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Saturn, with two small moons, as photographed by Voyager 2. 1979 – Far-out voyages The twin Voyagers begin landmark observations of the outer planets that last until 1989. The two spacecraft discover a total of 22 new satellites — three at Jupiter, three at Saturn, 10 at Uranus and six at Neptune — and send back stunning space images and revelations about the moons. Researchers spot more moons around Jupiter by tracking the motion of objects with a highpowered telescope. Year 2000: More moons Powerful electronic imagers integrated into ground-based telescopes on Earth in the late 1990s yield rich returns in the search for more extraterrestrial moons. Researchers find 11 new moons around Jupiter and 12 moons circling Saturn. The largest moon circling the largest planet in our solar system, icy Ganymede measures 5219 Km across. This moon of Jupiter, in the upper left-hand corner, is slightly larger than Saturn's giant satellite Titan (5150 Km), and its size overshadows the planets Mercury (4878 Km) and Pluto (~3000 Km). It is shown here with fellow Jovian moons Io, Europa and Callisto. (Earth’s Moon is 3476 Km in diameter. The Earth is 12,756 Km in diameter. Jupiter is 142,796 Km.) Speeding around a planet 11 times the size of Earth in less than two days takes its toll on Jupiter's geologically active moon Io. The gravitational stresses cause so much heat inside Io that volcanoes on the frozen world spew matter 185 miles into space, shooting more than a mile upward every two seconds. (Tidal forces, Jupiter’s main radiation belt and Io is strongly influenced by Jupiter’s magnetic field, wreaking havoc on the Sulfur and Iron in the core.) At first glance, Jupiter's moon Europa is a relatively simple world. But looks can be deceiving: Cracks cover the moon's icy surface, signals the possible presence of an underground liquid ocean. Since water appears to be key to life as we know it, scientists would not be surprised if Europa harbors some form of microscopic life. The temperature of the water is measured from 40-60 degrees below the surface. The Saturn moon Iapetus puzzled generations of astronomers by apparently vanishing every time it moved around to the western side of its home world. The mystery remained until Voyager flybys showed the moon to be almost perfectly two-toned: half sooty black, half sparkling white. Scientists theorize the moon's leading face was darkened after flying through a debris cloud created when a space rock collided with the neighboring moon Phoebe. The gravitational bond between Triton and its mother planet Neptune is so odd that astronomers believe the satellite's capture was particularly rough, with lots of heat generated in the process. However, the spacecraft Voyager 2 found Triton to be brutally cold, with temperatures plunging to minus 391 degrees Fahrenheit — frigid enough to turn nitrogen, carbon dioxide and methane into ice. With no atmosphere, our own moon experiences temperature swings in the extremes, soaring above 250 degrees Fahrenheit in full sun and dipping 450 degrees below zero in darkness. Besides the moonless planets of Mercury and Venus, other hot spots in the solar system include the volcanoes of Io, a moon circling Jupiter. Saturn's tiny moon Enceladus — about the width of Arizona — features a surface marked with craters, fissures, plains, volcanoes, cracks and ridges. Heavily covered in ice, the moon reflects around 90 percent of the sunlight that strikes its surface, giving it a brightness similar to that of freshly fallen snow. A thick atmosphere distinguishes the moon Titan from other satellites in our solar system. Even though this moon — Saturn's largest — is typically cloaked in a thick fog, researchers recently uncovered evidence that icy continents and seas of liquid methane, ethane or other hydrocarbons might cover its surface. They hope that the spacecraft Cassini, now en route to Saturn, might help reveal more. The martian moon Deimos, lower left, is so tiny you could walk across it in less than nine hours. Believed to be a captured asteroid, the satellite may soon lose its standing as the solar system's smallest moon, however. Powerful telescopes are finding even smaller shards orbiting Jupiter. Deimos is shown here with asteroid 951, top, and fellow martian moon Phobos.