In the Main Call for Proposals three teams were successful in applying for ToO time to observe new comets. When, just prior to launch of ISO, Comet Hale-Bopp (O/1995) was discovered, it was soon obvious this would be the comet. All ISO's instruments were used to observe comet Hale-Bopp during two visibility periods, March/April and September/October 1996 - due to visibility constraints it it could not be observed around the time of closest approach to the Earth (22 March) or perihelion (1 April 1997). Some results of these observations were published in the March 28 issue of Science devoted to Hale-Bopp (Vol 275, no. 5308), and in ISO INFO issue 10.
One team, lead by E. Grün of Heidelberg, Germany, obtained ISOPHOT measurements of the temperature of the dust cloud around Comet Hale-Bopp. In March 1996, when the comet was still more than 700 million kilometres from the Sun, the dust cloud was at minus 120 degrees C. When ISOPHOT made similar observations in October 1996 (see figure below), the comet was 420 million kilometres from the Sun, and the dust cloud had warmed to about minus 50 degrees C.
The SED of Hale-Bopp from 3 to 200 . Figure courtesy of E. Grün and processed by S. Peschke
Another team, lead by P. Lamy of Marseille, France, used ISOCAM to obtain a series of images of Comet Hale-Bopp in October 1996. Their analysis is continuing but preliminary images can be seen on the Solar System page of the isoweb science gallery at http://www.iso.vilspa.esa.es/science/.
The third team, lead by J. Crovisier of Paris-Meudon, France, obtained the spectrum of Hale-Bopp using all ISO's spectrometers. In March 1996, the PHOT-S spectrometer discovered that carbon dioxide was an important constituent of the comet's emissions of vapour. Observations by PHOT-S, SWS and LWS in September 1996, when the comet was over 400 million kilometres from the Sun, indicated that Hale-Bopp was shedding water vapour into space at 10 tonnes per second, carbon monoxide at 11 tonnes per second, and carbon dioxide at 5 tonnes per second. Altogether Comet Hale-Bopp's loss of these materials amounted at that time to 2.2 million tonnes per day. Counting molecules rather than mass, the water vapour, carbon monoxide and carbon dioxide were vaporizing in the ratio 10 to 6 to 2.
SWS observations of the 2.6 to 2.9 microns region (see figure below)
containing bands of water ( 
, 
and several hot bands) allowed a
determination of the H 2 O rotational temperature and its ortho-to-para
ratio (OPR) via the relative intensities of the lines. When water
molecules form at room temperatures, the nuclei of both hydrogen atoms spin
the same way 70% of the time. At very low temperatures of formation, as in
interstellar space, contrary directions of spin become commoner. The best
match to the ISO spectrum comes from a OPR ratio of 2.45 to 1, corresponding
to a temperature for the formation of the water molecules of 28.5 K.
Due to the extended ISO lifetime, comet Hale-Bopp will again become visible to ISO from mid-December onwards. It will be at approximately the same heliocentric distance as the first set of observations. Plans are being made for further observations during this visibility period.
More Hale-Bopp results are available in our Science Gallery.