1 Introduction

The Infrared Space Observatory (ISO) (Kessler et al. 1996) made during its lifetime between 1995 and 1998 more than 30 000 individual observations, ranging from objects in our own solar system right out to the most distant extragalactic sources. The solar system programme consisted of many spectroscopic and photometric studies of comets, asteroids, planets and their satellites at near- and mid-infrared (near-/mid-IR) wavelengths between 2.5 and 45 $\mu$m. At far-infrared (far-IR) wavelengths, beyond 45 $\mu$m, the programmes were limited to spectroscopic observations of the outer planets, 3 satellites (Ganymede, Callisto, Titan), 3 comets (P/Hale-Bopp, P/Kopff, P/Hartley 2) and 4 asteroids (Ceres, Pallas, Vesta, Hygiea). Far-IR photometry on solar system objects was mainly done for calibration purposes (Uranus, Neptune and a few asteroids) and scientific studies of extended sources (P/Hale-Bopp, P/Kopff, P/Wild 2, P/Schwassmann-Wachmann, Chiron, Pholus).

In addition to the dedicated programmes on individual sources, ISO also made parallel and serendipitous observations of the sky. ISOCAM observed the sky in parallel mode a few arc minutes away from the primary target at wavelengths between 6 and 15 $\mu$m (Siebenmorgen et al. 2000). The ISOPHOT Serendipity Survey (Bogun et al. 1996) recorded the 170 $\mu$m sky brightness when the satellite was slewing from one target to the next. LWS performed parallel (Lim et al. 2000) and serendipitous surveys (Vivarès et al. 2000) in the far-IR. These complementary surveys contain many interesting objects, but the scientific analysis only began recently. The two LWS surveys and the ISOCAM parallel survey just underwent a first data processing and could therefore not be considered in the following.

Source extraction methods for the ISOPHOT Serendipity Survey (ISOSS) have been developed by Stickel et al. (1998a, 1998b) for point-sources and by Hotzel et al. (2000) for extended sources. First scientific results were published recently (Stickel et al. 2000; Tóth et al. 2000; Hotzel et al. 2001) and work is ongoing to produce further catalogues of Serendipity Survey sources. To facilitate the production of source lists it is necessary to identify and exclude all SSOs from the survey data. This catalogue cleaning aspect was one motivation for the following analysis, but there are also the calibration and scientific aspects of the SSO investigations: a few well known asteroids and planets like Uranus and Neptune provide the possibility to test and extend the photometric calibration of ISOSS to higher brightness levels (Müller & Lagerros 1998). For asteroids with known diameters, the surface regolith properties can be derived from the emissivity behaviour in the far-IR where the wavelength is comparable to the grain size dimensions. Additionally, reliable far-IR fluxes of asteroids allow diameter and albedo determinations for the less well-known targets. For comets the far-IR information is useful for coma and tail modeling (Grün et al. 2001). The close connection between large particles and far-IR thermal emission also allows further studies of trail formation and the important processes of dust supply for the interplanetary medium.

In the following sections we present and discuss the ISOSS data with emphasis on solar system targets (Sect. 2). This also includes the data processing, point-source extraction and calibration aspects. An iterative and fast method to search for SSOs in large sets of slewing data is explained in Sect. 3. The encountered SSOs are then separated into 2 categories:
1) Well known asteroids and the planets Uranus and Neptune, which were used to test and extend the photometric calibration of ISOSS (Sect. 4).
2) Asteroids and comets, for which the far-IR fluxes were used a) to derive diameters and albedos (asteroids) or b) to give a qualitative and quantitative description of the observational results for future modeling (comets) (Sect. 5).
In Sect. 6 we summarize the results and give a short outlook to future projects.