  Thermal modeling of the LISA gravitational reference sensor (GRS) includes
such effects as outgassing from the proof mass and its housing and the
radiometer effect. Experimental data in conditions emulating the LISA GRS are
required to confidently predict the GRS performance. Outgassing and the
radiometer effect are similar in characteristics and are difficult to decouple
experimentally.
  The design of our torsion balance allows us to investigate differential
radiation pressure, the radiometer effect, and outgassing on closely separated
conducting surfaces with high sensitivity. A thermally controlled split copper
plate is brought near a freely hanging plate-torsion pendulum.We have varied
the temperature on each half of the copper plate and have measured the
resulting forces on the pendulum.
  We have determined that to first order the current GRS model for the
radiometer effect, outgassing, and radiation pressure are mostly consistent
with our torsion balance measurements and therefore these thermal effects do
not appear to be a large hindrance to the LISA noise budget. However, there
remain discrepancies between the predicted dependence of these effects on the
temperature of our apparatus.
