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Unsolved problems in physics: What causes the apparent residual sunward acceleration of the Pioneer spacecraft?

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

Both Pioneer 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. The effect can be modeled as a slight additional acceleration towards the Sun.

The anomaly has no universally accepted explanation. The explanation may be mundane, such as measurement error, thrust from gas leakage or uneven radiation of heat. However, it is also possible that current physical theory does not correctly explain the behaviour of the craft relative to the sun.

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Initial indications

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. If the positions of the spacecraft are predicted one year in advance based on measured velocity and known forces (mostly gravity), they are actually found to be some 400 km closer to the sun at the end of the 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[1] and by direct measurement.[2]

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 has reaction wheels as well as thrusters for attitude control, and during cruise could rely for long periods on the reaction wheels alone, thus enabling precision measurements. However, it also had radioisotope thermoelectric generators (RTGs) mounted close to the spacecraft body, radiating 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, roughly three times as large as the Pioneer acceleration. Unfortunately, the measured value is the sum of the uncertain thermal effects and the possible anomaly. Therefore the Cassini measurements neither conclusively confirm nor refute the existence of the anomaly.[3]

Some possible explanations

  • 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.[4]
  • 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 [5] (see modified inertia below). Likewise, the anomaly does not appear in the orbits of Neptune's moons, challenging the possibility that the Pioneer anomaly may be an unconventional gravitational phenomenon.[6]
    • 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.[7]
    • electromagnetic forces due to an electric charge on the spacecraft
  • New physics
    • clock acceleration between coordinate or Ephemeris time and International Atomic Time.[8]
    • 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: 10−11 m/s2 from the traditional Newtonian value.[9]
    • 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.[10] More recently, a model for modified inertia using Unruh radiation and a Hubble-scale Casimir effect has been proposed to explain the Pioneer anomaly,[11] and a possible test for evidence of modified inertia on Earth has been proposed.[12] It has also been suggested that a modification of inertia can explain the flyby anomaly.[13]

Investigation of the Pioneer data

The effect is so small that it could be a statistical anomaly caused by differences in the way data were 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 Planetary Society announced on June 1, 2006 that 30 years of Pioneer data had been saved. It announced on March 28, 2007 that analysis of the data was underway. On March 19, 2008, it announced that one source of acceleration, uneven thermal radiation had been found to explain some of the deviation, but not all.[14][15]

Further research avenues

It is possible, but not proven, that this anomaly is linked to the flyby anomaly.[16] 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.

The Pioneer spacecraft are no longer providing new data, (the last contact having been on 23 January 2003)[17] 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 the retrieved Pioneer data.
  • 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.[18]
  • A dedicated mission has also been proposed.[19] Such a mission would probably need to surpass 200AU from the Sun in a hyperbolic escape orbit.
  • Observations of asteroids around 20AU may provide insights if the anomaly's cause is gravitational.[20][21]
  • In November 2009, ESA's Rosetta spacecraft measured its velocity during flyby to within a few millimeters per second in an effort to explain the flyby anomaly. [22] However, no anomaly was found on this most recent flyby. [23]

Meetings and conferences about the anomaly

A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly.[24]

The Pioneer Explorer Collaboration was formed to study the Pioneer Anomaly and has hosted three meetings (2005, 2007, and 2008) at International Space Science Institute in Berne, Switzerland to discuss the anomaly, and discuss possible means for resolving the source.[25]

See also

Primary references

The original paper describing the anomaly
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.')"


  1. ^ Noerdlinger, P.D. and Petrosian, V., (1971). "The Effect of Cosmological Expansion on Self-Gravitating Ensembles of Particles,". The Astrophysical Journal, 168,: pp. 1,.  
  2. ^ Williams, J.G. and Turyshev, S.G. and Boggs, D.H., (2004,). "Progress in Lunar Laser Ranging Tests of Relativistic Gravity," (PDF). Physical Review Letters, (APS,) 93, (26,): pp. 261101,.  
  3. ^ John D. Anderson, Eunice L. Lau, Giacomo Giampieri. "Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft" (PDF).   Note: The corresponding arXiv pre-print John D. Anderson, Eunice L. Lau, Giacomo Giampieri. "Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft".   was withdrawn.
  4. ^ Pioneer Anomaly Project Update: A Letter From the Project Director by Slava G. Turyshev, March 28, 2007
  5. ^ '(Preprint) Can the Pioneer anomaly be of gravitational origin? A phenomenological answer' by Lorenzo Iorio
  6. ^ 'Does the Neptunian system of satellites challenge a gravitational origin for the Pioneer anomaly?' by Lorenzo Iorio
  7. ^ Pioneer spacecraft a step closer to being boring, April 13th, 2008 David Harris
  8. ^ Antonio F. Ranada (10 January 2005). "The Pioneer anomaly as acceleration of the clocks". Retrieved 2008-05-13.  
  9. ^ Jacob D. Bekenstein. "The modified Newtonian dynamics-MOND-and its implications for new physics". Contemporary Physics 47, 387 (2006). Retrieved 2007-10-01.  
  10. ^ M.Milgrom. "The Modified Dynamics as a vacuum effect". Phys. Lett. A253 (1999). Retrieved 2007-10-03.  
  11. ^ M.E.McCulloch. "Modelling the Pioneer anomaly as modified inertia". MNRAS 376, 338-342 (2007). Retrieved 2007-10-02.  
  12. ^ A.Yu.Ignatiev. "Is violation of Newton's second law possible?". Phys. Rev. lett. 98, 101101 (2007). Retrieved 2007-10-03.  
  13. ^ M.E.McCulloch. "Modelling the flyby anomalies using a modification of inertia.". MNRAS-letters, 389(1), L57-60.  
  14. ^ "The Pioneer Anomaly: Changing the Laws of Physics?". September 10, 2007.  
  15. ^ "Projects: Pioneer Anomaly". The Planetary Society. May 19, 2008.  
  16. ^ NASA Baffled by Unexplained Force Acting on Space Probes
  17. ^ The Pioneer Missions. NASA.
  18. ^ Michael Martin Nieto. "New Horizons and the Onset of the Pioneer Anomaly".  
  19. ^ "Pioneer anomaly put to the test". September 1, 2004.  
  20. ^ Stuart Clark (10 May 2005). "Lost asteroid clue to Pioneer puzzle". New Scientist.  
  21. ^ "Can Minor Planets be Used to Assess Gravity in the Outer Solar System?". 2 Jan 2006.  
  22. ^
  23. ^
  24. ^ "Conference on The Pioneer Anomaly - Observations, Attempts at Explanation, Further Exploration".  
  25. ^ "The Pioneer Explorer Collaboration: Investigation of the Pioneer Anomaly at ISSI". February 18, 2008. Retrieved January 10, 2009.  

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:

Further elaboration on a dedicated mission plan (restricted access)

External links


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