Far from the warm inner regions of the solar system, in orbit around the great gas giants are moons full of surprises. One has a riot of active volcanoes and a surface that is red, yellow, and brown because its blanket of sulfur compounds. Another has a thick atmosphere of nitrogen and lakes of methane. At least one has a deep watery underground ocean. There is even talk of life on some of these remote, icy satellites, and scientists would love to send out probes to target them.
Several NASA spacecraft have flown by or orbited Jupiter and Saturn and their impressive collections of moons. Galileo, launched in October 1989, studied Jupiter and retinue—a solar system in miniature—for eight years before, at the end of its mission, being intentionally plunged to its doom in the dense Jovian clouds. Cassini has been orbiting Saturn since Christmas Day, 2004. Three weeks after its arrival, it released the European Huygens probe which parachuted down onto Titan, sending back pictures and other data, both during the descent and from the surface.
Planetary astronomers and astrobiologists are keen to return for closer looks at some of the big moons of Jupiter and Saturn, especially those that hold the promise of biochemistry and possibly even of life itself. Several spacecraft, including orbiters and landers, have been proposed, but only one has currently left the drawing board.
The European Space Agency plans to launch its heavily-instrumented JUpiter ICy moons Explorer (JUICE) to investigate the four big moons of Jupiter—Callisto, Europa, Ganymede, and Io—in 2022 for arrival in 2030. Europa has attracted the most attention because of its subsurface ocean, but Callisto and Ganymede are thought to have similar vast expanses of water below an upper icy crust. One of the key objectives of the mission will be to measure the thickness of the frozen crust of Europa. JUICE was originally half of a two-craft endeavor that was to have included a NASA-built Europa orbiter, but NASA pulled out of the project due to budgetary issues.
Although JUICE is the only mission at present to be fully funded and with a planned launch date, there’s no shortage of concepts or research going on into the possibility of icy moon explorers. NASA’s Astrobiology Science and Technology for Exploring Planets (ASTEP) program has funded a three-year study into a project called the Planetary Lake Lander (PLL). The main goal is to investigate the problems potentially involved in exploring the hydrocarbon lakes and seas of Saturn’s giant moon Titan. One of the sites visited by the PLL science team members is Laguna Negra in the central Andes of Chile, where they have been testing technologies that could one day be used on Titan to look for lifeforms that could possibly have a different biochemical basis.
Meanwhile, heading toward its prime target—Pluto and its out-sized moon Charon—is the New Horizons probe, launched in 2006. New Horizons will arrive for its dramatic encounter on July 14, 2015. The current plan is for it to fly within 10,000 km of Pluto and 27,000 km of Charon, revealing clearly for the first time the faces of these small, far-off worlds.
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