Pioneer anomaly

The Pioneer anomaly or Pioneer effect refers to the observed deviation from expectations of the trajectories of various unmanned spacecraft visiting the outer solar system, notably Pioneer 10 and 11. As of 2005, 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.

Effect

Unsolved problem in physics:

What causes the apparent residual Sunward acceleration of the Pioneer spacecraft?

(more unsolved problems in physics)

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 caused 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.

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
a real deceleration:
- by gravitational forces from unidentified sources such as the Kuiper belt or dark matter
- drag from the interplanetary medium, including dust, solar wind and cosmic rays
- gas leaks, including helium produced by radioactive decay escaping from the spacecrafts' radioisotope thermoelectric generators (RTGs)
- radiation pressure of sunlight, the spacecraft's radio transmissions, or thermal radiation pressure from the RTGs
- electromagnetic forces due to an electric charge on the spacecraft
new physics
- clock acceleration (gr-qc/0410084) between coordinate or Ephemeris time and International Atomic Time.
- A modification of the law of gravity. The theory known as MOND (MOdified Newtonian Dynamics), states that at very low accelerations of order: 10⁻¹⁰ m/s² the force of gravity deviates from the traditional Newtonian value. However, since the accelerations of the Pioneer craft are still much larger than this, MOND cannot explain the Pioneer Anomaly.
- A modified theory of gravity (gr-qc/0511026) that incorporates quantum effects can explain a trio of puzzling astronomical observations – including the wayward motion of the Pioneer spacecraft in our solar system and Galaxy rotation curves without dark matter (as reported in the New Scientist Online).

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]

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

Primary references

(preprint) Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration, Anderson et al, Phys. Rev. Lett. 81, 2858-2861 (1998)

The original paper

(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 original authors. 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.')"

External links

STVG (Scalar-tensor-vector gravity) theory predicts Pioneer anomaly
(preprint) Self creation cosmology predicts the Pioneer anomaly as a time drift between ephemeris and atomic clocks.
Pioneer anomaly on arxiv.org
C-news (in Russian)
Planetary Society data recovery effort enables study
A Force to Reckon With What applied the brakes on Pioneer 10 and 11? Scientific American Magazine (October 2005)