An Anomaly from the Edge of the Solar System

by Paul Gilster on November 23, 2004

Those of us with still fresh memories of Voyager 2′s encounter with Neptune in 1989 find it gratifying that both Voyager probes are still returning good science. It’s even more remarkable that the Pioneer 10 and 11 probes are still in the thick of things, but anomalies in their journeys beyond the orbit of Pluto offer tantalizing clues of some unexplained phenomenon in the far ranges of the Solar System.

As this article in Nature points out, since 1980 the Pioneers have been returning radio signals that have kept shifting to shorter and shorter wavelengths. The implication: both spacecraft are decelerating, even if only by the slightest amount. Some are calling this the ‘Pioneer anomaly,’ and it may just point to a new principle in physics, perhaps involving exotic forces or undiscovered forms of matter.

On the other hand, it may have a much more mundane explanation, such as a fuel leak that could be affecting the probes’ progress. Either way, engineers faced with designing navigation systems for deep space need to know the answer, and a misson to follow the Pioneer craft is now being proposed by Slava Turyshev of NASA’s Jet Propulsion Laboratory in Pasadena, California. In a preprint available online at the ArXiv site, Turyshev and colleagues Michael Martin Nieto and John D. Anderson summarize the issue:

The inability to find a standard explanation for the anomaly, combined with the evident lack of suitable experimental opportunities, motivated an interest in developing a designated mission to study the detected signal… The mission could lead to a determination of the origin of the discovered anomaly; it could also characterize its properties to an accuracy of at least three orders of magnitude below the anomaly’s size. The mission must be capable to discover if the anomaly is due to some unknown physics or else to an on-board systematic. Either way the result would be of major significance. If the anomaly is a manifestation of new or unexpected physics, the result would be of truly fundamental importance. However, even if the anomaly turns out to be an unknown manifestation of an on-board systematic, its understanding would vitally affect the design of future precision space navigation, especially in deep space. Furthermore, technologies and mission design solutions envisioned for this experiment will be vital to many space missions that are to come.

Making this mission happen will mean designing a spacecraft with navigation and instrumentation capabilities beyond those of NASA’s Jupiter Icy Moons Orbiter, which is scheduled for a launch around 2015. A more realistic option may be to design new instrumentation that could be incorporated into the design of a mission already being planned. The Pioneers themselves won’t be able to help — both are far enough away that they no longer can communicate with Earth.

“Lessons Learned from the Pioneers 10/11 for A Mission to Test the Pioneer Anomaly” is now available here on the ArXiv server (PDF warning). Image credit (above): NASA.