy of Gravity Probe B

Regardless of the scientific outcome of the Gravity probe itself, the thirty year life of this research and space flight mission has provided the world with valuable benefits.  Ninety seven students received PhDs at Stanford and other universities working on this  project.  Technologies developed for the spacecraft have been used in other applications, such as the optical bonding and fused quartz technologies used on the gyroscopes.  Photo diode detector technology has helped to improve digital cameras for all of us.  The porous helium plug developed for Gravity Probe G has been used in other cryogenically sensitive missions such as IRAS and COBE.  Further, the attitude control technology in the spacecraft led to more accurate (1 centimeter) GPS now being used for automatic aircraft landing and automatic precision farming.  Scientists and teams associated with Gravity Probe B have won several awards, including the 2005 NASA group achievemnet award given to the whole team.  Gravity Probe B will remain into the future as one of the most memorable NASA missions in the history of the space program.

Further Information

The information contained in this article was obtained form the following sources:

http://einstein.stanford.edu/

http://www.nasa.gov/mission_pages/gpb/index.html

http://en.wikipedia.org/wiki/Gravity_Probe_B