THE mystery surrounding the Pioneer anomaly has deepened. The unexplained changes in acceleration seen in NASA's Pioneer 10 and 11 probes could be related to similarly odd shifts in the speed of other space probes, possibly pointing towards new physics.
In the 1980s, researchers at NASA noticed that the Pioneer 11 spacecraft was slowing down more quickly than expected as it neared the edge of the solar system. The effect persisted until NASA lost touch with the spacecraft in 1995. A similar effect showed up in the Pioneer 10 spacecraft, which was sent in the opposite direction. Finally, in 1998, John Anderson, then at the Jet Propulsion Laboratory (JPL) in Pasadena, California, and his colleagues made their finding public.
Since then, other space probes have exhibited unexplained changes in speed. When NASA's Galileo and NEAR spacecraft and ESA's Rosetta flew past Earth, they showed bigger than expected boosts in speed. The largest anomaly was recorded for NEAR, whose velocity changed 13 millimetres per second more than it should have. This excess is much larger than the expected errors in measurement. Anderson, who is now with Global Aerospace Corporation in Altadena, California, and his team think that the two effects might be related. They have re-analysed the Pioneer data and say that Pioneer 11's odd acceleration patterns seem to have begun right after its fly-by of Saturn in September 1979 (www.arxiv.org/ astro-ph/0608087).
The data for Pioneer 10 was not precise enough around the time of its fly-by of Jupiter to indicate whether its acceleration problems began with that encounter, says team member Michael Nieto of the Los Alamos National Laboratory in New Mexico. This may soon be remedied. JPL's Slava Turyshev recently unearthed archived data from Pioneer 10 and 11 (New Scientist, 3 June, p 46) and is reconstructing the missions in their entirety, which could help determine whether Pioneer 10's anomaly began with a fly-by too. "That's what I want to look at more precisely," says Nieto.
The researchers say that while it is possible that an overlooked effect from ordinary physics might account for the anomalies, something more exotic could also be involved. For example, the spacecraft trajectories could be influenced by the presence of dark matter in the solar system, says Nieto. Or maybe the laws of gravity need reworking. "We're just throwing it out as a possibility that the anomalies might have a single cause," says Anderson. "We thought it was really time to get the community thinking about it."
Peter Antreasian, a spacecraft navigation expert at JPL who along with Joseph Guinn first brought attention to the anomalies seen in Galileo and NEAR during their Earth fly-bys, believes that it will require a modified law of gravity or other new physics to explain it. He does not think it is connected to the Pioneer anomaly, since the force behind this seems always to point in the same direction, back towards the sun. In the Earth fly-bys, by contrast, "a directional force such as the Pioneer anomalous force would have been very evident in the radiometric data in the last few days before the approach", he says. Whatever causes this anomaly seems to make its impact just a few minutes before the closest approach to Earth, he adds.
Not everyone is convinced the Earth fly-by anomaly points to new physics. "It's like a farmer in Louisiana seeing a light in the sky and immediately screaming 'UFO!', whereas it could be a number of other things," says Myles Standish, who calculates trajectories of solar system bodies for JPL. He says he feels the Earth fly-by anomaly is almost certainly due to an error in measurement or an incomplete analysis using ordinary physics
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