United States. National Aeronautics and Space Administration

“ The United States has always been a nation that moves forward, pushing back the frontiers of the West, pushing back the frontiers of social and economic development, and now pushing back the frontiers of space. ”

“ Jupiter is the largest planet in our solar system, with a diameter 11 times that of Earth. Jupiter rotates very quickly, making one full rotation in just under ten hours. Composed primarily of hydrogen and helium, Jupiter’s colorfully banded atmosphere displays complex patterns highlighted by the Great Red Spot, a large, circulating atmospheric disturbance. Three of Jupiter’s 13 known satellites are also visible in this Voyager 1 photograph. The innermost large satellite, Io, can be seen in front of Jupiter and is distinguished by its bright, orange surface. ”

“ Europa, the brightest of Jupiter’s Galilean satellites, may have a surface of thin ice crust overlying water or softer ice, with large-scale fracture and ridge systems appearing in the crust. Europa has a density about three times that of water, suggesting it is a mixture of silicate rock and some water. Very few impact craters are visible on the surface, implying a continual resurfacing process, perhaps by the production of fresh ice or snow along cracks and cold glacier-like flows. ”

“ Jupiter and two of its planet-sized satellites, Io at left and Europa at right, are visible in this Voyager 1 picture. Jupiter’s four largest satellites—Io, Europa, Ganymede and Callisto—were discovered in 1610 by Galileo Galilei. The two outer Galilean satellites are Ganymede and Callisto, not shown in this picture. All four satellites probably formed about four billion years ago but their surfaces vary in age tremendously. Io and Europa have younger, more active surfaces than Ganymede and Callisto. Like our Moon, the satellites keep the same face toward Jupiter. ”

“ Looking beyond matters of technology and the environment, the historian may cite the latter part of the twentieth century as a time of explosive exploration, comparable to the 15th and 16th century exploration of the Earth’s oceans and the distant lands that bounded them. In a sense, exploration—whether it is physical or intellectual—provides its own rewards. ”

“ Unmanned spacecraft missions to the Sun increase our understanding of that most fundamental of all energy sources, paving the way for the efficient conversion of solar energy into many practical applications, and releasing us from dependence on ever-decreasing reserves of fossil fuels. Spacecraft circling the Earth study the upper atmospheric processes that play major roles in controlling our weather. These same spacecraft look down on Earth, aiding us with increasingly accurate forecasts of weather and crop productivity. ”

“ Historians writing a hundred or two hundred years from now may well look on the latter part of the twentieth century as another turning point in civilization. For the first time, we explored beyond Earth—first the Moon, then the neighboring planets, and finally the outermost planets, the very fringe of our solar system. ”

“ Callisto is the most heavily cratered planetary body in our solar system. In this Voyager 2 nine-frame mosaic, a special computer filter was used to provide high contrast in the surface topography. The impact structure visible at the upper right edge of the satellite is smaller than the largest one found by Voyager 1 but more detail is obvious; it is estimated that 15 concentric rings surround the bright center. Many hundreds of moderate-sized craters are also visible, a few with bright ray patterns. The limb is smooth, which is consistent with Callisto’s icy composition. ”

“ The atmosphere of Jupiter is colorful, with cloud bands of alternating colors. A major characteristic of the atmosphere is the appearance of regularly spaced features. Around the northern edge of the equator, a train of plumes is observed, which has bright centers representative of cumulus convection similar to that seen on Earth. At both northern and southern latitudes, cloud spots are observed spaced almost all the way around the planet, suggestive of wave interactions. ”

“ Long linear fractures or faults which crisscross Europa’s surface in various directions are over 1000 kilometers (600 miles) long in some places. Large fractures are 200 to 300 kilometers (125 to 185 miles) wide, wider than the crust is thick. Also visible are somewhat darker mottled regions that appear to have a slightly pitted appearance. No large craters (more than five kilometers in diameter) are identifiable in this Voyager 2 picture, indicating that this satellite has a very young surface relative to Ganymede and Callisto, although perhaps not as young as Io’s surface. ”

“ Conditioned to hyperbole, we fail to recognize those advances that are truly exceptional. We need a historian’s vantage point to identify the events that can literally change the course of civilization. ”

“ This infrared image of Jupiter was taken from Earth and shows heat radiating from deep holes in Jupiter’s clouds. Bright areas in the image are higher temperature regions than the dark areas and correspond to parts of the atmosphere that are relatively free of obscuring clouds. The Great Red Spot appears on the left limb, or edge of the planet, as a dark area encircled by a bright ring, indicating that the Spot is cooler than the surrounding region. ”

“ The largest aurora ever observed, nearly 29,000 kilometers (18,000 miles) long, appears in this Voyager 1 photograph, taken on the dark side of Jupiter six hours after closest encounter. The auroral lights are brighter than any northern lights seen on Earth. Jupiter’s north pole is approximately midway along the auroral arc. This timed exposure of the aurora also shows what appear to be lightning storms several thousand kilometers below the aurora. The strength of the lightning bolts is comparable to that of superbolts seen near cloud tops above Earth. ”

“ Amalthea, Jupiter’s innermost satellite, was discovered in 1892. It is so small and close to Jupiter that it is extremely difficult to observe from Earth. Amalthea’s surface is dark and red, quite unlike any of the Galilean satellites. The three Voyager 1 pictures and the one Voyager 2 picture following (seen against the disk of Jupiter) reveal a small, elongated object, about 265 kilometers (165 miles) long and 150 kilometers (90 miles) in diameter. Amalthea keeps its long axis pointed toward Jupiter as it orbits around the planet every 12 hours. ”

“ Ganymede, largest of Jupiter’s 13 satellites, has bright “young” ray craters; light, linear stripes resembling the outer rings of a very large, ancient impact basin; grooved terrain with many faults; and regions of dark, heavily cratered terrain. Among the Galilean satellites, Ganymede probably has the greatest variety of geologic processes recorded on its surface and may be the best example for studying the evolution of Jupiter’s inner satellites. ”

“ The color of the Great Red Spot may indicate that it extends deep into the Jovian atmosphere. Cloud-top lightning bolts, similar to those on Earth, have also been found in the Jovian atmosphere. At the polar regions, auroras have been observed. A very thin ring of material less than one kilometer (0.6 mile) in thickness and about 6000 kilometers (4000 miles) in radial extent has been observed circling the planet about 55,000 kilometers (35,000 miles) above the cloud tops. ”

“ Study of the primitive crusts of the Moon, Mars, and Mercury permits us to reconstruct the first billion years of Earth history, a time when chemical elements were being concentrated in activity ultimately leading to the formation of important ore deposits. ”

“ Io’s surface, less than ten million years old, is quite young compared to the other Galilean satellites and to other terrestrial bodies, such as Mercury and the Moon. The surface is composed of large amounts of sulfur and sulfur dioxide frost, both of which account for most of the surface color. This picture was taken by Voyager 1. Material deposited by the volcano (see following pictures) can be seen as a white ring near the center of Io. ”

“ He will point out the technological advances incorporated in unmanned spacecraft, sophisticated robots able to control their own activities and solve their own problems. He will mention the revolution in microelectronics—the art of fabricating complex electrical control circuits so small the eye cannot perceive them, a revolution accelerated by the requirement to conserve weight and generate performance in interplanetary spacecraft. ”

“ The gradually curving lines that press through the dark region suggest the presence of a large impact basin to the southwest, which has been obliterated by the subsequent formation of younger grooved terrain. ”

“ The lack of high ridges, ring mountains, or a large central depression suggests that the impacting body caused melting, some flow, and shock waves, and that the refreezing occurred in time to preserve the concentric shock rings. ”