Radio observations of Jupiter-family comets

Abstract

Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); ii) to investigate the chemical composition of comets; iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei. Comets are classified from their orbital characteristics into two separate classes: i) nearly-isotropic, mainly long-period comets and ii) ecliptic, short-period comets, the so-called Jupiter-family comets. These two classes apparently come from two different reservoirs, respectively the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and - possibly - their different origin, they may have different chemical and physical properties that are worth being investigated. The present article reviews the contribution of radio observations to our knowledge of the Jupiter-family comets (JFCs). The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly-isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, Jupiter-family comets are more easily accessed by space missions. However, unique opportunities to observe Jupiter-family comets are offered when these objects come by chance close to the Earth. About a dozen JFCs were successfully observed by radio techniques up to now. No obvious evidence for different properties between JFCs and other families of comets is found.