... The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of impact craters in the Solar System, but harbors no volcanoes or even any large mountains. ... Skylab Over Earth . ... Did a gamma-ray burst precede this supernova? ... Starting in the 1400s astronomers erected large instruments here to enable them to measure star and planet positions with increasing accuracy. ... Particularly strange is the green auroral ring caused by ionized oxygen high in the Earth's atmosphere. ...