The telescopic cameras were operating constantly as the ship cut across the orbit of the giant inner satellites -- every one of them larger than the Moon, every one of them unknown territory. Three hours before transit, Discovery passed only twenty thousand miles from Europa, and all instruments were aimed at the approaching world, as it grew steadily in size, changed from globe to crescent, and swept swiftly sunward.
Here were fourteen million square miles of land which, until this moment, had never been more than a pinhead in the mightiest of telescope. They would race past it in minutes, and must make the most of the encounter, recording all the information they could. There would be months in which they could play it back at leisure.
From a distance, Europa had seemed like a giant snowball, reflecting the light of the far-off Sun with remarkable efficiency. Closer observations confirmed this; unlike the dusty Moon, Europa was a brilliant white, and much of its surface was covered with glittering hunks that looked like stranded icebergs. Almost certainly, these were formed from ammonia and water that Jupiter's gravitational field had somehow failed to capture.
Only along the equator was bare rock visible; here was an incredibly jagged no-man's-land of canyons and jumbled boulders, forming a darker band that completely surrounded the little world. There were a few impact craters, but no sign of vulcanism; Europa had obviously never possessed any internal sources of heat.
There was, as had long been known, a trace of atmosphere. When the dark edge of the satellite passed across a star, it dimmed briefly before the moment of eclipse. And in some areas there was a hint of cloud -- perhaps a mist of ammonia droplets, borne on tenuous methane winds.
As swiftly as it had rushed out of the sky ahead, Europa dropped astern; and now Jupiter itself was only two hours away.