
	I am puzzled by the term "concept." Drag free may already have
been flown. It was the idea behind putting up a spacecraft that would
more accurately respond to motions from the Earth's gravity field and
ignore drag. It was proposed many years ago and involved a ball
floating between sensors whose job it was to signal to little
adjustment jets to keep the ball away from them. The ball itself would
then be in a drag free condition and respond only to gravity
anisotropies, whereas the spacecraft itself would be continuously
adjusting its position to compensate for drag.
