A Force to Reckon With; October 2005; Scientific American Magazine; by Alexander Hellemans; 2 Page(s)

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.