1. Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
2. Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking RH5 6NT, UK
3. The Centre for Planetary Sciences at UCL/Birkbeck, Gower Street, London WC1E 6BT, UK
4. Department of Physics, United States Naval Academy, 572C Holloway Rd, Annapolis, MD 21402, USA
5. Department of Astronomy, University of Maryland, College Park, MD 20742, USA
6. Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72, Tsarigradsko Chaussèe Blvd., Sofia BG-1784, Bulgaria