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 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, 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 sunwards acceleration of (8.74 ± 1.33) × 10⁻¹⁰ m/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:

• observational errors, including measurement and computational errors, in deriving the acceleration
• 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 fission-produced helium escaping from in the spacecrafts' radioisotope thermoelectric generators
  • 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

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. 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 [1]. A dedicated mission has also been proposed (most recently by ESA [2]); any such mission would probably need to surpass 20 AU from the Sun in a hyperbolic escape orbit.

References

• [3] 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
• [4] 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, in which they conclude that the observations remain unexplained
• [5] A Mission to Test the Pioneer Anomaly, Anderson et al, Int.J.Mod.Phys. D11 (2002) 1545-1551
• 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)
• [6] Conventional Forces can Explain the Anomalous Acceleration of Pioneer 10, Scheffer,Phys.Rev. D67 (2003) 084021. Argues that the "Pioneer Anomaly" is not so anomalous, and can be well explained by conventional physics.

External links

• "Opening New Doors", Seattle Times layman's article