How We Got There

Abstract

Our Solar System seems to be arranged in a strange way. The inner planets huddle near the Sun, circling fast in their tight orbits like vultures over Serengeti leftovers. But beyond Mars, everything spreads out. If our Sun were the size of the head of a pin, the four inner planets would circle within a region 22 in. from it. But on this scale, Jupiter’s orbit would keep the planet at 66 in., and Neptune would revolve around the pinhead Sun some 39 ft away. The distances to the outer planets seemed, to early spacecraft designers, a great chasm. Mars was just barely doable; the Red Planet could be reached in 6 months on a fast track. But a probe to Jupiter faced years in the harsh vacuum of space, and a voyage to Neptune promised the daunting agony of a long decade.