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

Image credit: NASA.
In the 1950s, those few individuals who made it their business to think seriously about interplanetary travel predicted that the immense gulfs between the planets would be crossed only through enormous efforts. To be sure, chemical-propellant rocket engines akin to those already in use on missiles at the time could probably accomplish round-trip journeys to the Moon and reach Mars and Venus; for worlds beyond these near neighbors, however, new technology and techniques almost certainly would be needed.

One proposed technique would effectively 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. When applied to the Solar System, this technique would see space travelers use the planets as "ports-of-call" where they could refuel, resupply, and wait while planets aligned to permit a minimum-energy transfer to their next destination.

This is as good a place as any to ask (and answer) an obvious question: why did 1950s planners feel obligated to include crews on board their interplanetary spacecraft? 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 severely 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 align so that a minimum-energy crossing becomes possible. After a propulsive escape from Earth orbit and four months spent coasting along a curving path about the Sun, the spacecraft would intersect Venus. Its crew would ignite rockets to slow the spacecraft so the planet's gravity could capture it into orbit, then they would rendezvous with a Venus-orbiting space station to refill its 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 orbit just as an opportunity for a minimum-energy transfer to Mercury ended. In that case, they would need to wait five months for Venus and Mercury to align again. The flight to Mercury would begin with propulsive escape from Venus orbit. The spacecraft would then spend three months coasting along a curving Sun-centered path. When it intersected Mercury, its crew would fire its rocket motors so that the little planet's gravity could capture it into orbit.

If they were the first humans to reach Mercury, then 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 lined up again, they would retrace their steps to Venus and 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 more daunting. Because Jupiter, Saturn, Uranus, Neptune, and Pluto orbit far from the Sun, their years are long, so opportunities to begin 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 could require years or even decades.

A spaceship from Earth bound for Uranus, for example, would have to wait for an opportunity to begin a minimum-energy transfer to Mars (they occur every 26 months). About six months after Earth departure, its curving Sun-centered orbital path would intersect Mars. The crew would then fire their spacecraft's rocket motors to slow down so the planet's gravity could capture it into orbit.

At a Mars-orbiting space station — perhaps on Phobos, the innermost martian moon — they would ready their spacecraft to take advantage of the next Mars-Jupiter minimum-energy transfer opportunity (they happen every 28 months). After a journey of about three years, the intrepid Uranus-bound crew would fire rocket motors to capture into orbit around banded Jupiter, where they would wait for the next minimum-energy Jupiter-Saturn transfer opportunity (they occur every 20 years). While they cooled their heels they might explore the giant world's sprawling family of moons. Thirteen were known in the 1950s, and almost nothing was known about any of them.

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 opportunity (they occur about every 54 years), the crew might refuel their spacecraft at the large moon Titan, for much of the 20th century the only planetary satellite known to have an atmosphere. Astronomers in the 1950s thought that Titan's air envelope was made of methane, which, when liquefied, constitutes 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 shortest duration possible, the one-way trip from Earth to Uranus would last at least 40 years.

Of course, these examples are somewhat disingenuous, for no one really expected that space travelers would restrict themselves to minimum-energy transfers. They would, it was assumed, exploit the ability to refuel at each planetary port-of-call to take on extra propellants and apply extra energy to each leg of their spacecraft's interplanetary trek. By so doing, voyage duration and the time between minimum-energy transfer opportunities could be reduced. Even so, round-trip voyages to worlds past Jupiter were likely to test the endurance of even the hardiest crews.

Confronted with these cold facts, many 1950s space writers felt certain that spacecraft exploration of the Solar System 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 tilted world would be "left to roll along in its icy solitude, remote, unwelcoming, and lonely beyond our understanding."

Even as these somewhat florid words saw print, however, propulsion engineers were hard at work developing new fast ways of reaching the planets. Part Two of this post will look at some of the spacecraft designs they proposed. Part Three will then describe discoveries that undermined their plans and threw open the entire Solar System to scientific exploration.


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.

More Information

The Challenge of the, Planets, Part Two: High Energy

The Challenge of the Planets, Part Three: Gravity

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