6 Conclusions and outlook

The purpose of the SSO extraction from the ISOSS was manifold: Calibration aspects, catalogue cleaning aspects and scientific aspects. The main achievements were clearly in the calibration section, were serendipitously seen asteroids and planets led to an improved flux calibration for ISOSS targets. Bright sources of the automatic point-source extraction procedure have now a solid calibration basis. It was also possible to establish new methods to calibrate source detections under a large variety of circumstances, including the important slew end positions and low slewing speeds.

The aspect of SSO cleaning from ISOSS catalogue lists will avoid wrong identifications and help follow up programmes of galactic and extra-galactic sources.

The outcome of the scientific analysis of SSO detections were modest due to the limitations of the ISOSS mentioned in Sect. 5. Despite all difficulties we could demonstrate that the far-IR fluxes of asteroids are important. Diameter and albedo estimates through TPM calculations are much more reliable than estimates based on visible brightness alone. An accurate H-value of 12.0 mag would allow for diameters ranging from 10.6 km (p_V=0.25) to 23.7 km (p_V=0.05), corresponding to a $\pm$40% uncertainty of the average. An additional thermal flux with a flux error of $\pm$20% allows a 4 times more accurate diameter determination.

The ISOSS results for Hale-Bopp are more valuable. They can now be used for additional comet modeling (e.g. models by Hanner 1983) for more reliable interpretation of grain properties and ice influences at different heliocentric distances. Comets are usually optically bright due to fresh ice surfaces, but in the far-IR the sublimated larger particles dominate the thermal emission. After many orbits around the sun these large particles form trails which were first measured by IRAS (Sykes 1986). For Hale-Bopp we found significantly more thermal emission post-perihelion than for comparable configurations pre-perihelion. Additionally we saw asymmetries due to the dust tail and an indicative detection of large particles concentrated towards the anti-orbital velocity (trail) direction.

The expectations for future far-IR and submillimetre projects on SSO related topics are large: SIRTF, SOFIA, ASTRO-F, HERSCHEL and others will have many dedicated programmes on asteroids, comets and planets, but will also see by chance interesting targets. Especially the ASTRO-F/FIS all sky survey in 4 photometric bands in the region 50 to 200 $\mu$m will serendipitously detect many SSOs. Our experience with ISOSS in terms of calibration through asteroids and planets, but also in identification of moving targets could then be of great benefit.

Acknowledgements

We would like to thank Elwood C. Downey for many helpful discussions and for providing xephemdbd for position calculation of SSOs.

The orbital elements were provided by Gareth Williams, IAU Minor Planet Center (http://cfa-www.harvard.edu/~iau/mpc.html), for 4 epochs during the ISO mission: 1996 Apr. 27, 1996 Nov. 13, 1997 Jun. 1 and 1997 Dec. 18. A modified version of the Uppsala N-body ephemeris programme was used for the final analysis of the pointing accuracy between slew and SSO position.

Thanks also to Eberhard Grün, Martha Hanner and Michael Müller who supported the comet analysis and interpretation and to Dietrich Lemke for many valuable comments.

The project was conducted at the ISOPHOT Data Centre, Max-Planck-Institut für Astronomie, Heidelberg, Germany and at the ISO Data Centre, Villafranca del Castillo, Spain. This project was supported by Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR) with funds of Bundesministerium für Bildung und Forschung, grant no. 50 QI 9801 3.