Comet observing program: Water in comets: water ice ~50% of bulk composition of cometary nuclei water vapor: sublimation drives cometary activity close to the sun main constituent (> 80%) of the gaseous coma water coma: expansion velocity ~ 0.8 km/s adiabatic cooling => cold: ~50 to 100 K => Strong (optically thick), narrow sub-mm lines, not observable from the ground H 2 O( ) line at GHz observed from space (main ground vibrational state, fundamental rotational line of water) - first detection: in comet C/1999 H1 (Lee) with SWAS satellite - Spectrally resolved for the first time: comet C/2001 A2 (LINEAR) with Odin (first astronomical target detected by Odin on April 27, 2001) - 2’ spatial resolution maps obtained with Odin on comets since June 20, Odin sensitivity: S/N > 7 in one orbit (1.5h) for Q H2O (r h =  =1 AU)=10 28 mol./s Observations of comets with Odin satellite

Telescope characteristics: Diameter: 1.1 m, off-axis secondary: HPBW = 127” at 557GHz, Beam_eff. =85% Sky subtraction: - “Dicke switching”: 2 reference beam offset by 44º - position switching (but loss of efficiency) - frequency switching at 119 GHz “Comet tracking” mode for moving targets Solar elongation constraint: 60º -120º Instrument characteristics Heterodyne Receivers: 5 (up-to) receivers running in parallel: GHz (O 2 ) (frequency splitter) (Tsys  600 K) 495, 549, 555, 572 GHz, tunable to ±8GHz (Achieved Tsys  3200 K) (polarization splitter and Source/Reference beam alternating) objectives: (CS, CI, O 2 ; 13 CO, H 2 16 O and H 2 18 O (2 Rx); CO, NH 3 ) Spectrometers: 3 backends: Acousto-optical Spectrometer: 1 GHz bandwidth, 1728 Channels Resolution: 1.2 MHz 2 Auto-Correlators: MHz bandwidth Resolution: 1.2MHz to 150kHz (  maximum spectral resolution = 80m/s at 557GHz) Astronomical Observations with the Odin satellite

Modelling H 2 O emission in comets Model exist (Bockelée-Morvan,1987; Chin et al., 2001) –Radiative excitation of vibrational bands by solar radiation –Collisional excitation –Coma in spherical expansion (~ 0.8 km/s) : line shapes. –Coma temperature (~ 60 K) estimated from molecular rotational lines (e.g. methanol) Line shapes and intensities deduced by numerical integration using a radiative transfer code. Model recently updated to consider collisional excitation by electrons (Biver, 1997) –Electron density scaled from 1/P Halley.

(Submitted to Icarus)

Activity of 19/P Borrelly monitored from OH (Nançay,1994&2001) and H 2 O (Odin,2001) observations ODIN