However, once they passed beyond the orbit of Uranus, the radio signals sent back by the aging craft have been shifted to progressively shorter wavelengths. This implies that they are decelerating slightly; no one knows why this is happening.

Some space scientists are calling for a deep-space mission to try to understand this behavior - called the Pioneer anomaly. Mundane reasons for this effect have been brought forward; it could be due to leakage of gaseous fuel from the thrusters, for example. However, some scientists wonder if the Pioneer anomaly may be due to some previously unknown forces that are only noticed outside the solar system.

(From Pioneer 10 aerospace physics)

A number of possible causes have been ruled out by Philip Laing and Anthony Liu, working under a NASA Office of Space Science grant:

• Errors in JPL's orbital determination software
• Gravity of the Kuiper belt and gravity from the galaxy
• Errors in the planetary ephemeris
• Errors in the values of the Earth's orientation, precession, and nutation
• Nongravitational effects from solar pressure and attitude control maneuvers
• Solar wind and interplanetary medium
• Nominal thermal radiation and plutonium half-life
• Drifting clocks, general relativity, and the speed of gravity
• Hardware problems at the tracking stations.

Slava Turyshev of NASA's JPL in Pasadena thinks that the issue is worth exploring even if it turns out to be a gas leak, which is considered the most likely cause. Working on this problem forces space engineers to rethink their methods for very precise navigation in space. There is a possibility that upcoming space missions may be modified to try to verify (or disprove) the idea that unknown physical forces are behind the Pioneer anomaly.

However, no explanation based on conventional physics has been found; the effect is so persistent that it could indicate some physical principle not yet considered in previous explanations of the motions of the planets. Meanwhile, Pioneer 10 is on its way to the giant red star Aldebaran. Estimated time of arrival - 2 million years from now.

As soon as I read this story, I thought about the classic 1950 sf novel Brain Wave by Poul Anderson. At the time of the story, the sun moves out of a field that pervades a huge area of space, one which depresses electrochemical and electromagnetic phenomena - including human neural function. The result of the solar system leaving this field is that everyone on Earth immediately gains about one hundred IQ points due to increased synaptic speed.