Pioneer Anomaly

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 Sun ward acceleration of (8.74 ± 1.33) × 10⁻¹⁰ m/s² for both spacecraft.

Data from the Galileo and Ulysses spacecraft are 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 measure accurately with Three-axis Stabilised craft such as the Voyagers .

Possible Explanations

Explanations for the discrepancy that have been considered include:

Observational Error s, 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 Generator s (RTGs)

--- Radiation Pressure of sunlight, the spacecraft's Radio transmissions, or Thermal Radiation pressure from the RTGs

--- Electromagnetic Force s due to an Electric Charge on the spacecraft

new Physics

--- clock acceleration {Link without Title} 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 low accelerations, the force of gravity deviates from the traditional Newtonian value.

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. {Link without Title}

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. Unfortunately ''New Horizons'' will almost surely have the same problem that precluded good data from the ''Cassini'' mission - the RTGs are mounted close to the spacecraft body, so thermal radiation from them, bouncing off the spacecraft, will produce a systematic thrust of a not-easily predicted magnitude, several times as large as the Pioneer effect.

A dedicated mission has also been proposed (most recently to AU from the Sun in a Hyperbolic Escape Orbit .

Observations of Asteroid s around 20 AU may provide insights if the anomaly's cause is gravitational (Page et al, 2005). {Link without Title}

MEETINGS AND CONFERENCES ABOUT THE ANOMALY

A meeting was held at the University Of Bremen in 2004 to discuss the Pioneer anomaly. {Link without Title}

Another meeting was held at International Space Science Institute in 2005 to discuss the anomaly, and discuss possible means for resolving the source. [http://www.issi.unibe.ch/teams/Pioneer/]
You can read 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)

(preprint) ''Study of the anomalous acceleration of Pioneer 10 and 11'', Anderson et al, Phys. Rev. D 65, 082004 (2002)

FURTHER READING

The ISSI meeting above has an excellent [http://www.issi.unibe.ch/teams/Pioneer/pa-literature.htm 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:

(preprint) ''A Mission to Test the Pioneer Anomaly'', Anderson et al, Int.J.Mod.Phys. D11 (2002) 1545-1551

(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

(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.

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.

	CATEGORIES ABOUT PIONEER ANOMALY

	gravity

	pioneer program

	unsolved problems in physics

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Pioneer Anomaly