  The low-frequency resolution of space-based gravitational wave observatories
such as LISA (Laser Interferometry Space Antenna) hinges on the orbital purity
of a free-falling reference test mass inside a satellite shield. We present
here a torsion pendulum study of the forces that will disturb an orbiting test
mass inside a LISA capacitive position sensor. The pendulum, with a measured
torque noise floor below 10 fNm/sqrt{Hz} from 0.6 to 10 mHz, has allowed
placement of an upper limit on sensor force noise contributions, measurement of
the sensor electrostatic stiffness at the 5% level, and detection and
compensation of stray DC electrostatic biases at the mV level.
