1 European Space Research and Technology Centre, Science and Robotic Exploration Directorate, European Space Agency, Noordwijk, Netherlands. mtaylor@cosmos.esa.int.
2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
3 European Space Astronomy Centre, Science and Robotic Exploration Directorate, European Space Agency, Madrid, Spain.
4 INAF-Osservatorio Astronomico, Trieste, Italy.
5 Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris, France.
6 Max-Planck-Institut für Kernphysik, Heidelberg, Germany.