Swimming In Newtonian Space

Michael Longo, a physicist at the University of Michigan, suggests that a satellite going around the Earth can change its orbit by proper gymnastic alteration of its shape, without the need for any external force other than gravity. This proposition has been previously made by invoking the properties of curved spacetime at the heart of general relativity which predicted a very tiny effect (Science, 28 February and 21 March 2003), but Longo shows that Newtonian physics predicts a much larger effect.

Consider, he says, a dumbbell in Earth orbit. The outer mass feels slightly less gravity than the inner mass. If the masses are pulled together the differential force on the separate masses will cause the center-of-mass of the dumbbell to move inward slightly. How slightly? For a 100-meter-sized dumbbell in a highly elliptical orbit, the object's perigee (the point of closest approach to Earth) can move inward 1 mm on each pass. In effect, the satellite could use solar energy or other stored energy to change its orbit, without the need for propellant. (Longo, American Journal of Physics, October 2004.)