Astronomy Picture of the Day

Explanation: NASA's robot spacecraft Galileo began its long voyage to Jupiter in October of 1989. In December of last year it arrived in the Jovian system, beginning its unprecedented, detailed exploration by dropping a probe into the gas giant's atmosphere. By early this morning it will have accomplished another milestone in its ambitious mission. Now in orbit around Jupiter, Galileo will make its first close flyby of Ganymede, Jupiter's (and the solar system's) largest moon at 2:29 a.m. EDT. As planned, approaching to within 524 miles, it will make a series of high resolution images of the surface which will reveal features as small as 33 feet across. This close-up colour image from the Voyager 2 flyby in 1979 previews sights Galileo will see in greater detail. Showing features as small as 3 miles across, it reveals a variety of terrain on Ganymede's icy surface, including impact craters with bright rays and long strips of light grooved structures suggesting large scale motions of the frozen crust. Galileo's flyby images will be stored onboard for playback and should be available during the week of July 10.