[1] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[2] Univ Cologne, Inst Phys 1, D-50937 Cologne, Germany
[3] Univ Massachusetts, Dept Astron, Amherst, MA 01003 USA
[4] NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
[5] Cornell Univ, Dept Astron, Natl Astron & Ionosphere Ctr, Ithaca, NY 14853 USA
[6] NASA, Ames Res Ctr, Moffett Field, CA 94035 USA
[7] Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA
[8] Keystone Oaks Planetarium, Pittsburgh, PA 15216 USA