E. Grün 1, S. B. Peschke 1, H. Krüger 1, M. Müller 1, M. S. Hanner 2, T. Y. Brooke 2, H. Böhnhardt 3, H. Campins 4, D. J. Osip 4, J. Crovisier 4, I. Heinrichsen 5, H. U. Keller 6, R. Knacke 7, P. Lamy 8, Ch. Leinert 9, D. Lemke 9, M. Stickel 9, C. M. Lisse 10, M. Solc 11, V. Vanysek 11[+], M. Sykes , & J. Zarnecki 12, [+] deceased
1 MPI-K, Heidelberg, Germany
2 JPL, Pasadena, USA
3 ESO, Santiago, Chile
4 U. Florida, Gainesville, USA
5 Observatoire de Paris, Meudon, France
6 ESA-VISPA, Spain
7 MPI-Ae, Lindau, Germany
8 Penn State University, Erie USA
9 LAS, Marseille, France
10 MPI-A, Heidelberg, Germany
11 U. Maryland, College Park, USA
12 Steward Obs., Tucson, USA
13 Charles University, Prague, Czech Republic
14 University of Kent, Canterbury, UK
Comet Hale-Bopp has been observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO), four times (March, April, September, and October 1996) before its perihelion passage at heliocentric distances of 4.92AU, 4.58AU, 2.93AU and 2.81AU, respectively and at 3.91AU on its way out (December 1997). Each time, multi-filter photometry covering the range between 3.6-175 micrometers with nine filters was performed in order to sample the spectral energy distribution of the comet. The analysis of comet observations with ISOPHOT had to consider (1) any offset of the comet's position from the center of the aperture, (2) the brightness distribution within the coma, (3) the zodiacal background, and (4) the spectral energy distribution of the comet's emission. The first two observations in Spring 1996 revealed a color temperature of 150K +/- 10K. For the second set of observations in Autumn 1996, the color temperature was 205K +/- 15K. Extensive spectral modelling of the observations has been used to infer the dust composition and size distribution. There is evidence that the dust composition and the size distributions varied between the observations.