Gravity, that fundamental force explained by Isaac Newton so well, may not be so constant afterall.
Recent probes hurtling across Earth's terminator have not behaved as expected, missing the tightly calculated predicted trajectory by a small margin. The incidence has been measured at least five times in cases around the Earth and also on the outbound slingshot characterizing the departure of Pioneer X and XI from the solar system, billions of miles -- and several light-minutes away.
This troubling occurrence has suggested to some scientists that the law of gravity is somehow incorrect and needs to be modified - if something is mostly true, but not always, than by definition it cannot be a law in the physical sciences, at least as presently conceived. Or, on the other hand, a new principle of physics exists that we have not accounted for up to now.
Attempting to force observation into a law and creating a 'fudge factor' was previously the case with the Ptolemaic system, which defined and expanded the concept of epicycles or tangent circles to account for perturbations in planetary orbits, in particular the planet Mars - until it was realized (by Kepler) that orbits were ellipses. Previously, it was believed that orbits must be circular (perfect).
more at space.com