Newfound Data Could Solve NASA's Great Gravity Mystery (SPACE.com)

I'm betting on Newton, and a physical reason for slowing.

When I was in grade school, I had a teacher tell me, (I think it was a teacher, that was a long, long time ago) that the earth's gravity is keeping the moon in our orbit the same as the sun's gravity is keeping the planets in orbit.

any truth to that?

Your teacher was right.

The Earth and Moon orbit each other because of their mutual gravity, but the Earth is a lot more massive than the Moon, so it's very nearly accurate to say that the Moon orbits the Earth because of the Earth's gravity.

In the same way, the Earth-Moon system orbits the Sun because of the Sun's gravity.

It is interesting to note that if the Sun suddenly disappeared, the Moon would continue to orbit the Earth. On the other hand, if the Earth suddenly disappeared, the lonely Moon would continue to orbit the Sun!

The solar wind may have good momentum in the inner solar system but, at some distance out, I would expect all that gas, ions and charged particals to bunch up into very thin clouds- thin as in the opposite of dense. Perhaps the Pioneer probes are simply experiencing drag.