Plans for the Future
In 1997, with the Mars probes already underway, preparations were being made in the United States, Europe, and elsewhere to launch probe missions to revisit Saturn and to rendezvous with a comet. Several more Mars missions were also on the drawing boards at NASA, as well as in Russia and Japan, for the coming decade. One of their major tasks will be to look for evidence of life on Mars—either presently living microorganisms or remnants of ones from the distant past. Among the spacecraft being planned by NASA was a lander that would bring a sample of Martian soil back to Earth sometime around 2010.
Saturn's moon Titan was also due for a return visit. The possibility that organic molecules exist on the surface of Titan led to speculation that this intriguing moon could serve as a laboratory for examining how life begins. A joint U.S.-European mission named Cassini/Huygens, scheduled for an October 1997 launch, was to explore this possibility further. Plans called for the American-built Cassini orbiter to reach Saturn in 2004. There, if all goes according to plan, it will release a European-built atmospheric probe, Huygens, that will plunge through Titan's atmosphere making chemical measurements all the way down to the moon's mysterious surface. Cassini will then begin a four-year orbital tour of Saturn, making repeated passes by Titan to map its surface with advanced radar. If the mission is successful, Cassini and Huygens could greatly contribute to scientists' understanding of how life emerged on Earth and how it may do so elsewhere.
Future probe missions will also continue to target extraterrestrial objects besides planets and moons. The last major encounter with a comet in deep space took place in 1986, when Halley's Comet was visited by the Giotto probe, sent by the European Space Agency, and two Soviet Vega spacecraft. But far more exploration is needed to tell scientists what comets are made of and whether, as some suspect, the ice they contain has remained largely unaltered since the formation of our solar system. To help answer these questions, the Stardust mission, scheduled by NASA for launch in 1999, is intended to intercept a comet named Wild-2 at a point inside the orbit of Mars in early 2004. Scientists especially want to know whether comets contain organic molecules. Some researchers theorize that Earth and other planets and moons where organic compounds abound obtained those compounds from collisions with comets. The Stardust probe is designed to collect cometary dust and return it to Earth in 2006. More ambitious comet missions were to follow.
There is also interest in sending a mission to Pluto, the last planet in the solar system that remains unexplored by a probe from Earth, and to the Kuiper Belt. The latter is a collection of icy debris, just beyond the orbit of Pluto, that is left over from the formation of the outer planets. A NASA project named Pluto Express was being developed in 1997 to achieve both these goals. Pluto Express would fly past Pluto and then enter the Kuiper Belt to explore what lies at the very edge of the solar system. Scientists now know that at least some of the comets that make their way into the inner solar system originate in the Kuiper Belt. They are pulled from their distant orbits beyond Pluto by the gravity of the giant planets. Pluto Express could be launched sometime in the early 2000's, reaching Pluto around 2010.
The relatively low cost and increasing sophistication of unmanned space probes ensure that they will serve as our eyes and ears in the solar system for some time to come. The next generation of probes will add to the already enormous body of knowledge about the solar system amassed by their predecessors. When human beings finally set foot on other planets and their moons, the terrain, geology, and atmospheric conditions of those worlds will already be known to them. Unmanned probes will have blazed the trail.