Drag-free satellite

Drag-free satellite is a special type of satellite that offers an environment that is as isolated as possible from the forces of nature. A properly designed drag-free-satellite proof mass is uncoupled from the rest of the Universe to a remarkable degree.

The classical drag-free satellite is actually two satellites in one. A small inner, usually spherical, proof mass is located in a cavity inside of a larger normal satellite. The cavity contains sensors, capacitive or optical, which measure the position of the proof mass with respect to the outer satellite. The main satellite has small thrusters or jet engines which are fired so that the satellite chases the proof mass which then always remains centered in the cavity.

Since the proof mass is shielded by the outer satellite from gas drag and solar radiation pressure, it follows a trajectory which is determined only by gravity and the very small internal disturbing forces from the main satellite. Hence the name drag-free satellite since there is no drag on the inner proof mass, and the drag on the main satellite is exactly compensated by the thrusters.

These satelites are used to conduct a number of important experiments in gravitational physics which require such isolated conditions. They can be categorized into the following types

1. A test of the Equivalence Principle, more accurately known as a test of the universality of free fall (i.e. independent of mass or composition).

2. The LISA mission to detect gravitational waves in space will explore Massive Black Holes throughout the Universe.

3. The Relativity Gyroscope experiment to search for a Scalar Field and measure Frame-Dragging. A proof mass which is spun up acts as a gyroscope with almost no disturbing torques. Such a gyroscope is accurate enough to perform these tests of General Relativity.

Drag-free satellites have been used to eliminate drag on the U.S. Navy's TRANSIT navigation satellites thus greatly lengthening the time with which their orbits could be accurately predicted. Six were flown in the 1970s and 1980s, three test vehicles and three operational flights.