What Is a Drag-Free Satellite?
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 jets which are fired so that
the satellite chases the proof mass which then always remains centered in
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
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. (Also see other Drag-Free
Experimental Gravitational Physics With Drag-Free
Drag-Free Satellites offer an environment with the lowest disturbing forces
in nature. A properly designed Drag-Free-Satellite proof mass is uncoupled
from the rest of the Universe to a remarkable degree.
There are three important experiments in Gravitational Physics which require
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).
The LISA mission to detect gravitational waves in space will explore
Massive Black Holes throughout the Universe.
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.
The U. S. Navy's Transit Navigation Satellites
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 1970's
and 1980's, three test vehicles and three operational flights. One was a
full three-axis system and 5 were single-axis. The three-axis flight was
the first and worked perfectly. The last three operational single-axis
vehicles also worked perfectly.
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