Lyman-alpha observations of comet 46 P Wirtanen with SWAN on SOHO:: H2O production rate near 1997 perihelion

The SWAN instrument on board SOHO is a Lyman-ce photometer able to map the sky intensity with a resolution of 1 degrees, primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background. In addition SWAN was extensively used to map the Lyman cc emission of several comets since launch in December 1995. Here we report observations of Comet 46 P/Wirtanen near perihelion. From the recorded Lyman a intensity the H2O production rate was derived for 45 observations from 21 December 1996-17 May 1997, with a peak of 1.6 +/- 0.4 x 10(28) mol/s just before perihelion. This should help to constrain the physical models of 46 P/Wirtanen for Rosetta mission planning purposes. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.