Ground-based and space sub-mm observations of comets

Microwave spectroscopy is a powerful tool to study the composition of cometary atmospheres. Since comet 1P/Halley in 1985, about two dozens of comets have been observed with this technique. Most of the 24 known cometary parent molecules were identified from observations of their rotational transitions with ground-based millimetre and submillimetre telescopes. Submillimetre spectroscopy also allowed the measurements of key isotopic species, such as HDO. The avaibility of space-borne submillimetre instrumentation opened a new window for the study of comets, especially concerning water. Water ice is the main constituent of comets and its sublimation is driving the activity of comets close to the Sun. Knowing precisely the water outgassing rate of a comet is essential in order to retrieve accurate molecular abundances and compare comets with each other. Water sub-mm lines are much stronger than those of any other species and optically thick in cometary comae. Because of the Earth atmosphere, sub-mm lines of water can only be observed from space. The Submillimeter Wave Astronomy Satellite (SWAS) made the first detection of the fundamental line of water at 557 GHz in 1999 in comet C/1999 H1 (Lee) (Neufeld et al. 2000, ApJ 539 L151). The Odin satellite, launched in Feb. 2001, is equipped with a 1.1 m sub-mm telescope. It has observed the 557 GHz line of water with high spectral resolution in 8 comets to date. Thanks to its frequency coverage, it has also made the first detection of the H_2¹⁸O isotopic species line at 548 GHz in comet 153P/Ikeya-Zhang in April 2002 (Lecacheux et al. 2003, A&A 402, L55). Further comet observations with Odin will enable the search for H_2¹⁷O and ammonia lines. In the future, these lines will be observed in-situ with the MIRO experiment aboard the Rosetta Spacecraft, and a more thorough investigation of the water sub-mm spectrum will be possible with the ESA Herschel Space Observatory. This will help in improving our knowledge of the processes that govern the excitation of water in cometary atmospheres. Herschel will allow us to investigate the water outgassing of a large number of comets over a wide range of heliocentric distances. With Herschel, it will be also possible to observe HDO in weakly active comets, such as Jupiter-family comets.