[1] Univ Chicago, Enrico Fermi Inst, Lab Astrophys & Space Res, Chicago, IL 60637 USA
[2] Lockheed Martin Astronaut, Denver, CO 80201 USA
[3] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
[4] Univ Washington, Dept Astron & Astrophys, Seattle, WA 98195 USA
[5] Open Univ, Planetary & Space Sci Res Inst, Milton Keynes MK7 6AA, Bucks, England