1 Introduction

Radio astronomical research on comets started with the apparition of Comet C/1956 R1 (Arend-Roland), which many of the radio observatories of the time tried to detect - unsuccessfully - at various frequencies (Lovell 1959). Two decades later Hobbs et al. (1975) reported the detection of a strong radio signal from Comet C/1973 E1 (Kohoutek), which was thought to originate from an Icy Grain Halo (IGH; Delsemme 1973; Gibson & Hobbs 1981,1983) of cm-sized icy grains. Subsequent attempts to verify this model with observations of other comets failed (e.g., Snyder et al. 1983; Hoban & Baum 1987). Altenhoff et al. (1983) reported the detection of thermal emission from the nucleus of Comet C/1983 H1 (IRAS-Araki-Alcock). Their results of the nuclear size were consistent, e.g., with the infrared (IR) observations of Hanner et al. (1985) and with radar observations of Harmon et al. (1989), but could not be confirmed by the interferometric radio observations of de Pater et al. (1985). Altenhoff et al. (1986,1989) monitored the radio emission of Comet 1P/Halley over several days and measured the extent of the radio halo confirming for the first time the presence of mm-sized ice/dust particles.

More recently the exceptional brightness of Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) allowed detailed radio observations. Altenhoff et al. (1999, Paper I), de Pater et al. (1997,1998), and Jewitt & Matthews (1997,1999) demonstrated the potential of radio- and millimeter-wavelength continuum observations for studies of the nuclear diameter and dusty halo. The interpretation of these observations is however complicated by spectral index variation in the halo emission, as seen with the dual frequency interferometer, by uncertainties in the frequency dependence of the absorption coefficient for the differently sized halo dust particles, and by the unexpected dependence on the observation frequency of the halo size (Paper I).

When Comet C/1999 S4 (LINEAR) (short: comet S4) was detected, Marsden 1999 predicted that it would become optically bright. We therefore initiated a series of observations at various radio telescopes, at frequencies ranging from 32 to 860 GHz. During these observations the cometary nucleus disintegrated, providing an unique opportunity to examine the comet and its composition.