A special group of solar system objects are extended structures such as comet trails or zodiacal dust. These sources do not require tracking by the satellite. Therefore they are treated like normal targets and the restrictions to AOTs, as is the case for solar-system targets, do not apply.
In the proposal, these objects must be identified by setting both Right Ascension and Declination to zero and requesting fixed time observations. This is used as a flag for the ISO Science Operations Team to handle the observations manually. Appropriate information on coordinates must be contained in the proposal.