In the 1950s and early 1960s, many who made it their business to consider the yawning gulfs between the planets foresaw that only through enormous efforts, spanning perhaps centuries, could those immensities be crossed. To be sure, rocket engines burning chemical propellants akin to those already in use in missiles could accomplish round-trip journeys to the moon, and probably also reach and return from Mars and Venus; beyond those near neighbors, however, new propulsion technology and techniques would be required.
One approach sought to mimic the European and Chinese Ages of Exploration, during which ships sought repair and resupply at exotic seaports and remote islands as they slowly made their way to distant destinations. The moon, the planets, and the moons of planets would serve as ever-moving stepping stones. Earth and Venus, for example, become aligned every 19 months so that advanced chemical or perhaps nuclear-thermal rocket engines could propel a spaceship on a minimum-energy course to a Venus-orbiting station from an Earth-orbiting spaceport or from a base on Earth’s moon. Upon arrival at Venus four or five months later, the crew would assist technicians based there as they rechecked, refueled, resupplied, and refurbished their spaceship.
When, months later, Venus and Mercury moved along their orbits so that they became aligned for a minimum-energy crossing, the crew would board their spaceship and move on to their ultimate destination. If they were the first crew to reach Mercury, they would look for valuable resources – chiefly rocket propellants – and perhaps establish the nucleus of a permanent base. When Mercury and Venus lined up again, they would begin to retrace their steps to Earth.
If a spaceship’s destination lay in the other direction – that is, beyond Mars, in the outer Solar System – then the challenges of interplanetary voyaging were far greater. Though spacecraft would not travel in straight lines between worlds – they would instead follow curving minimum-energy Hohmann transfer orbits about the Sun – the straight-line distances between Earth and the planets serve to illustrate the problem. Mercury is at its most distant about 120 million miles from Earth; Jupiter, on the other hand, approaches Earth no nearer than about 480 million miles.
Because Jupiter, Saturn, Uranus, Neptune, and Pluto orbit far from the Sun, their years are long, so opportunities for minimum-energy transfers between them occur infrequently. Jupiter orbits the Sun in a dozen Earth years, Saturn circles the Sun in 26, and Uranus has a year lasting 84 Earth years. A spaceship starting from Earth bound for Uranus would have to wait for an Earth-Mars minimum-energy transfer opportunity (these occur every 26 months). Upon arrival at Mars, they would ready their spaceship to take advantage of a Mars-Jupiter transfer opportunity (they happen every 28 months). The journey from Mars to Jupiter would require about six years.
The intrepid Uranus-bound crew would then have to wait for a minimum-energy Jupiter-Saturn transfer opportunity. These happen every 20 years. The journey from Jupiter to stunning Saturn would last a decade. While they awaited a Saturn-Uranus transfer opportunity (they occur every 54 years), the crew might refuel at Saturn’s moon Titan, which was known to have an atmosphere; 1950s astronomers thought it was made of methane, which could serve as rocket fuel. The journey from Saturn to Uranus would last 27 years. Hence, even if the wait time at every stop along the way were minimal, the one-way journey from Earth to Uranus would need at least 40 years.
Confronted with these kinds of numbers, most 1950s space writers felt certain that the planets were virtually off-limits. Patrick Moore, for example, wrote in 1955 that none of his readers would live to see Mars and Venus up close. As for the nearest and largest of the gas giants, Moore declared that “for generations of men to come, there can be no hope of seeing the wonders of Jupiter from close range.” He added that “[w]e must be content to look at the king of planets from a respectful distance, and leave him alone in his cold, proud glory.”
Even as these florid words saw print, propulsion engineers sought new fast ways of reaching the planets. Part Two of this post will look at some of the spacecraft designs they proposed; it will then describe the discoveries that undermined their plans and threw open the entire Solar System to scientific exploration.
Guide to the Planets, Patrick Moore, Eyre & Spottiswoode, London, 1955; pp. 141, 195.