…or at least as much as we think we do. Does the gravity model need to be adjusted?

In the one probe the researchers did not confirm a noticeable anomaly with, MESSENGER, the spacecraft approached the Earth at about latitude 31 degrees north and receded from the Earth at about latitude 32 degrees south. “This near-perfect symmetry about the equator seemed to result in a very small velocity change, in contrast to the five other flybys,” Anderson explained — so small no anomaly could be confirmed.

The five other flybys involved flights whose incoming and outgoing trajectories were asymmetrical with each other in terms of their orientation with Earth’s equator.

For instance, the NEAR mission approached Earth at about latitude 20 south and receded from the planet at about latitude 72 south. The spacecraft then seemed to fly 13 millimeters per second faster than expected. While this is just one-millionth of that probe’s total velocity, the precision of the velocity measurements was 0.1 millimeters per second, carried out as they were using radio waves bounced off the craft. This suggests the anomaly seen is real — and one needing an explanation.

Well, gravity just like evolution, is (in the words of anti-evolutionists) only a theory. It’s not reality–it’s simply an attempt to model it. And for most purposes, it does a pretty good job. But one of the reasons to do space, I think, is that it gives us new laboratories to make new discoveries about basic physics, the potential of which is unforeseeable.