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The Pioneer Anomaly

A Spinor Info page.

In 1972, NASA launched its most ambitious space probe to date: Pioneer 10 was designed for the first time to leave the inner solar system, cross the asteroid belt, perform close-up observations of Jupiter, and continue to fly out of the solar system on an hyperbolic escape trajectory. A year later, its sister craft, Pioneer 11 followed on an orbit that took it not just to Jupiter, but also to Saturn several years later.

The Pioneer spacecraft were spin stabilized, requiring very few attitude correction maneuvers to maintain an Earth-pointing orientation of their high gain antenna. The small number of maneuvers means that Pioneer 10 and 11 flew most of the time undisturbed.

Pioneer 10 survived for over 30 years, the last communication taking place in 2003. The life of Pioneer 11 was a little shorter; last communication took place in 1995, but the spacecraft ceased providing precision orbit measurements in 1990, due to a glitch in its radio communication subsystem.

The line-of-sight velocity of the Pioneer spacecraft was measured using radio Doppler observations. During their long operating life, the two Pioneer spacecraft provided a near continuous stream of Doppler data. This allowed precision orbit determination at a level that remains unsurpassed to this date.

When the Pioneer Doppler data set is fitted to models of the known gravitational forces in the solar system, there is a discrepancy. After all known effects (including small nongravitational forces) are accounted for, we find that the discrepancy remains; however, it can be eliminated if we assume the presence of a small, approximately constant, approximately sunward pointing acceleration. The magnitude of this acceleration is very small: about 10^–9 m/s², which is roughly one ten billionth of the gravitational acceleration on the surface of the Earth. Nevertheless, if this is a real acceleration, it'd mean that by the end of its useful life, Pioneer 10 was nearly half a million kilometers off course.

What can possibly cause this discrepancy? To date, the answer eludes us, although a strong possibility exists that the explanation is mundane: heat emitted by the spacecraft preferentially in one direction may be sufficient to cause an acceleration of the required magnitude. Our work to model the thermal behavior of Pioneer 10 and 11 is ongoing, relying, among other things, on a reconstruction of the engineering history of the spacecraft, made possible by recovered telemetry data.

There are, of course, other alternatives: the possibility that Pioneer has detected new physics beyond Einstein right here within the solar system is tremendously exciting.

The literature of the Pioneer anomaly is huge. Much of it is about attempts to explain the anomaly. Some important papers that deal with the anomaly itself are:

• Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration.
  J. D. Anderson and P. A. Laing and E. L. Lau and A. S. Liu and M. M. Nieto S. G. Turyshev, Physical Review Letters, Vol 81, No 14, 2858-2861 (1998).
  This is the original paper reporting on the discovery of the anomalous acceleration.
• Comment on "Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration".
  J. I. Katz, Physical Review Letters, Vol 83, No 9, 1892 (1999).
  One of two replies raising the possibility that the anomaly may be due to prosaic causes.
• A Prosaic Explanation for the Anomalous Acceleration Seen in Distant Spacecraft.
  Edward M. Murphy, Physical Review Letters, Vol 83, No 9, 1890 (1999).
  The second of two replies raising the possibility that the anomaly has a prosaic cause.
• Reply (to the comment by Katz on "Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration").
  J. D. Anderson and P. A. Laing and E. L. Lau and A. S. Liu and M. M. Nieto S. G. Turyshev, Physical Review Letters, Vol 83, No 9, 1893 (1999).
  Anderson and collaborators dispute the possibility that anisotropic heat radiation was sufficient to cause the anomaly.
• Reply (to the comment by Murphy on "Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration").
  J. D. Anderson and P. A. Laing and E. L. Lau and A. S. Liu and M. M. Nieto S. G. Turyshev, Physical Review Letters, Vol 83, No 9, 1891 (1999).
  Anderson and collaborators dispute the possibility that electrical heat can be responsible for the anomalous acceleration.
• Conventional Forces can Explain the Anomalous Acceleration of Pioneer 10.
  L. K. Scheffer, Physical Review D, Vol 67, No 8, 084021 (2003).
  A detailed study by Lou Scheffer of the conventional forces present on Pioneer 10, which seem to be sufficient to account for the anomalous acceleration.
• Study of the anomalous acceleration of Pioneer 10 and 11.
  J. D. Anderson and P. A. Laing and E. L. Lau and A. S. Liu and M. M. Nieto and S. G. Turyshev, Physical Review D, Vol 65, No 8, 082004 (2004).
  This is the "big paper", the most comprehensive study to date of the Pioneer anomaly.
• Independent Confirmation of the Pioneer 10 Anomalous Acceleration.
  Craig B. Markwardt, eprint arXiv:gr-qc/0208046.
  The first of several truly independent confirmations that the anomaly is indeed present in the Pioneer Doppler data.
• A study of the Pioneer anomaly: New data and objectives for new investigation.
  S. G. Turyshev and V. T. Toth and L. R. Kellogg and E. L. Lau and K. J. Lee, IJMPD Vol 15, No 1, 1-55 (2006).
  The first detailed account of newly recovered Doppler and telemetry data.
• The Pioneer anomaly: seeking an explanation in newly recovered data.
  V. T. Toth and S. G. Turyshev, Can. J. Phys. 84: 1063-1087 (2006).
  Further details about the new data, mostly focusing on the telemetry.
• The constancy of the Pioneer anomalous acceleration.
  Ø. Olsen, A&A 463, 393-397 (2007).
  Another independent confirmation of the Pioneer acceleration, with emphasis on studying its constancy.