The Challenge of the Planets, Part One: Ports-of-Call

NASA

In the 1950s, those few individuals who made it their business to think seriously about interplanetary travel foresaw that the immense gulfs between the planets would be crossed only through enormous efforts. To be sure, rocket engines burning chemical propellants akin to those already in use in missiles could probably accomplish round-trip journeys to the moon, and also reach and return from Mars and Venus; for worlds beyond those near neighbors, however, new techniques almost certainly would be required.
One technique proposed for reaching the planets sought to mimic the old European and Chinese voyages of exploration, which saw sailing ships seek repair and resupply at exotic seaports and remote islands as they made their way to distant destinations. Applied to the Solar System, this technique would see piloted rocket ships use the moon and the planets as ports-of-call where they could refuel, resupply, and wait while planets aligned to permit minimum-energy transfers to their next destinations.
This might be a good place to ask an obvious question: why did 1950s planners feel the need to send crews to the planets? In those days, it was widely assumed that spacecraft would need continuous repair to remain functional for long enough to reach another planet. The harsh vacuum and temperature variations in space, combined with micrometeoroids and radiation, meant that robot spacecraft would likely malfunction and, with the nearest repairmen millions of miles away, rapidly degrade and fail. In addition, automation, radio communications, and sensor capabilities remained sharply limited.
A voyage to Mercury employing the ports-of-call technique would begin with a trip from Earth to Venus. Every 19 months, the two planets become aligned so that a minimum-energy crossing becomes possible. After a brief period of acceleration and four months spent orbiting gradually closer to the Sun, the spacecraft’s path would intersect Venus and its crew would ignite rockets to slow it so that the cloudy planet’s gravity could capture it into orbit. There, they would rendevous with a Venus-orbiting space station to refill their spacecraft’s propellant tanks and take on fresh supplies.
Venus-Mercury minimum-energy transfer opportunities occur about every five months. If the crew were unlucky, they might reach Venus just as an opportunity for a minimum-energy transfer to Mercury ended. In that case, they would have to wait five months for Venus and Mercury to align again.
The flight to Mercury would begin with a brief period of acceleration. The spacecraft would then spend three months orbiting ever closer to the Sun. When it intersected Mercury, it would fire its rockets so that the little planet’s gravity could capture it into orbit.
If they were the first humans to reach Mercury, they would look for valuable resources – rocket propellants, to begin with – and perhaps establish the nucleus of a permanent base. Then, when Mercury and Venus again lined up, they would retrace their steps to Venus and then back to Earth.
If a spacecraft’s destination lay in the other direction – that is, beyond Mars, in the outer Solar System – then the challenges of interplanetary voyaging became much greater. 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. Because their orbits are far apart, travel between one cold outer world and the next using minimum-energy transfers can require years or even decades.
A spaceship bound for Uranus starting from Earth, for example, would have to wait for a minimum-energy transfer opportunity linking Earth and Mars (they occur every 26 months). Perhaps six months after Earth departure, the spacecraft’s orbit about the Sun would intersect Mars. The crew would then fire their spacecraft’s rocket motors to slow slow down so Mars’s gravity could capture it into orbit.
At a Mars-orbiting space station – perhaps on Phobos, the innermost martian moon – they would ready their spaceship to take advantage of a Mars-Jupiter minimum-energy transfer opportunity (these happen every 28 months). The journey from Mars to Jupiter would need about three years. The intrepid Uranus-bound crew would capture into orbit around banded Jupiter, where they would wait for a minimum-energy Jupiter-Saturn transfer opportunity (they happen every 20 years). No doubt they would explore the giant world’s sprawling family of planet-size moons.
The minimum-energy journey from Jupiter to ringed Saturn would last 10 years. While they waited in the Saturn system for a Saturn-Uranus minimum-energy transfer (which occur about every 54 years), the crew might refuel at the large moon Titan, which was known to have an atmosphere; 1950s astronomers thought that it was made of methane, which makes a reasonably efficient 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 of the least duration possible, the one-way minimum-energy journey from Earth to Uranus would last at least 40 years.
Of course, these examples are somewhat disingenuous, for no one really expected that spacecraft voyaging between planets would restrict themselves to minimum-energy transfers. The ability to refuel at each port-of-call would, it was assumed, be exploited to permit a spacecraft to take on extra propellants and apply extra energy to each leg of its interplanetary trek. By expending more than the minimum amount of propellant required, voyage durations and the time between minimum-energy transfer opportunities could be reduced. Even so, while chemical or nuclear-thermal propulsion systems were employed, round-trip voyages to worlds past Jupiter were likely to last a long time.
Confronted with these cold facts, many 1950s space writers felt certain that the close-up exploration of the planets would not begin until at least the 21st century. Patrick Moore, for example, wrote in 1955 that none of his readers would live to see Mars and Venus up close, and that no spacecraft would reach Jupiter or Saturn for generations. Though he cautioned against excessive pessimism, Moore declared that spacecraft might never reach Uranus, Neptune, and Pluto: he wrote, for example, that “we need not waste time working out the possibility of a journey to Uranus,” adding that the wildly tilted world would be “left to roll along in its icy solitude, remote, unwelcoming, and lonely beyond our understanding.”
Even as these florid words saw print, however, propulsion engineers were busy developing 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 discoveries that undermined their plans – and threw open the entire Solar System to scientific exploration.
Reference
The Exploration of Space, Arthur C. Clarke, Harper & Bros., New York, 1951, pp. 137-162.
Space Travel, Kenneth Gatland and Anthony Kunesch, Philosophical Library, New York, 1953, pp. 173-175.
Guide to the Planets, Patrick Moore, Eyre & Spottiswoode, London, 1955; pp. 141, 195.
The Exploration of the Solar System, Felix Godwin, Plenum Press, New York, 1960; pp. 152-161.