Henry Spencer is a computer programmer, spacecraft engineer and amateur space historian

Apollo 9 is almost forgotten now, as it was an Earth-orbit test flight sandwiched between more glamorous missions to the Moon. But it had an important part to play - the astronauts and mission planners thought it was the most difficult Apollo mission short of the actual landing. It lifted off 40 years ago today, on 3 March 1969.

Even though Apollo 9 would go no higher than Earth orbit, it was the first full checkout of the complete Apollo spacecraft, including the troubled Lunar Module (LM). It would also feature the first Apollo spacewalk - the only in-space test of the Apollo spacesuit (and its life-support backpack) before the landing. Jim McDivitt, Dave Scott, and Rusty Schweickart had a lot of work ahead of them. Anything that went badly wrong could jeopardise landing a man on the Moon on schedule.

The most obvious problem was that Apollo 9 ran late, because the LM ran late. The Apollo 5 unmanned test in January 1968 had flown an unfinished and below-spec LM, with only partial success. In early 1969, Apollo couldn't wait any longer: if there was to be any hope of a lunar landing that year, the LM had to be tested and qualified for flight right away.

Apollo 9 also ran late in a smaller way: it was originally set for launch on February 28th. However, with final preparations underway and the multi-day countdown already started, the entire crew came down with colds. With bad memories of Apollo 7 still fresh, launching a sick crew didn't seem like a good idea. Fortunately, Apollo 9 didn't have the tight launch windows of a lunar mission, so NASA just delayed the launch by a few days.

(In hindsight, part of the problem was obvious. The last couple of months before an Apollo launch were very hectic for the crew, and it wasn't surprising that tired, stressed astronauts tended to come down with minor ailments. The training schedule was revised to try to make the last few days before launch quieter, and to isolate the crew somewhat.)

One novelty of Apollo 9 was that spacecraft names reappeared. They had been banished by management since Gus Grissom, who commanded the 1965 Gemini 3 mission, named it "The Unsinkable Molly Brown". An homage to a survivor of the Titanic, the name reflected the fact that he'd nearly drowned when his Mercury capsule sank in 1961.

The two Apollo 9 spacecraft needed separate radio call-signs: they couldn't both be "Apollo". McDivitt's crew named them after their shapes: the LM was "Spider", the main spacecraft was "Gumdrop". (Management winced.)

With the crew recovering, the countdown resumed, and Apollo 9 lifted off on March 3rd. The first phase of the flight focused on manoeuvring the two spacecraft together. Either one might be called on to supply propulsion for the combination, and nobody had ever manoeuvred such an ungainly contraption in space before, so it needed testing.

The one real problem seen during this phase was a repeat of Apollo 8's spacesickness. Schweickart in particular was sick repeatedly, and this affected the next major step: the spacewalk.

The original plan had been for Schweickart to go out Spider's front hatch, and then climb along the outer surfaces of the two spacecraft to reach Gumdrop's hatch. This was now judged too ambitious, and he just went out onto Spider's "front porch", staying within easy reach of the hatch. In fact, he was feeling better by then, and it all went smoothly.

Among other novelties, this was the first completely self-contained spacewalk. All previous spacewalkers had received their air supply via an "umbilical" hose from the spacecraft, but Schweickart, testing a suit meant for use on the Moon, depended completely on his backpack.

With that out of the way, it was time to take the final step: the two spacecraft separated and manoeuvred independently. Despite worries, this generally went well. The one snag was that manoeuvring the LM precisely was more difficult than expected: the response to the controls was a bit sloppy.

Apollo 9's one big problem came when it was time to get the two spacecraft back together. McDivitt, flying Spider, had a terrible time with the docking manoeuvre. He had his head tipped far back, looking through a small window over his head, and he had to mentally turn the controls the same way.

The mirror-smooth surface of the other spacecraft, Gumdrop, reflected Earth and space and made its position very difficult to judge. There was a docking target for him to look at, but it was small and dimly lit, inside one of Gumdrop's windows. Spider had an optical sight to help, but its filters cut out too much light. And finally, Spider's sloppy manoeuvring was a most unwelcome complication now.

McDivitt, considered one of NASA's best pilots, finally did get the two spacecraft docked, but he commented afterward: "That wasn't a docking, that was an eye test."

Some of the difficulties could be cured. Revisions to the LM's control software fixed the sloppy handling, and the docking target and the optical sight were improved.

The window position and the shiny surfaces couldn't be changed, but the need to cope with them could: henceforth, except in emergencies, the astronaut who was left behind in the mothership would fly the docking manoeuvres. He had a much better view, and had few reflection problems because the LM had fewer large smooth surfaces.

Overall, this wasn't too bad. Many people had expected worse from the LM. Apollo 9's fixes would be checked out during the dress-rehearsal mission, Apollo 10, but the road to the Moon now seemed clear.

Image: The Apollo 9 Command/Service Modules were photographed from the Lunar Module (Credit: NASA-JSC)