Pioneer anomaly

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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 number 10 and 11 of the Pioneer program. At present, there is no universally accepted explanation for this phenomenon; while it is possible that the explanation will be prosaic—such as thrust from gas leakage—the possibility of entirely new physics is also being considered.

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[edit] Effect

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

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 causes a constant sunward acceleration of (8.74 ± 1.33) × 10−10 m/s2 for both spacecraft.

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 harder to measure accurately with three-axis stabilised craft such as the Voyagers.

[edit] Possible explanations

Explanations for the discrepancy that have been considered include:

[edit] 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 unanalysed pre-1987 data that may yield further insights. [1]
  • 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 in February, 2007.
  • A dedicated mission has also been proposed (most recently to ESA [2]); 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). [3]
  • 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. [4]

[edit] Meetings and conferences about the anomaly

A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly. [5]

Another meeting was held at International Space Science Institute in 2005 to discuss the anomaly, and discuss possible means for resolving the source. [6] There is a detailed report of the 2005 meeting on the Planetary Society's website at Pioneer Anomaly

[edit] Primary references

The original paper describing the anamoly
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.')"

[edit] 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)

[edit] See also

[edit] External links

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