a1 Research and Science Support Department, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands e-mail: Nicolas.Andre@cesr.fr; Bernard.Foing@esa.int
a2 Center for Earth, Planetary, Space and Astronomical Research, Open University, Milton Keynes MK7 6AA, UK e-mail: C.S.Cockell@open.ac.uk
a3 Current address Centre d'Etude Spatiale des Rayonnements, CNRS/Université Paul Sabatier, 9 avenue du colonel Roche, 31028 Toulouse, France