A close look

Abstract

The rapid pace of rocket development following the Second World War persuaded many people to the view that space travel was imminent. In the early 1950s, in a series of articles in the New York magazine Collier’s — magnificently illustrated by artist Chesley Bonestell — Wernher von Braun, who was arguably the world’s most experienced rocketeer, explained how a fleet of massive nuclear-powered vehicles would ferry hundreds of people to Mars. Sceptics, however, considered that even the launch of small probes to investigate the planets was pure fantasy. In 1925 W. Hohmann, a German mathematician, studied the energy requirements for travelling from Earth to Mars, and found that the minimum energy ‘transfer’ to Mars involved departing from Earth on an elliptical orbit with an aphelion, achieved some 250 days later, that was tangential to Mars’s orbit. The planet would have to be present when the spacecraft arrived, of course; so by calculating back from the time of interception and allowing for the planet’s motion around the Sun, the time of departure could be identified. In fact, while the craft was in transit Mars would travel an angle of about 130 degrees. The best time to mount such a voyage would be when Mars was at opposition, and ‘launch windows’ were available that opened about 50 days prior to opposition and lasted for several weeks. This also, of course, depended on the rocket’s capability. Because Mars has an elliptical orbit with the closest point of approach to Earth varying between 55 and 100 million kilometres, the ideal time to make such a flight would be at a perihelic opposition. Unfortunately, the necessary technology did not exist at the very close perihelic opposition of 1956, but having begun the Space Age on 4 October 1957 by launching Sputnik, the Soviet Union decided to dispatch two probes during the October 1960 window.