 -- Ever since the Challenger disaster in 1986, NASA has said that its space shuttles were magnificent machines, but conceded they were complicated and risky to fly. The shuttle Atlantis, which made a magnificent last launch today, proved the point again.

The launch almost didn't happen. NASA played chicken with Florida's famously temperamental weather – and won – but then had to stop the countdown with only 31 seconds to go because of a computer glitch. NASA resolved the problem with less than a minute to spare before Atlantis' target, the International Space Station, would have passed out of range.

The four astronauts – Commander Christopher Ferguson, along with Douglas Hurley, Sandra Magnus and Rex Walheim – have the mundane sounding task of docking with the station and dropping off supplies. But even the crew size – shuttles have historically flown with six or seven astronauts – is a reflection of the risks of spaceflight.

SEE MORE: 30 Years Of Space Shuttle Missions

After the shuttle Columbia was lost on its return to Earth in 2003, investigators said its seven astronauts could have been saved if NASA had been ready. The investigation board recommended that NASA keep a second shuttle at the ready at every launch after that. If history repeated itself and a shuttle's heat-shield tiles were damaged the way Columbia's were, the crew could take refuge on the space station until the second shuttle could come to their rescue. The emergency flights would have four astronauts.

Atlantis has four astronauts because NASA no longer can launch a rescue mission. It has literally run out of those giant orange fuel tanks and white booster rockets that get shuttles into orbit. If lightning were to strike twice, and Atlantis, like Columbia, could not get home safely, the astronauts would have to hitch rides, one by one, on Russian Soyuz capsules – so tiny that the three crew members sit in almost a fetal position.

Since the Russians have their own budget problems, and need to fly their own cosmonauts, the number of extra seats is limited. So forget about sending more astronauts. NASA has quietly cautioned that in the unlikely event of trouble, Ferguson and his crewmates might have to wait in orbit for up to a year.

"You have to prepare yourself for that eventuality," said Hurley in a prelaunch interview with ABC News, "because if it really did happen you can imagine the kind of flailing that would be going around on the ground had we not gone through this kind of training."

"We really do want to finish it like any other flight," said astronaut Walheim. "It should be a celebration, a celebration of the end of the flight and a celebration of the fact that we have had an incredible 30 year program, but on the other side it is going to be a little bit sad to think it is the last time the space shuttle is flying."