Why would the test masses fall when they are in orbit at constant height?
The test masses are always falling, in orbit. That’s why weightlessness is sometimes called free fall. It is important not to confuse falling with changing height. Like a projectile, a satellite is always falling, but it is moving sideways at the same time, so after it’s gone (for example) a kilometer horizontally, it’s fallen eight centimeters or so below perfectly straight travel. But the Earth is round, and its surface curves downwards eight centimeters in the same distance. So the satellite will be the same height above the Earth as when it started. Same thing in the next kilometer, and the next…until the satellite gets all the way around, at the same height it started at. But it’s fallen all the way, it just happened to miss hitting the ground. See also, The Hitchhiker’s Guide to the Galaxy. The same is true of the test masses inside the satellite. They are falling too, nothing is holding them up. The satellite is there mostly to protect them from the wind of their motion, which
