The nuclei of comets 126P/IRAS and 103P/Hartley 2

We report the detection of the nucleus of 126P/IRAS and 103P/Hartley 2 with the Infrared Camera of the Infrared Space Observatory (ISOCAM). 126P/IRAS was observed on 12 November 1996, when it was at r(h) = 1.71 AU from the Sun and Delta = 1.32 AU from the Earth. 103P/Hartley 2 was observed on 5 February 1998, when it was at r(h) = 1.21 AU from the Sun and Delta = 0.91 AU from the Earth. The observations were performed with the broadband LW 10 filter centered at 11.5 mum. The spatial resolution was adequate to separate the thermal emission of the nuclei from that of their respective comae. We combined the ISOCAM observations with measured water production rates, using a model that considers a spherical nucleus with a macroscopic mosaic of small and numerous active and inactive regions, and we derived a radius of 1.57 +/- 0.14 km and an active fraction at perihelion of 0.11 +/- 0.03 for 126P/IRAS, and a radius of 0.71 +/- 0.13 km and an active fraction of similar to 1 at perihelion and 0.30 +/- 0.11 at 1.11 AU post-perihelion for 103P/Hartley 2. These two examples illustrate the large diversity of activity pattern that exists among cometary nuclei.