From the archive, originally posted by: [ spectre ]
http://www.nasa.gov/mission_pages/gpb/index.html
http://space.newscientist.com/article.ns?id=dn11615&feedId=online-news_rss20
Gravity probe measures Earth’s dent in space-time
16 April 2007
A NASA mission that took 40 years to get off the drawing board has
finally measured how the Earth dents the fabric of space-time.
The first result from the Gravity Probe B satellite confirms a
prediction of Einstein’s general theory of relativity to a precision
of better than 1%. “For the first time, we have seen one of Einstein’s
effects directly,” says mission leader Francis Everitt of Stanford
University in California, US.
Launched in April 2004, the satellite used four precision-engineered
gyroscopes to measure two effects predicted by Einstein’s theory of
general relativity.
One, called the geodetic effect, predicts that the Earth’s mass leaves
a dent in space-time that should tilt each gyroscope by 0.0018°, or
6606 milliarcseconds, over the course of a year. A second, more subtle
effect, called frame dragging, predicts how much the Earth’s rotation
drags space-time around with it.
Previously, astronomers have measured both effects by firing laser
beams at mirrors left on the Moon by the Apollo astronauts. “The
Moon’s orbit acts as a gyroscope,” says Clifford Will, an expert on
general relativity at Washington University in St Louis, Missouri, US.
“But lunar ranging provides indirect measurements. Gravity Probe B
provides a direct measure that’s unique and new.”
Now, the spacecraft team has reported a measurement of the geodetic
effect that falls within the value predicted by Einstein but is still
not as precise as the lunar laser experiments.
‘Glimpses’ of frame dragging
“The results aren’t quite what we’d hoped for at this stage,” admits
Bill Bencze who is programme manager for the mission. That is due in
part to a series of solar flares in March 2005 that interrupted the
satellite’s observations and will limit the final accuracy of the
experiment.
However, the biggest challenge for the mission team is to correct for
some unexpected torques on the gyroscopes that change their
orientation and can mimic relativistic effects. Bencze is confident
that by December the team will understand these effects well enough to
improve the precision of their measurement by a factor of 20,
eventually achieving an accuracy of better than 0.01%.
So far, Everitt says the team has seen “glimpses” of frame dragging
but is not yet in a position to report a figure for the effect. They
hope to do that in December as well. “Frame dragging is the name of
the game,” says Will. “Question is: what will GP-B achieve at the end
of the day?”
The predictions of general relativity fall well within the probe’s
precision, though that might change when the team announces far more
accurate results. “It’s not a done deal that general relativity is
safe,” says Bencze, who points out that the team’s job is to make the
best measurements, rather than confirm general relativity. “If
Einstein is right, good for him,” says Bencze. “If not, too bad.”
The results were presented over the weekend at a meeting of the
American Physical Society in Jacksonville, Florida, US.
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http://science.nasa.gov/headlines/y2000/geodetic.htm
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WAS EINSTEIN RIGHT? SCIENTISTS PROVIDE FIRST PUBLIC PEEK AT GRAVITY
PROBE B RESULTS.
14 April 2007
“For the past three years a satellite has circled the Earth,
collecting data to determine whether two predictions of Albert
Einstein’s general theory of relativity are correct. Today, at the
American Physical Society (APS) meeting in Jacksonville, Fla.,
Professor Francis Everitt, a Stanford University physicist and
principal investigator of the Gravity Probe B (GP-B) Relativity
Mission, a collaboration of Stanford, NASA and Lockheed Martin, will
provide the first public peek at data that will reveal whether
Einstein’s theory has been confirmed by the most sophisticated
orbiting laboratory ever created.
“Gravity Probe B has been a great scientific adventure for all of us,
and we are grateful to NASA for its long history of support,” said
Everitt. “My colleagues and I will be presenting the first results
today and tomorrow. It’s fascinating to be able to watch the Einstein
warping of spacetime directly in the tilting of these GP-B gyroscopes –
more than a million times better than the best inertial navigation
gyroscopes.”The GP-B satellite was launched in April 2004. It
collected over a year’s worth of data that the Stanford GP-B science
team has been poring over for the past 18 months. The satellite was
designed as a pristine, space-borne laboratory, whose sole task was to
use four ultra-precise gyroscopes to measure directly two effects
predicted by general relativity. One is the geodetic effect-the amount
by which the mass of the Earth warps the local space-time in which it
resides. The other effect, called frame-dragging, is the amount by
which the rotating Earth drags local space-time around with it.
According to Einstein’s theory, over the course of a year, the
geodetic warping of Earth’s local space-time causes the spin axes of
each gyroscope to shift from its initial alignment by a minuscule
angle of 6.606 arc-seconds (0.0018 degrees) in the plane of the
spacecraft’s orbit. Likewise, the twisting of Earth’s local space-time
causes the spin axis to shift by an even smaller angle of 0.039 arc-
seconds (0.000011 degrees) – about the width of a human hair viewed
from a quarter mile away – in the plane of the Earth’s equator. GP-B
Scientists expect to announce the final results of the experiment in
December 2007, following eight months of further data analysis and
refinement. Today, Everitt and his team are poised to share what they
have found so far-namely that the data from the GP-B gyroscopes
clearly confirm Einstein’s predicted geodetic effect to a precision of
better than 1 percent. However, the frame-dragging effect is 170 times
smaller than the geodetic effect, and Stanford scientists are still
extracting its signature from the spacecraft data. The GP-B instrument
has ample resolution to measure the frame-dragging effect precisely,
but the team has discovered small torque and sensor effects that must
be accurately modeled and removed from the result.
“We anticipate that it will take about 8 more months of detailed data
analysis to realize the full accuracy of the instrument and to reduce
the measurement uncertainty from the 0.1 to 0.05 arc-seconds per year
that we’ve achieved to date down to the expected final accuracy of
better than 0.005 arc-seconds per year,” says William Bencze, GP-B
Program Manager. “Understanding the details of this science data is a
bit like an archeological dig: a scientist starts with a bulldozer,
follows with a shovel, and then he finally uses dental picks and
toothbrushes to clear the dust away from the treasure. We are passing
out the toothbrushes now.”
The two discoveries
Two important discoveries were made while analyzing the gyroscope data
from the spacecraft: 1) the “polhode” motion of the gyroscopes damps
out over time, and 2) the spin axes of the gyroscopes were affected by
small classical torques. Both of these discoveries are symptoms of a
single underlying cause: electrostatic patches on the surface of the
rotor and housing. Patch effects in metal surfaces are well known in
physics, and were carefully studied by the GP-B team during the design
of the experiment to limit their effects. Though previously understood
to be microscopic surface phenomena that would average to zero, the GP-
B rotors show patches of sufficient size to measurably affect the
gyroscopes’ spins.
The gyroscope’s polhode motion is akin to the common “wobble” seen on
a poorly thrown (American) football, though it shows up in a much
different form for the ultra-spherical GP-B gyroscopes. While it was
expected that this wobble would exhibit a constant pattern over the
mission, it was found to slowly change due to minute energy
dissipation from interactions of the rotor and housing electrostatic
patches. The polhode wobble complicates the measurement of the
relativity effects by putting a time-varying wobble signal into the
data.
The electrostatic patches also cause small torques on the gyroscopes,
particularly when the space vehicle axis of symmetry is not aligned
with the gyroscope spin axes. Torques cause the spin axis of the
gyroscopes to change orientation, and in certain circumstances, this
effect can look like the relativity signal GP-B measures. Fortunately,
the drifts due to these torques has a precise geometrical relationship
to the misalignment of the gyro spin/vehicle symmetry axis and can be
removed from the data without directly affecting the relativity
measurement.
Both of these discoveries first had to be investigated, be precisely
modeled and then be carefully checked against the experimental data
before they are removed as sources of error. These additional
investigations have added more than a year to the data analysis, and
this work is still in process. To date, the team has made very good
progress in this regard, according to its independent Science Advisory
Committee, chaired by relativistic physicist Clifford Will of
Washington University in St. Louis, Mo., that has been monitoring
every aspect of GP-B for the past decade.
In addition to providing a first peek at the experimental results at
the APS meeting, the GP-B team has released an archive of the raw
experimental data. The data will be available through the National
Space Sciences Data Center at the NASA Goddard Space Flight Center
beginning in June 2007.
Conceived by Stanford Professors Leonard Schiff, William Fairbank and
Robert Cannon in 1959 and funded by NASA in 1964, GP-B is the longest
running, continuous physics research program at both Stanford and
NASA. While the experiment is simple in concept – it utilizes a star,
a telescope and a spinning sphere – it took more than four decades and
$760 million to design and produce all the cutting-edge technologies
necessary to bring the GP-B satellite to the launch pad, carry out
this “simple” experiment and analyze the data. On April 20, 2004, GP-B
made history with a perfect launch from Vandenberg Air Force Base in
California. After a four-month initialization and on-orbit check-out
period, during which the four gyroscopes were spun up to an average of
4,000 rpm and the spacecraft and gyro spin axes were aligned with the
guide star, IM Pegasi, the experiment commenced. For 50 weeks, from
August 2004 to August 2005, the spacecraft transmitted more than a
terabyte of experimental data to the GP-B Mission Operations Center at
Stanford. One of the most sophisticated satellites ever launched, the
GP-B spacecraft performed magnificently throughout this period, as did
the GP-B Mission Operations team, comprised of scientists and
engineers from Stanford, NASA and Lockheed Martin, said Stanford
Professor Emeritus Bradford Parkinson, a co-principal investigator
with John Turneaure and Daniel DeBra, also emeritus professors at
Stanford. The data collection ended on Sept. 29, 2005, when the helium
in spacecraft’s dewar was finally exhausted. At that time, the GP-B
team transitioned from mission operations to data analysis.
Over its 47-year lifetime, GP-B has advanced the frontiers of
knowledge, provided a training ground for 79 doctoral students at
Stanford (and 13 at other universities), 15 masters degrees, hundreds
of undergraduates and dozens of high school students who worked on the
project. In addition, GP-B spawned over a dozen new technologies,
including the record-setting gyroscopes and gyro suspension system,
the SQUID (for Superconducting QUantum Interference Device) gyro
readout system, the ultra-precise star-pointing telescope, the
cryogenic dewar and porous plug, the micro-thrusters and drag-free
technology and the Global Positioning System-based orbit determination
system. All of these technologies were essential for carrying out the
experiment, but none existed in 1959 when the experiment was
conceived. Furthermore, some technologies which were designed at
Stanford for use in GP-B, such as the porous plug that controlled the
escape of helium gas from the dewar, enabled and were used in other
NASA experiments such as COBE (the COsmic Background Explorer, which
won this year’s Nobel prize) WMAP (for Wilkinson Microwave Anisotropy
Probe) and the Spitzer Space Telescope.
The experiment’s final result is expected on completion of the data
analysis in December of this year. Asked for his final comment,
Francis Everitt said: “Always be suspicious of the news you want to
hear.”
NASA’s Marshall Space Flight Center manages the GP-B program and
contributed significantly to its technical development. 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, Stanford’s major subcontractor, designed,
integrated and tested the spacecraft and built some of its major
payload components, including the dewar and probe that houses the
science instrument. NASA’s Kennedy Space Center, Fla., and Boeing
Expendable Launch Systems, Huntington Beach, Calif., was responsible
for the launch of the Delta II.”