Schafer proposes to demonstrate 2 different methods for actively cooling our 5-7.5 kg/m2 areal density Silicon Lightweight Mirrors (SLMS?) technology for future NASA far infrared and sub-millimeter missions. In Phase I, direct internal cooling will be demonstrated by directly flowing liquid nitrogen through the continuous open cell core of the SLMS?. Indirect external cooling will be demonstrated by flowing liquid nitrogen through a CTE matched C/SiC manifold that is bonded about the circumference of the SLMS?. During Phase II we plan to exercise our Space Act Agreement to helium test the SLMS? at NASA MSFC XRCF. Previously, under Contract No. NAS8-01174 entitled, "Silicon Lightweight Mirrors (SLMs) for Ultraviolet and Extreme Ultraviolet Imaging Mirrors" performed for Drs. Andrew Keys and Phil Stahl, Schafer produced a SLMS? UV Demonstrator Mirror (UVDM) with 9.8 kg/m2 areal density, 0.021 waves rms HeNe figure accuracy, and 4 ? rms surface roughness. The SLMS? UVDM was cryo-tested from 300 K to 27 K at NASA MSFC in the 4-foot XRCF chamber on 2 separate occasions, once uncoated and once with a multiple-layer dielectric coating. We demonstrated extreme dimensional stability with a relative deformation < /60 rms HeNe for the 275 K temperature excursion.