Gravity Probe B with solar panels folded

Gravity Probe B (GP-B) is a satellite-based mission which launched in 2004. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. The spaceflight phase lasted until 2005, and data analysis is currently underway (as of May 2008) and may continue to into 2010^[1]. Its aim is to measure spacetime curvature near Earth, and thereby the stress-energy tensor (the distribution, and especially the motion, of matter) in and near Earth, and thus to test related models in application of Einstein's general theory of relativity. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The stress-energy tensor (sometimes stress-energy-momentum tensor is a Tensor quantity in Physics that describes the Density and Flux Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916

Initial results confirm the expected geodetic effect to an accuracy of about 1%. The geodetic effect represents the effect of the curvature of Spacetime, predicted by General relativity, on a spinning moving body The expected frame-dragging signal is similar in magnitude to the current noise level (the noise being dominated by currently unmodeled effects). Albert Einstein 's theory of General relativity predicts that rotating bodies drag Spacetime around themselves in a phenomenon referred to as frame-dragging Work is continuing through September 2008, and 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.

Overview

Gravity Probe B was a relativity gyroscope experiment funded by NASA. A gyroscope is a device for measuring or maintaining orientation, based on the principles of Angular momentum. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Efforts were headed up by the Physics department at Stanford University with Lockheed Martin as the primary subcontractor. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Leland Stanford Junior University, commonly known as Stanford University or simply Stanford, is a private Research university located in Lockheed Martin ( is a large multinational Aerospace manufacturer and advanced technology Company formed in 1995 by the merger of

Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976. Gravity Probe A (GP-A was a space-based experiment to test Einstein 's theory of General relativity performed jointly by the Smithsonian Astrophysical Observatory

Some preliminary results were presented at a special session during the American Physical Society (APS) meeting, 14-17 April 2007. The American Physical Society was founded in 1899 and is the World 's second largest organization of physicists behind the Deutsche Physikalische Gesellschaft. 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 dragging effect are expected. Albert Einstein 's theory of General relativity predicts that rotating bodies drag Spacetime around themselves in a phenomenon referred to as frame-dragging

The mission plans were to test two unverified predictions of general relativity:

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 km (400 statute miles) altitude and crossing directly over the poles. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States So free are the gyroscopes from disturbance that they provided a near-perfect space-time reference system. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS They were 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 or gravitomagnetism, a field generated by the rotation of Earth and similar to magnetism in electrodynamics. Albert Einstein 's theory of General relativity predicts that rotating bodies drag Spacetime around themselves in a phenomenon referred to as frame-dragging Gravitomagnetism (sometimes Gravitoelectromagnetism, abbreviated GEM) refers to a set of formal analogies between Maxwell's field In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials.

Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20 percent and 10 percent respectively,^[2][3] whereas Gravity Probe B aims to measure the effect to a precision of 1 percent. LIDAR ( Li ght D etection a nd R anging is an optical remote sensing technology that measures properties of scattered light to find range and/or LAGEOS, or Laser Geodynamics Satellites are a series of scientific research Satellites designed to provide an orbiting laser ranging benchmark for geodynamical A recent analysis of Mars Global Surveyor data has claimed to have confirmed the effect to a precision of 0. The Mars Global Surveyor ( MGS) was a US Spacecraft developed by NASA and the Jet Propulsion Laboratory and launched 5%^[4],although the accuracy of this claim is disputed^[5].

The probe has also detected the so-called geodetic effect, a much larger effect caused by space-time being 'curved' by the mass of the Earth. The geodetic effect represents the effect of the curvature of Spacetime, predicted by General relativity, on a spinning moving body A gyroscope's axis when parallel transported around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. In Geometry, parallel transport is a way of transporting geometrical data along smooth curves in a Manifold. 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 April 19, 2004 at Vandenberg Air Force Base but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. Events 1012 - Martyrdom of Alphege in Greenwich London. 1529 - At the Second Diet of Speyer "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Vandenberg Air Force Base is a United States military installation with a Spaceport, in Santa Barbara County, California, United States. 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 April 20 at 9:57:23 a. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII. m. PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 a. m. (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 most nearly perfect spheres ever created by humans. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe ^[6] A fused quartz gyroscope for the Gravity Probe B experiment which differs from a perfect sphere by no more than 40 atoms of thickness, refracting the image of Einstein in background. Fused quartz and fused silica are types of Glass containing primarily Silica in amorphous (non- Crystalline form A gyroscope is a device for measuring or maintaining orientation, based on the principles of Angular momentum. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny Refraction is the change in direction of a Wave due to a change in its Speed. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical

The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi^[7]), a binary star in the constellation Pegasus. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. IM Pegasi ( HR 8703 HD 216489 SAO 108231 HIP 112997 is a variable Binary star system estimated to be 329 Light A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Pegasus (ˈpɛɡəsəs is a northern Constellation, named after the mythological winged horse Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both. A polar orbit is an Orbit in which a Satellite passes above or nearly above both poles of the body (usually a planet such as the Earth, but possibly

The gyroscopes are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (−271 degrees Celsius, −456 degrees Fahrenheit). Superfluidity is a phase of matter or description of Heat capacity in which unusual effects are observed when Liquids, typically of Helium-4 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The Celsius Temperature scale was previously known as the centigrade scale. Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 Near-absolute zero temperatures are required in order to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive. Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41 Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance

The gyroscopes are the most nearly spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms (less than 10 nanometers). History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny If scaled to the size of the earth, the tallest mountains would be 2. 4 meters (eight feet) high. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit ^[8] They are composed of fused quartz and coated with an extremely thin layer of niobium. Fused quartz and fused silica are types of Glass containing primarily Silica in amorphous (non- Crystalline form Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41 The gyros' spin axes are sensed by monitoring the magnetic field of the superconductive niobium layer with SQUIDs. Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species

IM Pegasi 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 equator of the sky coordinates. The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the The equatorial coordinate system is probably the most widely used Celestial coordinate system, whose equatorial coordinates are Declination (\delta Also important was its well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. Radio astronomy is a subfield of Astronomy that studies celestial objects at radio frequencies. 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. The United States Department of Defense ( DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government Leland Stanford Junior University, commonly known as Stanford University or simply Stanford, is a private Research university located in 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 engineering research into the basic requirements and tools for the satellite. Engineering is the Discipline and Profession of applying technical and scientific Knowledge and

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. NASA 's Space Shuttle, officially called the Space Transportation System ( STS) is the Spacecraft currently used by the United States

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

Main article: Gravity Probe B mission timeline

This is a list of major events for the GP-B experiment. The Gravity Probe B mission timeline describes the events during the flight of Gravity Probe B, the science phase of its experimental campaign and the analysis of the recorded

• April 20, 2004
  • Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII. "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again "
• August 27, 2004
  • GP-B entered its science phase. Events 479 BC - Greco-Persian Wars: Persian forces led by Mardonius are routed by Pausanias, the Spartan "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " 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.
• August 15, 2005
  • The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode. Events 778 - The Battle of Roncevaux Pass, at which Roland is killed Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar.
• September 26, 2005
  • The calibration phase ended with liquid helium still in the dewar. Events 46 BC - Julius Caesar dedicates a Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. 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
• April 14, 2007
  • Announcement of best results obtained to date. Events 43 BC - Battle of Forum Gallorum: Mark Antony, besieging Julius Caesar 's assassin Decimus Junius Brutus in Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Francis Everitt gave a plenary talk at the meeting of the American Physical Society announcing initial results:^[9] "The data from the GP-B gyroscopes clearly confirm Einstein's predicted geodetic effect to a precision of better than 1 percent. The American Physical Society was founded in 1899 and is the World 's second largest organization of physicists behind the Deutsche Physikalische Gesellschaft. 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. " (Source: Gravity Probe B web site ^[10])

Future

On February 9, 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. Events 474 - Zeno crowned as co-emperor of the Byzantine Empire. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. 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 motion by inducing currents in the housing electrodes, causing the motion to change with time. The details of a spinning body may impose restrictions on the motion of its Angular velocity vector ω. 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 GPB may differ from the official results, it may take several more years for of all the data received by GPB 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) scepticism in the scientific community"[1].

See also

• Frame-dragging
• Geodetic effect
• Gravity
• Gravitomagnetism
• Tests of general relativity
• Timeline of Gravitational Physics

References

• Hecht, Jeff. [2], New Scientist, May 20, 2008, accessed May 20, 2008. Events 325 - The First Council of Nicaea &ndash the first Ecumenical Council of the Christian Church is held 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 325 - The First Council of Nicaea &ndash the first Ecumenical Council of the Christian Church is held 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common

External links

• Gravity Probe B web site at NASA
• Gravity Probe B Web site at Stanford
• Graphic explanation of how Gravity Probe B works
• NASA GP-B launch site
• Vandenberg AFB launch schedule
• NASA article on the technologies used in Gravity Probe B
• Frame Dragging
• General Relativistic Frame Dragging
• Layman's article on the project progress
• Latest LAGEOS results reported at BBC news