We report here the observations of the hydrogen cloud in Lyman alpha emission of Comet 67P/Churyumov-Gerasimenko at the last three passages through its perihelion, in 1996, 2002 and 2009. This comet became the target of ESA space mission Rosetta in 2003, and is therefore of particular interest. The data were collected with the SWAN instrument on board SOHO, the main mission of which is to record all-sky images of interplanetary neutral hydrogen, in order to monitor the solar wind latitude distribution. For the 2009 perihelion, a dedicated campaign was implemented and allowed to collect 26 smaller and more refined images around the position of the comet, from which a unique series of H2O production rates could be derived by comparison with a model, for the period from 2 to 50 days after perihelion. To our knowledge, these are the only determination of H2O production rate for 2009. In addition, we could retrieve the H emission at the two previous perihelia in 1996 and 2002, thanks to serendipitous observations of comets as part of the all-sky monitoring program: 4 and 10 respectively for 1996 and 2002. While the perihelion distance to sun decreased continuously (1.300, 1.292, 1.246 AU respectively for 1996, 2002, 2009), the average production Q(H2O) just after perihelion did not increase accordingly: 1.30×1028, 1.70×1028, an5.65×1027 mol s-1. Comparison with 1982 IUE measurement of 6×1027 mol s-1 suggests no rapid aging of the comet,nd we may expect a similar level of activity at the next perihelion in August 2015. The production rate in 2009 showed a peak around 16.4 days post-perihelion, with a suggestion of a second peak at ~40 days, and a somewhat reduced rate in between (though SWAN measurements are incomplete). These measurements may help in planning of Rosetta activities around the next perihelion.
Bertaux et al.
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