Observations of the trans-Neptunian objects 1993 SC and 1996 TL66 with theInfrared Space Observatory

Observations at 90 mu m of the trans-Neptunian objects (TNOs), 1993 SC and 1996 TL66, using the Infrared Space Observatory (ISO) are reported. Five in dividual observations of 1993 SC were acquired giving a 2.7 sigma detection (a confidence level of 99.6%). The signal level of 11.46 +/- 4.24 mJy has been modeled using a standard thermal model (STM) and gives an effective ra dius of 164(-33)(+29) km and a geometric albedo of 0.022(-0.006)(+0.013). E stimated radii and albedos using the fast rotator approximation and the the rmophysical model are also presented. Two individual observations of 1996 T L66 are also reported. A clear signal of 39.77 +/- 11.62 mJy at 90 mu m was recorded. However, the position of the signal on the detector does not cor respond to the position expected. A detailed investigation has not revealed a satisfactory explanation. Assuming that ISO was mispointed and that the origin of the signal is 1996 TL66, application of the STM gives an effectiv e radius of 316(-49)(+42) km and a geometric albedo of 0.030(-0.007)(+0.012 ). This is in good agreement with expectations based on the assumption that the surfaces of TNOs are similar to those of cometary nuclei. The results for 1996 TL,, and 1993 SC indicate that TNOs are large, spherical, and very dark objects. A main-belt asteroid, 1997 SU15, was also detected giving an effective radius of 1.13 +/- 0.04 km and a geometric albedo of 0.25 +/- 0. 02. An estimate of the relative dependence of the zodiacal light background in the ecliptic on elongation angle at 90 mu m was also determined.