Herschel is taking part in an extraordinary worldwide astronomical campaign to study comet Hartley 2 before, during, and after the flyby by the NASA EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) mission, with closest approach on 4 November 2010. Hartley 2 passed just 0.11 AU (16.4 million km) from Herschel on 20 October 2010.
Herschel has obtained unique sensitive far-infrared continuum images constraining the size of the large dust particles, while spectra reveal the distribution of water molecules released from the nucleus as about 230 kg of ices evaporate every second.
Read the article on the ESA website
Also, see the HssO note for more information
Or, the article: "The Odin satellite observes water in comet 103P/Hartley 2".