Amalthea, one of the moons of Jupiter, photographed by Voyager 1, 1979.
© National Aeronautics & Space Administration / Science & Society
Jupiter has four main moons, discovered by Galileo, but many smaller ones as well (47 Jovian moons are currently known). Amalthea is one of these leser satellites and is the third closest to Jupiter itself. It measures 145 x 91 x 83 miles (232 x 146 x 134 km) and orbits within Jupiter's faint ring system. Amalthea is the reddest object in the solar system, and gives off more heat than it receives from the Sun, probably due to the influence of Jupiter's powerful magnetic field. The two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 made its closest approach to Jupiter of 278,000 kilometres in March 1979 before flying on to Saturn.