A problem with gravity

This has been in the news lately.

Here's a story from Nature from last September. It discusses both John Anderson and his Pioneer anomaly and Marty Milgrom, an Israeli physicist who has an alternative explaination (besides dark matter) for why galaxies don't seem to behave properly. The discrepencies seem to be similar.

Physicists are looking at ways to study a tiny but mysterious force acting on a pair of spacecraft at the edge of the Solar System.

One research team is seeking support to investigate flight data from the early stages of the Pioneer 10 and 11 missions, in a bid to understand why the craft are now travelling slightly more slowly than mission planners envisaged. A second, far more ambitious proposal would launch a dedicated space mission to study the effect, which has left the craft hundreds of kilometres closer to the Sun than had been anticipated.

The Pioneer anomaly, as the effect is known, was first detected about eight years after NASA launched the two craft to probe the outer Solar System in 1972 and 1973, respectively. It suggests that an unexpected force — about a hundred million times weaker than gravity's force on the surface of the Earth — has been pulling the craft back towards the Sun.

Many physicists think the slow-down is caused either by some unknown physical change in the craft themselves, increasing their drag, or by errors in the techniques used to track them.

But others have suggested that gravitational theory needs to be modified to account for the effect. This view was boosted in 2002, when an investigation into possible causes, led by John Anderson, a physicist at the Jet Propulsion Laboratory (JPL) in Pasadena, California, failed to find an alternative explanation (J. D. Anderson et al. Phys. Rev. D 65, 082004; 2002). The team rejected numerous effects, such as possible leaks of heat or gas from the crafts' plutonium thermoelectric generators.

In the absence of a conventional explanation, physicists have produced speculative alternatives. Mordehai Milgrom (The MOND pages) of the Weizmann Institute of Science near Tel Aviv in Israel, for example, has developed a modified version of classical gravity in which the strength of the gravitational field decreases more slowly than newtonian theory suggests. Milgrom produced his theory to account for discrepancies in the motion of galaxies, an effect normally attributed to the presence of dark matter, but physicists have pointed out that it could also account for the Pioneer anomaly.