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Mars, however, is only one of NASA's planetary targetsand a relatively close one at that. In 1972, the space agency will send two Pioneer spacecraft on a flyby of Jupiter, largest planet in the solar system. A year later, another Mariner will try the first multiple-planet probe. After a sweep of Venus, it will use the Venusian gravity to boost itself on toward Mercury, the sun's closest and smallest satellite. In the late 1970s, the so-called "outer planets" will be so favorably aligned that a spacecraft passing Jupiter could use its gravity to push on toward Saturn, Uranus and Neptune a "grand tour" that would cover billions of miles and take as long as ten years.
The prospects for man's first leap into the solar system will surely be enhanced by the success of such unmanned missions. Not only will they prove the feasibility of interplanetary travel, but they will help arouse the public support necessary for such journeys. To be sure, Americans will continue to agonize over the cost of the program which NASA says will come to no more than .5% to 1% of the gross national product (currently running at $900 billion) a year. And the question of priorities will remain relevant as long as such earthly imperfections as poverty and pollution persist. Still, as Science-Fiction Writer Isaac Asimov says, "Man has always had the other side of the hill to worry about"and he always will. This week the other side of the hill is the moon. Before this century ends, it will almost certainly be Mars and beyond.