... Mapping Mars . ... Orbiting about once every two hours at an altitude of over 200 miles, instruments onboard MGS now regularly explore the Martian surface and atmosphere. ... The Vernal Equinox occurs Saturday, March 20, marking the first day of Spring for the Northern Hemisphere. ... During late March and early April 1997, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on 1997 March 24th. ...