Pioneer anomaly - Definition

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 2004, 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.

The effect is seen in radio Doppler and ranging data. This is interpreted as corresponding to a very small constant Sunwards deceleration of the order of 8.74 x 10^-8 cm/s² (0.874 nm/s²) for both spacecraft.

Data from the Galileo and Ulysses spacecraft is also indicative of 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 accurately measure with three-axis stabilised craft such as the Voyagers.)

Explanations for the discrepancy that have been considered include:

• a "real" deceleration:
  • by conventional gravitational forces, including from:
    • the Kuiper belt
    • dark matter
  • solar wind and radiation pressure
  • reaction force from radio transmissions
  • drag from the interplanetary medium
  • gas leaks from the spacecraft (including helium from fission in RTGs)
  • thermal radiation pressure (from RTGs and elsewhere)
  • electromagnetic forces due to spacecraft charge
• errors (e.g., measurement, computational) in deriving the acceleration
• new physics

The Pioneer spacecraft are no longer providing 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 borne fruit. A dedicated mission to investigate this has been proposed (most recently to ESA [1] (http://physicsweb.org/article/world/17/9/3)); any such mission would probably need to surpass 20 AU from the Sun in a hyperbolic escape orbit.

References

• [2] (http://arxiv.org/abs/gr-qc/9808081) 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) (http://prola.aps.org/abstract/PRL/v81/i14/p2858_1) - the original paper
• [3] (http://arxiv.org/abs/gr-qc/0104064) Study of the anomalous acceleration of Pioneer 10 and 11, Anderson et al, Phys. Rev. D 65, 082004 (2002) (http://link.aps.org/abstract/PRD/v65/p082004) - a lengthy survey of several years of debate by the original authors, in which they conclude that the observations remain unexplained
• [4] (http://www.arxiv.org/abs/gr-qc/0205059) A Mission to Test the Pioneer Anomaly, Anderson et al, Int.J.Mod.Phys. D11 (2002) 1545-1551 (http://www.worldscinet.com/142/11/1110/S0218271802002876.html)
• Finding the origin of the Pioneer anomaly, Nieto & Turyshev (2004), Class. Quantum Grav. 21 4005-4023 (http://stacks.iop.org/0264-9381/21/4005) - further elaboration on a dedicated mission plan (restricted access)

External links

• "Opening New Doors" (http://seattletimes.nwsource.com/html/nationworld/2002138196_pioneer02.html), Seattle Times layman's article
• "Gravity May Lose Its Pull" (http://www.latimes.com/news/local/la-sci-pioneer21dec21,0,3834452.story?coll=la-home-local), LATimes