The electric universe folks say the constant small acceleration toward the sun is due to the spacecraft picking up a static electric charge on its outside that is attracted toward the sun's slighly positive charge, the spacecraft becomes a partially negative body moving in a circular orbit through a spherically symetric weak electric field. The nice thing about that theory is it explains why the devation from keppler's predictions take the form of a small constant acceleration in the direction of the sun. I read recently that the latest solar wind observations show a net electron backdraft toward the sun. That would support the notion of a mild positive charge to the sun, and begin to tie together the whole theory.