Pioneer : Anomaly Still Anonymous

http://www.sciam.com/article.cfm?art...4583414B7F0000

"One of the most intriguing mysteries in physics is the "Pioneer
anomaly," the slowing down of two spacecraft by an unknown force. NASA
launched Pioneer 10 and 11 in 1972 and 1973, respectively, and the
craft returned stunning images of Jupiter and Saturn. But as both
spacecraft continued their voyages at speeds of roughly 27,000 miles
per hour, astronomer John Anderson of the Jet Propulsion Laboratory in
Pasadena, Calif., noticed anomalies in telemetry data dating from as
far back as 1980. With continued analysis, researchers determined that
the spacecraft had been slowing down at a constant rate: each year they
fell 8,000 miles short of their calculated positions. The strange
behavior sparked several theories, but the lack of data made culling
the ideas difficult. Now a proposal to analyze telemetry from the early
years could literally point toward the correct explanation.

"The most obvious theory was that something on the spacecraft
themselves created a braking force--leaking gas or heat radiation,
perhaps. Over the years, however, researchers increasingly viewed this
hypothesis as less likely, and some physicists began to explore
possible flaws in Newton's laws and relativity. Others posited that
dark matter was the culprit: it might exert a gravitational or drag
force. A third theory embraces the idea that a minute acceleration
exists in the velocity of light, which might result in the appearance
that the probes are slowing down: if light travels faster, telemetry
signals arrive faster, and the craft seem to be closer.

"Anderson and theorist Michael M. Nieto of Los Alamos National
Laboratory have proposed a way to filter the ideas, noting the
interesting fact that the direction of the anomalous force would be
different for each theory. If the force points toward the sun, then it
should be a gravitational effect. If it points toward Earth, it should
be an anomaly relating to the velocity of light. If it points in the
direction of motion, it should be a drag force or a modification of
inertia. And finally, if it points along the spin axis of the probes,
it should indicate a force generated by the craft. ..."

(Alexander Hellemans, "A Force to Reckon With: What applied the brakes
on Pioneer 10 and 11? ", Scientific American, 10 October 1995)


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