It is 21 December 1968, 7.50am, Cape Kennedy, Florida. The Apollo 8 crew – Frank Borman, Jim Lovell and Bill Anders – are strapped into their couches, some 110 metres (363ft) above the ground at the top of the first manned Saturn 5 rocket – the most powerful machine ever built. As the final seconds tick down to launch there is little to say and little more they can do. Some four million litres of fuel is about to ignite beneath them. They are, as the watching BBC TV commentator helpfully put it, “sitting on the equivalent of a huge bomb”.

There is every reason to be concerned. During the previous unmanned test of the Saturn 5, a few months earlier, severe vibrations and g-forces shortly after launch would have likely killed anyone on board. Although the rocket has since been modified, Borman’s wife has been discretely warned by Nasa that her husband has about a 50/50 chance of surviving the mission.

The performance of the Saturn 5 rocket isn’t the only thing worrying Nasa management. Apollo 8 is a mission of firsts – a giant leap forwards in the race to land a man on the Moon. It will be the first manned spacecraft to leave Earth orbit, the first to orbit the Moon and the first to return to Earth at a staggering 40,000km/h (25,000mph). The mission is a calculated gamble by the space agency to beat the Soviet Union to our closest neighbour.

You may also like:

“It was a very, very bold decision,” says Teasel Muir-Harmony, Apollo Curator at the National Air and Space Museum in Washington DC. “Everyone within the agency knew it was an extraordinarily risky mission and there was a lot of criticism, most famously by British astronomer Sir Bernard Lovell, of the United States putting human life at risk.”

In fact, Apollo 8 was never intended to be so ambitious. It was originally planned as the first test of the Apollo lander in Earth orbit, but production of the lander was running late. On top of that, the CIA warned that intelligence suggested the Soviets were about to attempt their own manned flight around the Moon (you can read how close they really were here.)