We are developing infrared-blocking filters for millimeter and sub-millimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultra-low-density aerogel substrate has a low index of refraction, removing the need for an anti-reflection coating and allowing for ultra-broadband operation across the full range from zero frequency to 10 THz. The size of the scattering particles can be tuned to give variable cutoff frequencies. This technology would be key in enabling a broad range of missions, including Inflation Probe and future SOFIA instruments. Small scale prototypes have been made and measured to have excellent rejection of infrared radiation and variable cutoff frequency. Further technology development is required to (1) demonstrate the ability to scale the technology up to the full, roughly 30-cm-diameter size needed for Inflation Probe, (2) test the thermal filtering performance at the full size, and (3) investigate materials to extend the technology to higher frequencies.