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Title: The Pioneer Anomaly
Authors: Jose A. de Diego, Dario Nunez

Analysis of the radio-metric data from Pioneer 10 and 11 spacecrafts has indicated the presence of an unmodelled acceleration starting at 20 AU, which has become known as the Pioneer anomaly. The nature of this acceleration is uncertain. In this paper we give a description of the effect and review some relevant mechanisms proposed to explain the observed anomaly. We also discuss on some future projects to investigate this phenomenon.

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Pioneer anomaly
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Beyond the edge of the solar system, something has gradually dragged two of America's oldest space probes -- Pioneer 10 and Pioneer 11 -- a quarter-million miles off course. Astrophysicists have struggled 15 years in vain to identify the infinitesimal force at play. The Pioneer anomaly, as it is called, throws a monkey wrench into celestial mechanics.

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In April 2008, Slava Turyshev used tracking and telemetry data from one hundred different measurements on the Pioneer 11 spacecraft, including the varied temperatures within the spacecraft caused by the different metals and materials used to build the spacecraft.
The spacecraft is made with such materials as Teflon, aluminium, Mylar, aluminium-based paints, Kapton. The plutonium, which provides power for the spacecraft, also generates heat.
The team found that Pioneer 11 gives off more heat in certain directions than it does in other directions. The varied rates of heat emissions on the spacecraft are large enough to force the spacecraft off of its nominal trajectory.

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What is making NASA's twin Pioneer spacecraft mysteriously drift off course, apparently defying the laws of physics? A rigorous new analysis suggests ordinary heat emission can at least partly explain the wayward probes' strange trajectories.

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Anomalies spotted in spacecraft flight
Could a mystery force be altering their course?
The speed and direction of some spacecraft are being mysteriously altered as they pass near the Earth. This unexplained velocity shift hints that there is either something missing from current models used for space navigation, or, possibly, something missing from our understanding of physics.

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The Pioneer anomaly or Pioneer effect is the observed deviation from expectations of the trajectories of various unmanned spacecraft visiting the outer solar system, notably Pioneer 10 and Pioneer 11.

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A new look at the trajectories for various spacecraft as they fly past the Earth finds in each case a tiny amount of surplus velocity. For craft that pursue a path mostly symmetrical with respect to the equator, the effect is minimal. For craft that pursue a more unsymmetrical path, the effect is larger. In the case of the NEAR asteroid rendezvous craft, for instance, the velocity anomaly amounts to 13 mm/sec. Although this is only one-millionth of the total velocity, the precision of the velocity measurements, carried out by looking at the Doppler shift in radio waves bounced off the craft, is 0.1 mm/sec, and this suggests that the anomaly represents a real effect, one needing an explanation.

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Title: Jupiter, Saturn and the Pioneer anomaly: a planetary-based independent test
Authors: Lorenzo Iorio

In this paper we use the ratio of the corrections to the standard Newtonian/Einsteinian secular precessions of the longitudes of perihelia of Jupiter and Saturn, recently estimated by the Russian astronomer E.V. Pitjeva by fitting almost one century of data with the EPM ephemerides, to make an independent, planetary-based test of the hypothesis that the Pioneer anomaly (PA), as it is presently known in the 5-10 AU region, is of gravitational origin. Accounting for the errors in the determined apsidal extra-rates and in the values of the PA acceleration at the orbits of Jupiter and Saturn the answer is negative. If and when the re-analysis of the entire Pioneer 10/11 will be completed more firm conclusions could be reached. Moreover, it would also be important that other teams of astronomers estimate independently their own corrections to the perihelion precessions.

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Ed ~ It should be said that current MOND theories are now considered unlikely by most scientists.

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On March 2, 1972, NASA launched the Pioneer 10 spacecraft to study Jupiter, which it passed Dec. 3, 1973. It travelled on to become the first spacecraft to depart our solar system, destined for a star system far, far away, Aldebaran (al-DEB-uh-ran).
About mid-evening, face due east and look nearly halfway up the sky and you will see a bright, first magnitude reddish-orange star, Aldebaran, the ninth brightest star in our northern hemisphere.

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Title: Pioneer anomaly: the velocity vector orientation and a drift in the proper time
Authors: Vikram H. Zaveri
(Version v2)

A relativistic theory is proposed to explain the anomalous accelerations of Pioneer 10/11, Galileo and Ulysses spacecrafts. The theory points out at the limitations of the weak field approximation and proposes a drift in the proper time of the spacecraft outside the framework of general relativity. In this theory the proper time of a body is associated with the gravitational frequency shift of the constituent fundamental particles of the body. The frequency shift changes the energy level of the body which gets reflected in its motion. This change in energy level causes the time like geodesics to deviate from that of the standard theoretical models. We introduce proper time in the line element of a metric theory according to a fixed set of rules laid down by general relativity for introducing deviation in the flat Minkowski metric. The frequency shift for bodies of different composition traversing different trajectories however, is not the same and this gets reflected in its motion as an unmodelled anomalous effect. This association of proper time with the gravitational frequency shift of the body requires the flat Minkowski metric to deviate in different ways for different two body systems. This solves the problem of anomalous acceleration in a very simple way. Gravitational redshift of light, bending of light and perihelic precession of planets are within the permissible limits. Another independent proposal involving angle between the heliocentric radial vector and the tangential velocity vector is discussed. This yields a component of acceleration which is of the same order of magnitude as that of the Pioneer anomaly but has a large variation over distances.

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The 'Pioneer anomaly' the mystifying observation that NASA's two Pioneer spacecraft have drifted far off their expected paths cannot be explained by tinkering with the law of gravity, a new study concludes.
 Kjell Tangen, a physicist at the firm DNV in Hovik, Norway, says tweaking the law of gravity in a variety of ways cannot explain the anomaly while also getting the orbits of the outer planets right. After modifying gravity in ways that would match the Pioneer anomaly, he inevitably got wrong answers for the motion of Uranus and Pluto.

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