Introduction

Tight control of payload or component temperatures to values on the order of 1 C has been routinely accomplished on many programs during the past few decades. In recent years, however, some civilian and military payloads have been developed that demand much more stringent temperature stability for precision optical systems and high-accuracy clocks. For optical systems, even milliKelvin (mK) changes in temperature can result in thermal deformations in the range of nanometers and picometers that can translate into unacceptable errors in the pointing accuracy of optical elements. The Space Interferometry Mission (SIM), the Next Generation Space Telescope (NGST), and the Terrestrial Planet Finder (TPF) are examples of state-of-the-art programs with such strict deformation requirements on optical surfaces. Even tighter temperature control can be required for time-reference clocks where temperature changes of only tenths of a milliKelvin can cause unacceptable drift in time measurement. The hydrogen maser clock (HMC) program is an example of a time-reference system with such demanding thermal-control requirements.

Given that an aluminum atom has a diameter on the order of 0.3 nanometers, maintaining the relative position of the optical elements on a spacecraft...