Gravity Probe B (
GP-B) is a
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
-based mission which launched on 20 April 2004. The spaceflight phase lasted until 2005, and data analysis is expected to continue through 2010. Its aim is to measure
spacetime curvatureIn physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space being three-dimensional and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions...
near
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, and thereby the
stress-energy tensorThe stress-energy tensor is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and non-gravitational force fields...
(which is related to the distribution and the motion of matter in space) in and near Earth. This will provide a test of
general relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics. It unifies special relativity and Newton's law of universal gravitation, and describes gravity as a...
and related models.
Initial results confirmed the expected
geodetic effectThe geodetic effect represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body...
to an accuracy of about 1%. The expected
frame-draggingAlbert Einstein's theory of general relativity predicts that rotating bodies drag spacetime around themselves in a phenomenon referred to as frame-dragging. The rotational frame-dragging effect was first derived from the theory of general relativity in 1918 by the Austrian physicists Josef Lense...
effect was similar in magnitude to the current
noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
level (the noise being dominated by initially unmodeled effects). Work is continuing potentially to as late as March 2010, to model and account for these sources of unintended signal, thus permitting extraction of the frame-dragging signal if it exists at the expected level. By August 2008 the uncertainty in the frame-dragging signal had been reduced to 15%, and the December 2008 NASA report indicated that the geodetic effect was confirmed to better than 0.5%.
Overview
Gravity Probe B was a relativity
gyroscopeA gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. A mechanical gyroscope is essentially a spinning wheel or disk whose axle is free to take any orientation...
experiment funded by
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
. Efforts were led by
Stanford UniversityThe Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university located in Stanford, California, United States...
physics department with
Lockheed MartinLockheed Martin is a multinational aerospace manufacturer, global security and advanced technology company formed in 1995 by the merger of Lockheed with Martin Marietta. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area. Lockheed Martin employs 146,000 people...
as the primary subcontractor. Mission scientists view it as the second gravity experiment in space, following the successful launch of
Gravity Probe AGravity Probe A was a space-based experiment to test the theory of general relativity, performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration...
(GP-A) in 1976.
Some preliminary results were presented at a special session during the
American Physical SocietyThe American Physical Society was founded in 1899 and is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen science journals, including the world renowned Physical Review andPhysical Review Letters, and...
meeting, 14–17 April 2007. NASA initially requested a proposal for extending the GP-B data analysis phase through December 2007. The data analysis phase was further extended to September 2008, and possibly March 2010, when definitive science results on the
frame-draggingAlbert Einstein's theory of general relativity predicts that rotating bodies drag spacetime around themselves in a phenomenon referred to as frame-dragging. The rotational frame-dragging effect was first derived from the theory of general relativity in 1918 by the Austrian physicists Josef Lense...
effect are expected. The mission plans were to test two unverified predictions of general relativity: frame-dragging and the
geodetic effectThe geodetic effect represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body...
.
The experiment planned to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 650
kmThe kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second....
(400 mi) altitude, crossing directly over the poles. The gyroscopes were so free from disturbance that they provided a near-perfect space-time reference system. They were intended to measure how space and time are "warped" by the presence of the Earth, and by how much the Earth's rotation "drags" space-time around with it. This is the so-called frame-dragging effect, an example of
gravitomagnetismGravitomagnetism , refers to a set of formal analogies between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein field equations for general relativity...
. It is an analog of
magnetismIn physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
in classical electrodynamics, only this time caused by rotating masses rather than rotating electric charges.
Previously, only two analyses of the laser-ranging data obtained by the two
LAGEOSLAGEOS, or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth.-Function and operation:...
satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, whereas Gravity Probe B aims to measure the effect to a precision of 1%. However, subsequent analysis showed that the level of total uncertainty of the tests conducted with the two LAGEOS satellites has likely been greatly underestimated. A recent analysis of
Mars Global SurveyorThe Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on , the...
data has claimed to have confirmed the effect to a precision of 0.5%, although the accuracy of this claim is disputed.
Also the Lense–Thirring effect of the Sun has been recently investigated in view of a possible detection with the inner planets in the near future.
The probe has also detected the so-called
geodetic effectThe geodetic effect represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body...
, a much larger effect caused by space-time being 'curved' by the mass of the Earth. A gyroscope's axis when
parallel transportIn geometry, parallel transport is a way of transporting geometrical data along smooth curves in a manifold. If the manifold is equipped with an affine connection , then this connection allows one to transport vectors of the manifold along curves so that they stay parallel with respect to the...
ed around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. The angle 'missing' may be thought of as the amount the gyroscope 'leans over' into the slope of the space-time curvature. A more precise explanation for the space curvature part of the geodetic precession is obtained by using a nearly flat cone to model the space curvature of the Earth's gravitational field. Such a cone is made by cutting out a thin 'pie-slice' from a circle and gluing the cut edges together. The spatial geodetic precession is a measure of the missing 'pie-slice' angle. Gravity Probe B should measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.
The launch was planned for 19 April 2004 at
Vandenberg Air Force BaseVandenberg Air Force Base is a United States military installation with a spaceport, in Santa Barbara County, California, United States. It is also a census-designated place with a population of 6,151 as of the 2000 census. The base is named in honor of former Air Force Chief of Staff General...
but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On 20 April, at 9:57:23 AM
PDTA time zone is a region of the earth that has uniform standard time, usually referred to as the local time. By convention, time zones compute their local time as an offset from UTC . Local time is UTC plus the current time zone offset for the considered location.-Introduction:Time zones are...
(16:57:23
UTCCoordinated Universal Time , ) is a time standard based on International Atomic Time with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation...
) 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 lasted 16 months.
Experimental setup
The
Gravity Probe B experiment comprises four gyroscopes and a reference
telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
sighted on HR8703 (also known as
IM PegasiIM Pegasi is a variable binary star system approximately 329 light-years away in the constellation of Pegasus. With an apparent magnitude of 5.65, it is visible to the naked eye. The wide public awareness of it, however, is due to its use as the guide star for the Gravity Probe B general...
), a
binary starA binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
in the constellation
PegasusPegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
. In
polar orbitA polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the equator...
, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.
The gyroscopes are housed in a dewar of
superfluidSuperfluidity is a phase of matter or description of heat capacity in which unusual effects are observed when liquids, typically of helium-4 or helium-3, overcome friction by surface interaction when at a stage at which the liquid's viscosity becomes zero...
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
, maintaining a temperature of under 2 kelvins . Near-
absolute zeroAbsolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible and cannot be reached by artificial or natural means, because it is impossible to decouple a system fully from the rest of the universe...
temperatures are required in order to minimize molecular interference, and enable the
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
and
niobiumNiobium , or columbium , is the chemical element with the symbol Nb and the atomic number 41...
components of the gyroscope mechanisms to become
superconductiveSuperconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral...
.
At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s (less than ). If scaled to the size of the earth, the tallest mountains would be high. They are composed of
fused quartzFused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...
and coated with an extremely thin layer of
niobiumNiobium , or columbium , is the chemical element with the symbol Nb and the atomic number 41...
. A primary concern is minimizing any influence on their spin, so the gyroscopes must never touch their containing compartment. They are held suspended with electric fields, spun up using a flow of helium gas, and their spin axes are sensed by monitoring the magnetic field of the superconductive niobium layer with
SQUIDSuperconducting Quantum Interference Devices are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions....
s. (A spinning superconductor generates a magnetic field precisely aligned with the rotation axis - see
London momentThe 'London moment' is a quantum-mechanical phenomenon whereby a spinning superconductor generates a magnetic field whose axis lines up exactly with the spin axis....
.)
IM PegasiIM Pegasi is a variable binary star system approximately 329 light-years away in the constellation of Pegasus. With an apparent magnitude of 5.65, it is visible to the naked eye. The wide public awareness of it, however, is due to its use as the guide star for the Gravity Probe B general...
was chosen as the guide star 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
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
of the
sky coordinatesThe Equatorial Coordinate System is a popular method of mapping celestial objects. It functions by projecting the Earth's geographic poles, equator, and ecliptic onto the celestial sphere...
. Also important was its well understood motion in the sky, which was helped by the fact that this star emits relatively strong
radio signalsRadio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, but subsequent advances have identified a number of different sources of radio emission...
. As a preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over the last few years to understand its motion as precisely as needed.
History
The conceptual design for this mission was first proposed by an MIT professor, George Pugh, who was working with the U.S. Department of Defense in 1959 and later discussed by Leonard Schiff (Stanford) in 1960 at Pugh's suggestion. It was proposed to NASA in 1961, and they supported the project with funds in 1964. This grant ended in 1977 after a long phase of
engineeringEngineering is the discipline, art and profession of acquiring and applying technical, scientific and mathematical knowledge to design and implement materials, structures, machines, devices, systems, and processes that safely realize a desired objective or inventions.The American Engineers' Council...
research into the basic requirements and tools for the satellite.
In 1986 NASA changed plans for the
shuttleThe Space Shuttle, part of the Space Transportation System , is a spacecraft operated by NASA for orbital human spaceflight missions. It began operations in the 1980s and is scheduled to be retired from service in 2010 after 134 launches...
, 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.
Mission timeline
This is a list of major events for the GP-B experiment.
- 20 April 2004: Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
- 27 August 2004: GP-B entered its science phase. On mission day 129 all systems were configured to be ready for data collection, with the only exception being gyro 4, which needed further spin axis alignment.
- 15 August 2005: The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.
- 26 September 2005: The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of the last of the liquid helium.
- February 2006: Phase I of data analysis complete
- September 2006: Analysis team realised that more error analysis, particularly around the polhode motion of the gyros, was necessary than could be done in the time to April 2007, and applied to NASA for an extension of funding to the end of 2007.
- December 2006: Completion of Phase III of data analysis
- 14 April 2007: Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the American Physical Society
The American Physical Society was founded in 1899 and is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen science journals, including the world renowned Physical Review andPhysical Review Letters, and...
announcing initial results: "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." — Gravity Probe B website
Future
On 9 February 2007, it was announced that a number of unexpected signals had been received and that these would need to be separated out before final results could be released. In April it was announced that the spin axes of the gyroscopes were affected by torque, in a manner that varied over time, requiring further analysis to allow the results to be corrected for this source of error. Consequently, the date for the final release of data has been pushed back from April 2007 to December 2007, and subsequently to September 2008, and possibly March 2010. In the data for the frame-dragging results presented at the April 2007 meeting of the American Physical Society, the random errors were much larger than the theoretical expected value and scattered on both the positive and negative sides of a null result, therefore causing skepticism on whether any useful data could be extracted in the future to test this effect.
In June 2007, a detailed update was released explaining the cause of the problem, and the solution that was being worked on. Although electrostatic patches caused by non-uniform coating of the spheres was anticipated, and was thought to have been controlled for before the experiment, it is now known that the final layer of the coating on the spheres defined two halves of slightly different potential, which gave the sphere an electrostatic axis. This created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect. In addition, it dissipated energy from the
polhode motionThe details of a spinning body may impose restrictions on the motion of its angular velocity vector, ω. The curve produced by the angular velocity vector on the inertia ellipsoid, is known as the polhode, coined from Greek meaning "path of the pole"...
by inducing currents in the housing electrodes, causing the motion to change with time. This meant that a simple time-average polhode model was insufficient, and a detailed orbit by orbit model was needed to remove the effect. As it was anticipated that "anything could go wrong", the final part of the flight mission was calibration, where amongst other activities, data was gathered with the spacecraft axis deliberately mis-aligned for 24 hours, to exacerbate any potential problems. This data proved invaluable for identifying the effects. With the electrostatic torque modelled as a function of axis misalignment, and the polhode motion modelled at a sufficiently fine level, it is hoped to isolate the relativity torques to the originally expected resolution.
Stanford has agreed to release the raw data to the public at an unspecified date in the future. It is likely that this data will be examined by independent scientists and independently reported to the public well after the September 2008 release. Because future interpretations of the data by scientists outside of GP-B may differ from the official results, it may take several more years for of all the data received by GP-B to be completely understood.
NASA review
A review by a panel of 15 experts commissioned by NASA has recommended against extending the data analysis phase beyond 2008. They warn that the required reduction in noise level (due to classical torques and breaks in data collection due to solar flares) "is so large that any effect ultimately detected by this experiment will have to overcome considerable (and in our opinion, well justified) skepticism in the scientific community".
Data Analysis after NASA
NASA funding and sponsorship of the program ended on 30 September 2008, but GP-B has secured alternative funding that will enable the science team to continue working at least through December 2009. On 29 August 2008, the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress The ensuing SAC report to NASA states:
See also
- Frame-dragging
Albert Einstein's theory of general relativity predicts that rotating bodies drag spacetime around themselves in a phenomenon referred to as frame-dragging. The rotational frame-dragging effect was first derived from the theory of general relativity in 1918 by the Austrian physicists Josef Lense...
- Geodetic effect
The geodetic effect represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body...
- Gravity
- Gravitomagnetism
Gravitomagnetism , refers to a set of formal analogies between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein field equations for general relativity...
- Tests of general relativity
At its introduction in 1915, the general theory of relativity did not have a solid empirical foundation. It was known that it correctly accounted for the "anomalous" precession of the perihelion of Mercury and on philosophical grounds it was considered satisfying that it was able to unify Newton's...
- Timeline of Gravitational Physics
Timeline of gravitational physics and general relativity* 800s - Ja'far Muhammad ibn Mūsā ibn Shākir hypothesizes that the heavenly bodies and celestial spheres are subject to the same laws of physics as Earth, unlike the ancients who believed that the celestial spheres followed their own set of...
External links