Not long after his return from Gemini IX-A, astronaut Gene Cernan was summoned to Deke Slayton’s office and posed an unusual question.

‘‘Geno, how soon can you be ready to fly again?’’

‘‘Just say the word, Deke. When?’’

“Right now. Would you be willing to jump from backup to prime? Fly [Gemini] XII with Lovell?”

The year 1966 had certainly been a dramatic one for Cernan. When it began, he and Tom Stafford confidently looked forward to flying Gemini XII – the last manned mission in the series – themselves. Then, with awful suddenness, the deaths of Elliot See and Charlie Bassett in February pushed them from backup to prime crew on Gemini IX. Following his return from his first spaceflight, Cernan had been given a ‘dead-end’ slot, with Gordo Cooper, to back up Jim Lovell and Buzz Aldrin on Gemini XII. Now, with barely two-thirds of the year gone, Slayton was offering to break his own crew-rotation system, bumping Aldrin from the mission. Cernan’s first question to Slayton was a simple one. Why?

The reason was the Astronaut Manoeuvring Unit (AMU) – the Air Force-built rocket armchair which Cernan was originally detailed to test in June 1966 – whose military sponsors were pushing strongly to fly again on Gemini’s last mission. Without giving much away, Slayton told Cernan simply that he was the best man to fly the AMU, which was probably true, but a number of contributory factors centred on Buzz Aldrin himself: a man of mathematical and engineering genius, the first astronaut to possess a doctorate, an unquestioned expert in the field of space rendezvous. . . and a constant worry to Slayton. Aldrin had already raised eyebrows during Gemini IX-A, specifically those of Bob Gilruth and Chris Kraft, when he advocated having Cernan cut the lanyards of The Blob. Although not an outrageous suggestion, Slayton acquiesced, Aldrin’s advice was a little too adventurous in light of NASA’s limited EVA expertise.

Gilruth, Kraft and Slayton were not the only ones with worries about Aldrin. In his autobiography, Cernan hinted strongly that Aldrin’s intelligence was tempered by a seeming inability to stick to one topic: he had a tendency to fly off at tangents and drastically re-engineer everything, at a time when NASA had little time to do so. Astronauts and their wives would roll their eyes when Aldrin collared them, even over coffee, and engaged them in hours-long discussions of the intricacies of celestial navigation and mechanics. Coupled with reports of his performance in the Gemini simulators, it was Slayton’s judgement that the AMU test flight should be entrusted to Cernan, rather than Aldrin.

In his own defence, Aldrin would blame the decision on problems experienced by both Cernan and Dick Gordon on their EVAs: exhaustion, fogged-up visors and a difficulty in performing even simple tasks. ‘‘An urgent meeting of senior officials concerned with the Gemini XII EVA,’’ Aldrin wrote, ‘‘was held at the end of September and… they decided arbitrarily that I stood a poor chance of putting the innovative AMU backpack to good use. They felt the risks outweighed the benefits.’’

Despite the risks to his colleague’s career, Cernan accepted Slayton’s invitation on the spot – ‘‘when Deke asked if you would take a mission, there was only one answer’’ – and would have flown Gemini XII had not the decision been made that an AMU test was too risky. Gemini XII’s EVAs would focus instead on less dramatic evaluations of a spacewalker’s performance outside the pressurised confines of his spacecraft. Edwin Eugene Aldrin Jr, nicknamed ‘Dr Rendezvous’ behind his back, retained his place on the mission. Born in Glen Ridge, New Jersey on 20 January 1930, the son of an Army Air Corps pilot and a mother whose maiden name, propitiously, happened to be ‘Moon’, Aldrin’s development, even into adulthood, was very much guided by his father.

Naturally, in light of his father’s career, the man who would someday fly Gemini XII and become the second person to walk on the Moon was brought up with aviation in his blood. He first flew aboard an aircraft with his father in 1932, when he was barely two years old. (As a child, he earned the nickname ‘Buzz’ from his young sister, who, unable to pronounce ‘brother’, called him her ‘buzzer’.) Graduation was followed by enrolment in a military ‘poop school’ – aimed at preparing him for the Naval Academy at Annapolis – although Aldrin sought the Military Academy at West Point. Despite his father’s outspoken preference for the Navy, which he considered ‘‘took care of its people better’’, his son persisted and eventually won his reluctant approval.

When Aldrin graduated third in his class from West Point in 1951, his father’s immediate reaction was a question: who had finished first and second? He was not accepted for a coveted Rhodes postgraduate scholarship and instead entered the Air Force, earning his pilot’s wings later that same year after initial training in Bryan, Texas. During the conflict in Korea, Aldrin was attached to the 51st Fighter Wing, flying F-86 Sabres, and by the time hostilities ended in the summer of 1953 he had no fewer than 66 combat missions in his military logbook. Just a month before the end of the war, one of Aldrin’s gun-camera photographs – a Russian pilot ejecting from his stricken MiG – ended up in Life magazine.

In total, Aldrin returned from Korea having shot down three MiGs. Back in the United States, he became a gunnery instructor at Nellis Air Force Base in Nevada and in 1955 was accepted into Squadron Officer School in Montgomery, Alabama. At around the same time, he met and married Joan Archer and shortly thereafter became the father of a son, James. Professionally, his military career prospered: he was assigned as an aide to General Don Zimmerman, the dean of the new Air Force Academy, then moved to Germany in 1956, flying the F-100 Super Sabre as part of the 36th Fighter-Day Wing, stationed in Bitburg. During this time, he became a father twice more: to Janice and Andrew.

Before pursuing his next ambition of test pilot school, Aldrin, like his Bitburg flying comrade Ed White, sought to gain further education and was accepted into the Massachusetts Institute of Technology on military detachment for a doctorate of science degree in astronautics. His 259-page ScD thesis, completed in 1963, just months before his selection as an astronaut, was entitled ‘Line of Sight Guidance Techniques for Manned Orbital Rendezvous’. He chose the topic, he later wrote, because he felt it would have practical applications for the Air Force and aeronautics, although it also drew the attention of NASA, which was by now looking at lunar-orbital rendezvous for its Apollo effort.

Aldrin dedicated his thesis to future efforts in human exploration, wistfully remarking ‘‘if only I could join them in their exciting endeavours’’. By this time, of course, his application for the 1963 astronaut class was already being processed; he had tried to gain admission the previous year, seeking a waiver for his lack of test – piloting experience, but this time achieved success. A concern about his liver function, thanks to a bout of infectious hepatitis, did not prevent Aldrin from becoming one of the 14 astronauts named to the world that October.

Assigned to work on mission planning, his early days saw him focusing his attention on rendezvous and re-entry techniques… and, gradually, as each Gemini crew was named, he became increasingly frustrated that he was receiving no flight assignment. At one stage, he even approached Deke Slayton to stress his confidence in his own abilities – that his qualifications and understanding of orbital rendezvous far exceeded those of anyone else in the office – and was politely told that his comments would be noted.

Shortly thereafter, in early 1966, Aldrin and Jim Lovell were assigned as the backup team for Gemini X. His heart sank. Taking into account Slayton’s three – flight rotation system for backup-to-prime crews, there would be no Gemini XIII to which Aldrin and Lovell could aspire. It was, in effect, a ‘dead-end’ assignment. “Apparently, petitioning Deke – an arrogant gesture by ‘Dr Rendezvous’ – had not been well-received by the stick-and-rudder guys in the Astronaut Office,’’ Aldrin wrote. ‘‘By being direct and honest rather than political, I’d shafted myself.’’ All that changed on the last day of February, when See and Bassett were killed and their Gemini IX backups were pushed into prime position. In mid-March, Lovell and Aldrin were named as the new Gemini IX backups, with a formally-unannounced (but anticipated) future assignment as the prime Gemini XII crew.

For Aldrin, whose Nassau Bay backyard bordered that of the Bassetts, it was a devastating way to receive his long-desired flight assignment. Three weeks after the accident, he and Joan visited Jeannie Bassett to tell her the news. ‘‘I felt terrible,’’ he wrote, ‘‘as if I had somehow robbed Charlie Bassett of an honour he deserved.’’ Jeannie responded with quiet dignity and characteristic grace: her husband, she explained, felt that Aldrin ‘‘should have been on that flight all along. . . I know he’d be pleased’’.