[RightWhale]

Could there be a component of gravity that doesn't follow the inverse square law, but something somewhat less than the inverse square?

Or, perhaps there is a dispersed gravity source within the solar system that hasn't been noticed yet, not a planet, but a cloud of matter, gas possibly.

Perhaps it is electrostatic in nature, the ship could have picked up a large charge by now.

[blam]

The Great Attractor?

[El Gato]

If all the probes are experiencing about the same amount of anomalous accelleration, but presumably are heading off in different directions, then whatever is causing it must be either something associated with the solar system itself, or be some isotropic (same in all directions) property of the local interstellar neighborhood. All these probes are still fairly close to the solar system, so that any effect from other (known that is) stars and such would be fairly small, and with gravity effects from them well accounted for at any rate. To get a feel for the scale of things: Pioneer 10 is only 11 light hours out, with Voyager 1 being somewhat farther out. The nearest star is something like 4 light *years* away, the earth is about 8 light *minutes* from the sun.