"The Planetary Society offers a fascinating look at a little-known issue that some in spaceflight circles are calling the "flyby anomaly" – the apparent tendency of spacecraft swinging past Earth to speed up slightly, more than can be accounted for by known forces.

The phenomenon, which has been under discussion since being observed during a flyby of the Jupiter-bound Galileo probe in late 1990, will be the subject of a paper appearing in Physical Review Letters early next month. But as yet, researchers have no answers.

During that Galileo pass, on the craft's way to Jupiter, Jet Propulsion Laboratory trackers found that the probe was traveling slightly faster at the end of its swing past Earth than their calculations had predicted. Initially they blamed the inaccuracy of their own instruments, but when the flybys of NASA's NEAR probe in 1998 and the European Rosetta probe in 2005 each showed a similar, unexplained effect, they began scratching their heads in earnest.

According to the Planetary Society article, JPL researchers say the anomaly is too large to be explained by known effects related to Einstein's general theory of relativity. But they don't have an alternative answer. Hence the paper – they've done considerable mathematical work describing the effect, but don't know what causes it, and want other ideas.

From the article:

    "The formula doesn't suggest anything to us" (JPL engineer John Anderson) readily admitted, but perhaps some physicists will be able to come up with an explanation.

Researchers Investigate New Cosmic Mystery: The Flyby Anomaly [Planetary Society]"

News Source: Wired Science