Rendezvous around the Moon

Exactly 40 years ago, Apollo 10 blasted off Earth; its mission in the words of Commander Tom Stafford "to sort out the unknowns and pave the way for a lunar landing". Continuing his series of essays marking the 40th anniversary of the moonshots, Dr Christopher Riley reflects on Nasa's method for landing on the Moon and the Apollo legacy of space rendezvous.

In May 1969, with only seven months to go before the end of the decade, the first Lunar Module to fly in orbit around the Moon was powered up and readied for undocking from the Command Module.

Astronauts Tom Stafford, John Young and Gene Cernan were about to test out a technique for lunar landing which had first been proposed in 1916 by a Russian mechanic called Yuri Kondratyuk.

Kondratyuk's thesis described how a small landing craft could leave a mothership in lunar orbit to ferry its crew to the surface and back - a technique later referred to as Lunar Orbit Rendezvous or LOR.

Forty years after Apollo it's easy to see this method of landing a man on the Moon as the only way it could have been done. But the idea of bringing two vehicles together in space above the Moon had originally been rejected outright by Nasa as simply too difficult.

In 1961, with Kennedy's challenge still ringing in their ears, the agency had favoured a far simpler approach.

From the 1950s, the principal concept for a flight to the Moon involved a streamlined rocket blasting vertically off the Earth, flying straight there and then landing vertically tail first on a column of rocket thrust.

After lunar exploration was completed, it would then perform a similar vertical launch from the Moon and a final vertical landing back on Earth.

Popular in both science fiction and with military feasibility studies at the time, this "Direct Ascent", as it was dubbed, also seemed an obvious solution for Apollo.

But Direct Ascent was not as straight forward as it first appeared. It would need a completely new and truly immense rocket called the Nova to do the job; twice as powerful as the Saturn V and perhaps as tall as the Empire State Building. Secondly, no-one really knew how the astronauts, sitting near the top, could land a very tall rocket on the lunar surface tail first.

Rocket pioneer Wernher von Braun had been championing an alternative concept called Earth Orbit Rendezvous (EOR) since his US Army days in the late 1950s.

His method would eliminate the need for one giant rocket, requiring instead a number of relatively smaller Juno V booster rockets, later to be known as Saturn Vs.

These would launch into orbit the collective hardware necessary to assemble a giant vehicle to travel on to the Moon and return to Earth.

But whilst EOR solved the problem of building a giant rocket on Earth, the difficulty of landing it on the surface of the Moon remained.

In addition, multiple Saturn V rocket launches would be needed for each Moon shot and so EOR would be expensive.

What von Braun and the others had missed was that it was not necessary for the entire spacecraft to land on the Moon. This had been Kondratyuk's breakthrough thought.