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==> In this proposal, more time is being requested for PLAMY.COMETNU1
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Because they are extremely difficult to observe, cometary nuclei are poorly
known objects in spite of their pristine nature and of their potential as
probes to the origin and evolution of the solar system. We propose to study
the nuclei of 4 short-period comets, P/Encke, P/Tempel-Tuttle, P/Wild2 and
P/Elst-Pizzaro which may be considered a transition object (active asteroid
?), by measuring their thermal emission in the 8-15 micron spectral  band as
a function of time over approximately 10 hours (expected mean value  of their
rotational periods). As demonstrated by ground-based  observations under
favorable conditions of a few comets devoid of coma  (e.g., comet
P/Arend-Rigaux), the thermal light curve allows to determine,  under minimal
assumptions, the sizes and shapes of the nuclei and to study  their
rotational  properties. We shall also observe, as a "by-product",  the
thermal emission from the innermost dust coma and shall study possible dust
structures and follow their temporal evolution. Our top priority goes to 3
cometary nuclei for which we have a similar and complementary program with
the Hubble Space Telescope: P/Encke and P/Tempel-Tuttle for which we
submitted a proposal for the forthcoming cycle 7 and P/Wild2, the target of
the STARDUST mission, for which we shall shorthly sollicit Discretionary
Director Time. Although simultaneous observations are excluded because of
very different constraints, combining the thermal (ISO) and the visible
(HST) observations, after proper correction for different viewing geometries,
will allow to obtain additional information and constraints on these nuclei.
In particular, it will be possible to derive their albedo which will give  us
information on their degree of mantling and on the composition of the
mantle. Extending our knowledge and understanding of cometary nuclei as made
uniquely possible by ISO (and HST) is essential in the context of the space
exploration of primitive bodies (ROSETTA and STARDUST missions).