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A famous picture in the English edition of Newton’s “Principia” shows cannon balls being fired from the top of a mountain. If they go fast enough, their trajectory curves downward no more steeply than the Earth curves away underneath it – they go into orbit. This picture is still the neatest way to explain orbital flight. Newton calculated that, for a cannon-ball to achieve an orbital trajectory, its speed must be 18000 miles per hour – far beyond what was then achievable.

Indeed, this speed wasn’t achieved until 1957, when the Soviet Sputnik was launched. Four years later Yuri Gagarin was the first human to go into orbit. Eight years after that, Neil Armstrong made his “one small step”. The Apollo programme was a heroic episode. And it was a long time ago – ancient history to today’s young people.

Had the momentum of the 1960s been maintained over the next 40 years, there would be footprints on Mars by now. But after Apollo, the political impetus for manned spaceflight was lost.

The most crucial impediment to space flight stems from the intrinsic inefficiency of chemical fuel, and the consequent requirement to carry a weight of fuel far exceeding that of the payload. This is a generic constraint, based on fundamental chemistry. If a planet’s gravity is strong enough to retain an atmosphere, at a temperature where water doesn’t freeze, and metabolic reactions aren’t too slow, the energy required to lift a molecule from it will require more than one molecule of chemical fuel.

Launchers will get cheaper when they can be designed to be more fully reusable. It will then be feasible to assemble, in orbit, even larger artifacts than the International Space Station. But so long as we are dependent on chemical fuels, interplanetary travel will remain a challenge.

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