Apollo 10 was the mission that almost made it to the Moon. Designed as the full dress rehearsal for Apollo 11, the lunar module “Snoopy” approached to just under 15 km from the Moon’s surface with astronauts Thomas Stafford and Eugene Cernan aboard. It was May 22, 1969 – 50 years ago – and the last demonstration needed before NASA could send a man to the Moon. The penultimate step in road from the one-person Mercury, the two-person Gemini and the three-person Apollo missions demonstrating all the necessary technology steps needed in order to take a human from the Earth to the Moon and get them back safely.
Apollo 8, over Christmas 1968 had demonstrated it was possible to take humans to the Moon, orbit it and then come back to Earth. Apollo 9, in March 1969 then qualified the operations of the command module with the lunar module, including its separation, flight away and then return and re-docking, all while in Earth orbit. Apollo 10 was to take this one step further, going all the way to the Moon, and initiating a descent towards the surface, before return.
This dress rehearsal took the lunar module to the point where powered descent would begin for the actual landing that Apollo 11 would later do. Practicing this approach orbit required refining knowledge of the lunar gravitational field needed to calibrate the descent guidance system to within the 2 km needed for a landing. Except for this final stretch, the mission was designed to duplicate how a landing would have gone, both in space and for ground control, putting NASA’s flight controllers and extensive tracking and control network through a rehearsal.
The ascent stage was loaded with the amount of fuel and oxidiser it would have had remaining if it had lifted off from the surface and reached the altitude where the Apollo 10 ascent stage fired. So had the astronauts decided to carry on and land on the Moon they wouldn’t have been able to take off again!
Interesting fact: the Apollo 10 crew hold the record as the humans who have traveled the furthest away from their homes back on Earth, reaching to over 400,000 away! They reached the furthest point in their orbit around the far side of the Moon while the Moon was close to its furthest distance from Earth, and at about the same time that the Earth’s rotation put their own home town of Houston nearly a full Earth diameter further away!
Article by Michael Burton, Director of the Armagh Observatory and Planetarium. Further information from the wikipedia article on Apollo 10.