Pioneer anomaly

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

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. Both spacecraft are escaping from the solar system, and are slowing down under the influence of the Sun's gravity. Upon very close examination, however, they are slowing down slightly more than expected from influence of all known sources. The effect can be modelled as a slight additional acceleration towards the Sun.

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.

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[edit] Effect

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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 appears to cause a constant sunward acceleration of (8.74 ± 1.33) × 10−10 m/s2 for both spacecraft. This results in the spacecraft being about 5,000 km closer to the sun than they should be, every year. The magnitude of the Pioneer effect is numerically quite close to the product of the speed of light and the Hubble constant, but the significance of this, if any, is unknown.

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 much harder to measure accurately with craft that use thrusters for attitude control. These spacecraft, such as the Voyagers, acquire small and unpredictable changes in speed as a side effect of the frequent attitude control firings. This 'noise' makes it impractical to measure small accelerations such as the Pioneer effect. The Cassini mission also had reaction wheels for attitude control, thus avoiding this particular problem, but also had radioisotope thermoelectric generators (RTGs) mounted close to the spacecraft body, spewing kilowatts of heat in hard-to-predict directions. The measured value of unmodelled acceleration for Cassini is (26.7 ± 1.1) × 10−10 m/s2, but this is the sum of (uncertain) thermal effects and the possible anomaly. Therefore measurements of Cassini do not conclusively either confirm or refute the existence of the anomaly[1].

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[2] 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.[3].

It is possible, but not proven, that this anomaly is linked to the flyby anomaly[4]. Although the circumstances are very different (planet flyby vs. deep space cruise), the overall effect is similar - a small but unexplained velocity change is observed on top of a much larger conventional gravitational acceleration.

[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) 7 independent analyses have shown the effect[5].
  • 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, an acceleration does not show up in the orbits of the outer planets, so any generic gravitational answer would need to violate the equivalence principle [6] (see modified inertia below).
    • 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, reflecting from the back of 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.
      • A recent presentation at the APS April 2008 meeting suggests that differential heating may account for as much of 1/3 rd of the observed acceleration.[7]
    • electromagnetic forces due to an electric charge on the spacecraft
  • new physics
    • clock acceleration between coordinate or Ephemeris time and International Atomic Time.[8]
    • A modification of the law of gravity. The theory MOND (MOdified Newtonian Dynamics) proposes that the force of gravity deviates to a very different force law at very low accelerations of order: 1.0−11 m/s2 from the traditional Newtonian value [9].
    • Modified inertia. MOND can also be interpreted as a modification of inertia, perhaps due to an interaction with vacuum energy, and that such a trajectory-dependent theory could account for the different accelerations apparently acting on the orbiting planets and the Pioneer on their escape trajectories [10]. More recently, a model for modified inertia using a Unruh radiation and Hubble-scale Casimir effect has been proposed to explain Pioneer anomaly,[11] and a possible test for evidence of modified inertia on Earth has been proposed.[12]. It was also suggested that this same model can explain the flyby anomaly.[13]
    • Down-scaling of photon frequency as a consequence of integrable Weyl geometry. This connects the Pioneer anomaly with the Hubble parameter via conformal geometry, and in a sense obviates the need for an assumed expansion of space in favor of a local "down-scaling" of photon frequency - an effect similar to "tired light" but originating in the geometry of spacetime rather than as a higher-order gravitational effect.[14].
    • Extending the Hubble law (which relates the increase (redshift) of the wavelength of a photon from another galaxy to the expansion of the universe) to the realm of unbounded massive particles, the particle's associated de Broglie wavelength will be redshifted due to the expansion. This redshift corresponds to a decrease in the particle's momentum over time. Thus, the Pioneer spacecrafts' anomalous accelerations may be a counter example to the hypothesis of dark matter in the Milkyway galaxy [15].
    • A revised theory of gravity in which the spin of an object changes its effective value of G, an effect not predicted by Newton or Einstein.[16]

[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) [1]. [2], [3] The Pioneer Explorer Collaboration is also re-creating all telemetry data, currently 40 GB of it, copying it into a common format for comparison. When complete, this effort will allow all of the navigational and sensor data to be directly compared for the entire recorded flight time. As of September 10, 2007 this effort was nearly complete. [4]
  • 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. In particular, despite any large systematic bias from the RTG, the 'onset' of the anomaly at or near the orbit of Saturn might be observed[17].
  • 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]

The Pioneer Explorer Collaboration has hosted two meetings at International Space Science Institute in Berne, Switzerland to discuss the anomaly, and discuss possible means for resolving the source.

The first meeting was held on November 7-11 2005; a detailed report is available on the Planetary Society's website at Pioneer Anomaly.

There was a second working meeting on February 19-23, 2007. A third meeting was held on 18-22 February 2008.

[edit] Primary references

The original paper describing the anomaly
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

  1. ^ John D. Anderson, Eunice L. Lau, Giacomo Giampieri. Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft. Note: The corresponding arXiv pre-print John D. Anderson, Eunice L. Lau, Giacomo Giampieri. Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft. was withdrawn.
  2. ^ Pioneer Explorer Collaboration
  3. ^ Computer sleuths try to crack Pioneer anomaly
  4. ^ NASA Baffled by Unexplained Force Acting on Space Probes
  5. ^ Pioneer Anomaly Project Update: A Letter From the Project Director by Slava G. Turyshev, March 28, 2007
  6. ^ '(Preprint) Can the Pioneer anomaly be of gravitational origin? A phenomenological answer' by Lorenzo Iorio
  7. ^ Pioneer spacecraft a step closer to being boring, April 13th, 2008 David Harris
  8. ^ Antonio F. Ranada (10 Jan 2005). The Pioneer anomaly as acceleration of the clocks. Retrieved on 2008-05-13.
  9. ^ Jacob D. Bekenstein. The modified Newtonian dynamics-MOND-and its implications for new physics. Contemporary Physics 47, 387 (2006). Retrieved on 2007-10-01.
  10. ^ M.Milgrom. The Modified Dynamics as a vacuum effect. Phys. Lett. A253 (1999). Retrieved on 2007-10-03.
  11. ^ M.E.McCulloch. Modelling the Pioneer anomaly as modified inertia. MNRAS 376, 338-342 (2007). Retrieved on 2007-10-02.
  12. ^ A.Yu.Ignatiev. Is violation of Newton's second law possible?. Phys. Rev. lett. 98, 101101 (2007). Retrieved on 2007-10-03.
  13. ^ M.E.McCulloch. Can the flyby anomalies be explained by a modification of inertia?.
  14. ^ Erhard Scholz (14 August 2007). Another look at the Pioneer anomaly. Retrieved on 2008-05-13.
  15. ^ F.J. Oliveira. Is the Pioneer anomaly a counter example to the dark matter hypothesis?. Int. Jour. Theo. Phys. DOI:10.1007/s10773-007-9434-y (2007).
  16. ^ Anthony D Osborne and N Vivian Pope (2008). Light Speed, Gravitation and Quantum Instantaneity. phi Philosophical Enterprises. ISBN 978-0950379067. . See also the author's web site.
  17. ^ Michael Martin Nieto. New Horizons and the Onset of the Pioneer Anomaly.

[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