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

Contents 1 Effect 1.1 Possible explanations 1.2 Research avenues 2 Meetings and conferences about the anomaly 3 Primary references 4 References 5 Further reading 6 See also 7 External links

[edit] Effect

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.

The effect is so small that it could be a statistical anomaly caused by differences in the way data was 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 Pioneer Explorer Collaboration is expecting to address this concern around June 2007. They are currently re-outputting all of the Pioneer data from the spacecraft's entire recorded flight time into a single common format. Using this data it should be possible to determine if the acceleration is directed Earthward, suggesting data collection artifacts, or Sunward, suggesting a real effect that requires explanation.^[1]

[edit] 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
  • Significant errors in computation are not likely since (at current count) 5 independent analyses have shown the effect.
• 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, the acceleration does not show up in the orbits of the outer planets, so any gravitational answer would need to violate the equivalence principle.^[2] Another direction of insight is the mass of the Milky Way galaxy, which – when taken to account – yields the exact acceleration change for Pioneer 10 as that observed. ^[3]
  • 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, as if directed into 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 the wrong sign, as are the spacecraft's radio emissions.
    • The others are prime suspects, as presented at the second ISSI meeting in Berne, Feb 2007.
  • 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 the force of gravity deviates to a very different force law at very low accelerations of order: 10⁻¹⁰ m/s² from the traditional Newtonian value.

[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 unanalyzed pre-1987 data that may yield further insights. 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 (according to Turyshev the data was found on about 400 magnetic tapes and includes 30 years of data for Pioneer 10 as well as 20 years of data for Pioneer 11) [2]. [3], [4] The self-described Pioneer Explorer Collaboration' is also re-creating all telemetry data, currently 40 GB of it, copying it into a common format for comparison. It is expected this effort will culminate around June 2007, allowing all of the navigational and sensor data to be directly compared for the entire recorded flight time.

• 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.

• A dedicated mission has also been proposed (most recently to ESA [5]); 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). [6]

[edit] Meetings and conferences about the anomaly

A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly. [7]

Another meeting was held at International Space Science Institute in 2005 to discuss the anomaly, and discuss possible means for resolving the source. [8] There is a detailed report of the 2005 meeting on the Planetary Society's website at Pioneer Anomaly

There was a second working meeting at ISSI in Berne, February 19-23, 2007.

[edit] 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 describing the anomaly

• (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 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] References

[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:

• Scientific American, vol 279, #6, December 1998, 26-27.
• (preprint) A Mission to Test the Pioneer Anomaly, Anderson et al, Int.J.Mod.Phys. D11 (2002) 1545-1551
• [9] A Route to Understanding of the Pioneer Anomaly, Slava G. Turyshev, Michael Martin Nieto, and cJohn D. Anderson (2004)
• (preprint) A Mission to Explore the Pioneer Anomaly, Dittus et al. (2005)
• 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)

• (preprint) Utilizing Minor Planets to Assess the Gravitational Field in the Outer Solar System, Page et al, 2005
• "Opening New Doors", Seattle Times layman's article
• (preprint) Conventional Forces can Explain the Anomalous Acceleration of Pioneer 10, Scheffer, Phys.Rev. D67 (2003) 084021. One of several arguments that the "Pioneer Anomaly" can be well explained by conventional physics.
• (preprint) Using Early Data to Illuminate the Pioneer Anomaly, M. M. Nieto, J. D. Anderson (2005).
• Scientific American, vol 293, #4, October 2005, 24-25.
• J. R. Brownstein and J. W. Moffat (2006). "Gravitational solution to the Pioneer 10/11 anomaly". Classical and Quantum Gravity 23: 3427-3436. . Arxiv.org Preprint (gr-qc/0511026)
• Masreliez C. J., The Pioneer Anomaly - A cosmological explanation. (2005) Ap&SS, v. 299, no. 1, pp. 83-108

[edit] See also

• Flyby anomaly
• STVG
• modified Newtonian dynamics
• General relativity
• Unsolved problems in physics

[edit] External links

• "Pioneering Gas Leak? The strange motions of two space probes have mundane explanations--probably" Scientific American (December 1998)
• "A Force to Reckon With: What applied the brakes on Pioneer 10 and 11?" Scientific American (October 2005)
• A New Insight into the Pioneer 10/11 Gravitation anomaly, an effect attributable to retarded energy deployment in the gravitational field (August, 2002)
• "Gravity theory dispenses with dark matter" - STVG (Scalar-tensor-vector gravity) theory claims to predict Pioneer anomaly
• Planetary Society data recovery effort enables study
• Possible solution based on composition differences.
• Pioneer anomaly on arxiv.org The .ru site which has annoying "pop-up" ads
• (preprint) Self-creation cosmology predicts the Pioneer anomaly as a time drift between ephemeris and atomic clocks.
• Space.com: The Problem with Gravity: New Mission Would Probe Strange Puzzle
• C-news (in Russian)
• Newfound Data Could Solve NASA's Great Gravity Mystery