Pioneer anomaly

Pioneer anomaly

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. Both spacecraft are escaping from the solar system, and are slowing down under the influence of the Sun's gravity. Upon very close examination, however, they are slowing down slightly more than expected from influence of all known sources. The effect can be modelled as a slight additional acceleration towards the Sun.

At present, there is no universally accepted explanation for this phenomenon; while it is possible that the explanation will be mundane—such as thrust from gas leakage—the possibility of unknown physics is also being considered.


The effect is seen in radio Doppler and ranging data, yielding information on the velocity and distance of the spacecraft. When all known forces acting on the spacecraft are taken into consideration, a very small but unexplained force remains. It appears to cause a constant sunward acceleration of (8.74 ± 1.33) × 10−10 m/s2 for both spacecraft. This results in the spacecraft being about 5,000 km closer to the sun than they should be, every year. The magnitude of the Pioneer effect is numerically quite close to the product of the speed of light and the Hubble constant, but the significance of this, if any, is unknown (Gravitationally bound objects such as the solar system, or even the galaxy, do not partake of the expansion of the universe - this is known both from theory [cite article |title=The Effect of Cosmological Expansion on Self-Gravitating Ensembles of Particles,
author=Noerdlinger, P.D. and Petrosian, V.,
journal=The Astrophysical Journal,
] and by direct measurement [cite article
title=Progress in Lunar Laser Ranging Tests of Relativistic Gravity,
author=Williams, J.G. and Turyshev, S.G. and Boggs, D.H.,
journal=Physical Review Letters,
] .)

Data from the Galileo and Ulysses spacecraft indicate a similar effect, although for various reasons (such as their relative proximity to the Sun) firm conclusions cannot be drawn from these sources. These spacecraft are all partially or fully spin-stabilised; the effect is much harder to measure accurately with craft that use thrusters for attitude control. These spacecraft, such as the Voyagers, acquire small and unpredictable changes in speed as a side effect of the frequent attitude control firings. This 'noise' makes it impractical to measure small accelerations such as the Pioneer effect. The Cassini mission also had reaction wheels for attitude control, thus avoiding this particular problem, but also had radioisotope thermoelectric generators (RTGs) mounted close to the spacecraft body, spewing kilowatts of heat in hard-to-predict directions. The measured value of unmodelled acceleration for Cassini is (26.7 ± 1.1) × 10−10 m/s2, but this is the sum of (uncertain) thermal effects and the possible anomaly. Therefore measurements of Cassini do not conclusively either confirm or refute the existence of the anomaly [cite web |url= |title=Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft |author=John D. Anderson, Eunice L. Lau, Giacomo Giampieri Note: The corresponding arXiv pre-print cite web |url= |title=Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft |author=John D. Anderson, Eunice L. Lau, Giacomo Giampieri was withdrawn. ] .

The effect is so small that it could be a statistical anomaly caused by differences in the way data was collected over the lifetime of the probes. Numerous changes were made over this period, including changes in the receiving instruments, reception sites, data recording systems and recording formats. The Pioneer Explorer Collaboration [ [ Pioneer Explorer Collaboration] ] is expecting to address this concern around June 2007. They are currently re-outputting all of the Pioneer data from the spacecraft's entire recorded flight time into a single common format. Using this data, it should be possible to determine if the acceleration is directed Earthward, suggesting data collection artifacts, or Sunward, suggesting a real effect that requires explanation. [ [ Computer sleuths try to crack Pioneer anomaly] ] .

It is possible, but not proven, that this anomaly is linked to the flyby anomaly [ [ NASA Baffled by Unexplained Force Acting on Space Probes] ] . Although the circumstances are very different (planet flyby vs. deep space cruise), the overall effect is similar - a small but unexplained velocity change is observed on top of a much larger conventional gravitational acceleration.

Possible explanations

Explanations for the discrepancy that have been considered include:
* observational errors, including measurement and computational errors, in deriving the acceleration.
**Approximation/statistical errors
**Significant errors in computation are not likely since (at current count) 7 independent analyses have shown the effect [ [ Pioneer Anomaly Project Update: A Letter From the Project Director] by Slava G. Turyshev, March 28, 2007] .
* a real deceleration not accounted for in the current model, such as:
** gravitational forces from unidentified sources such as the Kuiper belt or dark matter. However, an acceleration does not show up in the orbits of the outer planets, so any generic gravitational answer would need to violate the equivalence principle [ [ '(Preprint) Can the Pioneer anomaly be of gravitational origin? A phenomenological answer'] by Lorenzo Iorio] (see modified inertia below). The observed cosmological constant also cannot explain the Pioneer anomaly because it is many orders of magnitude too small and has the wrong sign (repulsive instead of attractive).
** drag from the interplanetary medium, including dust, solar wind and cosmic rays. However, the measured densities are too small to cause the effect.
** gas leaks, including helium from the spacecrafts' radioisotope thermoelectric generators (RTGs)
** radiation pressure of sunlight, the spacecraft's radio transmissions, or thermal radiation pressure from the RTGs (See Radioisotope rocket), or asymmetrical radiation of the heat from the spacecraft electronics, reflecting from the back of the spacecraft’s dish-like main antenna, causing a recoil like sunlight striking a solar sail.
***The pressure of sunlight is too small at this distance, and points into the wrong direction. The same applies to the spacecraft's radio emissions.
***The others are prime suspects, as presented at the second ISSI meeting in Berne, Feb 2007.
***A recent presentation at the APS April 2008 meeting suggests that differential heating may account for as much of 1/3 rd of the observed acceleration. [ [ Pioneer spacecraft a step closer to being boring] , April 13th, 2008 David Harris]
** electromagnetic forces due to an electric charge on the spacecraft
* New physics
** clock acceleration between coordinate or Ephemeris time and International Atomic Time. [cite web |url= |title=The Pioneer anomaly as acceleration of the clocks |author=Antonio F. Ranada |date=10 Jan 2005 |accessdate=2008-05-13]
** A modification of the law of gravity. The theory MOND (MOdified Newtonian Dynamics) proposes that the force of gravity deviates to a very different force law at very low accelerations of order: 1.0−11 m/s2 from the traditional Newtonian value cite web|url=|title=The modified Newtonian dynamics-MOND-and its implications for new physics|author=Jacob D. Bekenstein|publisher=Contemporary Physics 47, 387 (2006)|accessdate=2007-10-01 ] .
** Modified inertia. MOND can also be interpreted as a modification of inertia, perhaps due to an interaction with vacuum energy and such a trajectory-dependent theory could account for the different accelerations apparently acting on the orbiting planets and the Pioneer craft on their escape trajectories cite web|url=|title=The Modified Dynamics as a vacuum effect|author=M.Milgrom|publisher=Phys. Lett. A253 (1999)|accessdate=2007-10-03 ] . More recently, a model for modified inertia using Unruh radiation and a Hubble-scale Casimir effect has been proposed to explain the Pioneer anomalycite web|url=|title=Modelling the Pioneer anomaly as modified inertia|author=M.E.McCulloch|publisher=MNRAS 376, 338-342 (2007)|accessdate=2007-10-02 ] , and a possible test for evidence of modified inertia on Earth has been proposedcite web|url= |title=Is violation of Newton's second law possible? |author=A.Yu.Ignatiev |publisher=Phys. Rev. lett. 98, 101101 (2007)|accessdate=2007-10-03 ] . It has also been suggested that a modification of inertia can explain the flyby anomaly.cite web|url= |title=Modelling the flyby anomalies using a modification of inertia. |author=M.E.McCulloch|publisher=MNRAS-letters, 389(1), L57-60]
** [ Aethereal Gravitation Theory] (AGT) as proposed by Sanford-Wood (2008) predicts that gravitational acceleration is increased for a body moving at velocity across the gravitational gradient. The theoretical magnitude of this effect for the Pioneer craft is in close accordance with observation.
** Down-scaling of photon frequency as a consequence of integrable Weyl geometry. This connects the Pioneer anomaly with the Hubble parameter via conformal geometry, and in a sense obviates the need for an assumed expansion of space in favor of a local "down-scaling" of photon frequency - an effect similar to "tired light" but originating in the geometry of spacetime rather than as a higher-order gravitational effect. [cite web |url= |title=Another look at the Pioneer anomaly |author= Erhard Scholz |date= 14 August 2007 |accessdate=2008-05-13] .
** Extending the Hubble law (which relates the increase (redshift) of the wavelength of a photon from another galaxy to the expansion of the universe) to the realm of unbounded massive particles, the particle's associated de Broglie wavelength will be redshifted due to the expansion. This redshift corresponds to a decrease in the particle's momentum over time. Thus, the Pioneer spacecrafts' anomalous accelerations may be a counter example to the hypothesis of dark matter in the Milkyway galaxy cite web|url=|title=Is the Pioneer anomaly a counter example to the dark matter hypothesis?
author=F.J. Oliveira|publisher=Int. Jour. Theo. Phys. DOI:10.1007/s10773-007-9434-y (2007)
] .
** A new model of the universe called the Scale Expanding Cosmos Theory (SEC), also known as the Expanding Spacetime Theory (EST). This theory explains the anomaly simply as cosmic drag, and it also removes the need for dark energy and dark matter. [cite conference |title=The Scale Expanding Cosmos Theory and a New Conception of the Progression of Time |url= |author=Masreliez, C. Johan |publisher=Astronomical Society of the Pacific |booktitle=Astrophysical Ages and Times Scales, ASP Conference Series Vol. 245 |date=2001 |editor=Ted von Hippel, Chris Simpson, and Nadine Manset |pages=601]

Research avenues

The Pioneer spacecraft are no longer providing new data and Galileo was deliberately burned up in Jupiter's atmosphere at the end of its mission. So far, attempts to use data from current missions such as Cassini have not yielded any conclusive results. There are several remaining options for further research:

* Further analysis of archived Pioneer data. The current analysis is based primarily on data from 1987 and later; there remains unanalyzed pre-1987 data that may yield further insights. The Planetary Society issued an appeal to its worldwide membership and raised the funding needed to study the Pioneer anomaly. Scientists and engineers led by Slava G. Turyshev at JPL were able to recover much of the more-than-30-year navigational histories of both spacecraft, including data from their Jupiter and Saturn encounters in the 1970s (according to Turyshev the data was found on about 400 magnetic tapes and includes 30 years of data for Pioneer 10 as well as 20 years of data for Pioneer 11) [] . [] , [] The Pioneer Explorer Collaboration is also re-creating all telemetry data, currently 40 GB of it, copying it into a common format for comparison. When complete, this effort will allow all of the navigational and sensor data to be directly compared for the entire recorded flight time. As of September 10, 2007 this effort was nearly complete. []

* The "New Horizons" spacecraft to Pluto is spin-stabilised for much of its cruise, and there is a possibility that it can be used to investigate the anomaly. "New Horizons" may have the same problem that precluded good data from the "Cassini" mission - its RTG is mounted close to the spacecraft body, so thermal radiation from it, bouncing off the spacecraft, may produce a systematic thrust of a not-easily predicted magnitude, several times as large as the Pioneer effect. Nevertheless efforts are underway to study the non-gravimetric accelerations on the spacecraft, in the hopes of having them well modeled for the long cruise to Pluto after the Jupiter fly-by that occurred in February 2007. In particular, despite any large systematic bias from the RTG, the 'onset' of the anomaly at or near the orbit of Saturn might be observed [cite web|author = Michael Martin Nieto| url= |title= New Horizons and the Onset of the Pioneer Anomaly] .

* A dedicated mission has also been proposed (most recently to ESA [] ); any such mission would probably need to surpass 200 AU from the Sun in a hyperbolic escape orbit.

* Observations of asteroids around 20 AU may provide insights if the anomaly's cause is gravitational (Page et al, 2005). []

Meetings and conferences about the anomaly

A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly. []

The Pioneer Explorer Collaboration has hosted three meetings at [ International Space Science Institute] in Berne, Switzerland to discuss the anomaly, and discuss possible means for resolving the source.

The first meeting was held on November 7-11 2005; a detailed report is available on the Planetary Society's website at [ Pioneer Anomaly] .

There was a [ second working meeting] on February 19-23, 2007. A third meeting was held on [ 18-22 February 2008] .

Primary references

* Cite journal
author = John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, Slava G. Turyshev
title = Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration
url =
journal = Phys. Rev. Lett.
volume = 81
pages = 2858–2861
year = 1998
doi = 10.1103/PhysRevLett.81.2858
[ (preprint)] : The original paper describing the anomaly
* [ (preprint)] "Study of the anomalous acceleration of Pioneer 10 and 11", Anderson et al, [ Phys. Rev. D 65, 082004 (2002)] : A lengthy survey of several years of debate by the authors of the original 1998 paper documenting the anomaly. The authors conclude: "Until more is known, we must admit that the most likely cause of this effect is an unknown systematic. (We ourselves are divided as to whether 'gas leaks' or 'heat' is this 'most likely cause.')"


Further reading

The ISSI meeting above has an excellent [ reference list] divided into sections such as primary references, attempts at explanation, proposals for new physics, possible new missions, popular press, and so on. A sampling of these are shown here:

* "Scientific American", vol 279, #6, December 1998, 26-27.
* [ (preprint)] "A Mission to Test the Pioneer Anomaly", Anderson et al, [ Int.J.Mod.Phys. D11 (2002) 1545-1551]
* [] "A Route to Understanding of the Pioneer Anomaly", Slava G. Turyshev, Michael Martin Nieto, and John D. Anderson (2004)
* [ (preprint)] "A Mission to Explore the Pioneer Anomaly", Dittus et al. (2005)
* "Finding the origin of the Pioneer anomaly", Nieto & Turyshev (2004), [ Class. Quantum Grav. 21 4005-4023] : Further elaboration on a dedicated mission plan (restricted access)
* [ (preprint)] "Utilizing Minor Planets to Assess the Gravitational Field in the Outer Solar System", Page et al, 2005
* [ "Opening New Doors"] , Seattle Times layman's article
* [ (preprint)] "Conventional Forces can Explain the Anomalous Acceleration of Pioneer 10", Scheffer, [ Phys.Rev. D67 (2003) 084021] . One of several arguments that the "Pioneer Anomaly" can be well explained by conventional physics.
* [ (preprint)] "Using Early Data to Illuminate the Pioneer Anomaly", M. M. Nieto, J. D. Anderson (2005).
* "Scientific American", vol 293, #4, October 2005, 24-25.
*. Preprint ( [ gr-qc/0511026] )
* Masreliez C. J., " [ The Pioneer Anomaly - A cosmological explanation] ". (2005) Ap&SS, v. 299, no. 1, pp. 83-108

See also

* Flyby anomaly
* Galaxy rotation problem
* modified Newtonian dynamics
* General relativity
* Unsolved problems in physics

External links

* [ "Pioneering Gas Leak? The strange motions of two space probes have mundane explanations--probably"] "Scientific American" (December 1998)
* [ "A Force to Reckon With: What applied the brakes on Pioneer 10 and 11?"] "Scientific American" (October 2005)
* [ "Gravity theory dispenses with dark matter"] - STVG (Scalar-tensor-vector gravity) theory claims to predict Pioneer anomaly
* [ Planetary Society data recovery effort enables study]
* [ Shows number of publications about the Pioneer anomaly on, by year.]
* [ The Problem with Gravity: New Mission Would Probe Strange Puzzle]
* [ "Wanted - Einstein Junior"] "the Economist"(March 2008)
* [ Aethereal Gravitation Theory] (AGT)

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