GRAVITATIONAL PHYSICS SPACE PROBES SCIENTIFIC MYSTERIES A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site Pioneer anomaly Both Pioneer 10 and 11 are slightly off course The Pioneer 10 and Pioneer 11 deep space probes are not where they are supposed to be. Radio telemetry received from the spacecraft, over a period of many years before contact was lost with both probes, indicated that they are slowing down slightly more than expected with the result that each year they travel about 5,000 km (3,000 miles) less than mission controllers had projected. The slowing down is small, amounting to a deceleration towards the Sun of (8.74 ± 1.33) 10-10 m/s2, i.e. than a nanometer (a billionth of a meter) per second per second. This is equivalent to just one ten-billionth of the gravity at Earth’s surface. However, although tiny, the effect has been persistent over several decades. When mission controllers last heard from Pioneer 10, it was a quarter of a million miles from where it was supposed to be – roughly the distance from the Earth to the Moon. When NASA lost touch with Pioneer 11, several years earlier, it was heading for a similar deviation. The anomalous deceleration of the twin Pioneers is inferred from a small, constant, anomalous Doppler shift in the frequency of radio signals received from the spacecraft. Various explanations have been put forward to account for the so-called Pioneer anomaly. Although it is suspected that there may be a systematic origin to the effect, such as thrust from a gas leak, none has been found. Some scientists have therefore considered more esoteric explanations, including the possibility that our understanding of gravity made need to be revised. Origins of the mystery The trajectories of Pioneer's 10 and 11 and Voyager's 1 and 2 up to 1992 Pioneer 10 was launched in 1972 and became the first spacecraft to fly past Jupiter. Its identical sister craft, Pioneer 11, was launched the following year and subsequently flew past both Jupiter and Saturn. Both are now heading out of the Solar System but in completely different directions. For details of the spacecraft and where they are heading see the pages on the respective probes. Since about 1980 evidence has been accumulating for what has become known as the Pioneer anomaly. This evidence was first brought to light and, has subsequently been analyzed, by John Anderson (Pioneer 10 Principal Investigator for Celestial Mechanics) and his colleagues at the Jet Propulsion Laboratory in Pasadena. Their first published results appeared in 1998; 1 these prompted a number of articles in publications such as Scientific American (December 1998 issue) and Newsweek magazine (Oct. 4, 1999 issue) which brought the phenomenon to widespread public attention. Anderson and coworkers wrote a second paper, which appeared in 2002, discussing the anomaly further.2 The JPL team pointed to data from the Ulysses and Galileo spacecraft which suggest a similar effect may be at work on these probes. However, it has been hard to draw firm conclusions in the case of Ulysses and Galileo partly because they have not travelled as far from the Sun as have Pioneers 10 and 11. The Voyager 1 and Voyager 2 spacecraft, which have now gone even further from the Sun than the Pioneers, are not a helpful as might be expected in contributing to the investigation of the Pioneer anomaly because of the way in which they are stabilized. Unlike the Pioneers, which are spin-stabilized, the Voyagers have what is known as three-axis stabilization. This results in an greater uncertainty in the spacecrafts' theoretical positions. The uncertainty is sufficiently high as to mask any deceleration similar in magnitude to that seen in the Pioneer probes. Possible explanations A variety of possible causes of the Pioneer anomaly have been considered, ranging from the mundane to the exotic. These include: observational errors, including measurement and computational errors, in deriving the acceleration approximation/statistical errors possible corruption to the radio Doppler data gas leaks, including helium produced by radioactive decay escaping from the spacecrafts' radioisotope thermoelectric generators (RTGs) thermal radiation from the RTGs interactions between the solar wind and the spacecraft radiation pressure resulting from the tiny transfer of momentum when solar photons impact the spacecraft electromagnetic forces due to an electric charge on the spacecraft wobbles and other changes in Earth's rotation perturbations from the gravitational attraction of planets and smaller bodies in the solar system, including the Kuiper Belt drag from the interplanetary medium, including dust, solar wind and cosmic rays the influence of dark matter clock acceleration between coordinate or Ephemeris time and International Atomic Time new or revised theories of gravity It remains unknown which if any of these explanations is correct. No spacecraft behavior or previously unknown property of the outer solar system has been shown yet to explain the anomalous decelerations. Hence, although scientists expect that a mundane explanation remains most likely, they have been forced to consider theories that would involve a change in our understanding of the laws of physics. Further research Thus far only data from about 1987 on has been examined in detail. The reason for this is that earlier data received from the Pioneers is stored on tapes in a format that can only be handled by antiquated computers. NASA was unwilling to allocate resources to analyze this data. Consequently, Anderson and his colleagues approached the The Planetary Society, requesting funds in order that the data could be recovered and made available to researchers wishing to look further into the Pioneer anomaly. The Planetary Society appealed to its members and received the support needed for the recovery to go ahead. This process is now going ahead and analysis is expected to start soon. Other lines of research may involve close observation of other spacecraft heading for the outer solar system, including New Horizons, which is bound for Pluto and the Kuiper Belt. Unfortunately, New Horizons suffers from a similar drawback in this respect to the Cassini spacecraft – namely, that its RTGs are mounted close to the spacecraft's body, so infrared 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. Another suggestion, made by Gary Page of George Mason University and his colleagues, is to use remote asteroids to test if an unknown gravitational factor is the cause of the anomaly. They have identified 15 asteroids that might be subjected to the mysterious force; all of the asteroids' orbits stretch far into the outer solar system where the anomaly has made itself felt. Of the 15 candidates, the most promising is 1995SN55. This 370-kilometre-wide rock has spent the past 54 years in the anomaly zone, so it should have experienced the largest perturbation, and, interestingly, it is not where predictions say it should be. References Anderson, J. D., P. A. Laing, E. L. Lau, A. S. Liu, M. M. Nieto, and S. G. Turyshev."Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an apparent anomalous, weak, long-range acceleration." Phys. Rev. Lett. 81 (1998) 2858-2861. Anderson, J. D., P. A. Laing, E. L. Lau, A. S. Liu, M. M. Nieto, and S. G. Turyshev. "Study of the anomalous acceleration of Pioneer 10 and 11." Physical Review D 65 (2002) 082004. 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