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Measurements of Iapetus as seen at 20 and 2.2 μm in the shadow of Saturn's ring are given, providing the thermal response to a rapidly varying heat input. The 20 μm thermal emission follows the 2.2 μm flux input closely. The observations, plus a simple diffusion calculation, imply that the surface of Iapetus is made of material having a very small thermal inertia, probably / ≾ 10Jm^(-2)s^(-½)K^(-1).

Comet Kohoutek has been observed at wavelengths between 1.25 and 12.5 microns before and after perihelion passage extending to comet-sun distances of 1 AU. The luminosity and the variation of brightness with comet-sun distance in the infrared are extraordinarily similar to those of Comet Ikeya-Seki (1965f). Apart from an apparent 'silicate' emission feature near 10 microns, the spectrum of the comet between 3.5 and 12.5 microns is close to that expected from emission by grey particles. Hotter particles and scattered sunlight produce the bulk of the 1.25- to 3.5-micron emission.