With The Lunar Module's life support systems coming on line, the immediate threat of death to the crew had been suspended, and it was time to start thinking about how to get the astronauts home.
Jerry Bostick was the chief of the flight dynamics branch, the part of mission control that looks after a spacecraft's trajectory--where it is, where it's going, where it should be, and how to get it there. The controllers of the flight dynamics branch sat in the front row of mission control, which they had proudly dubbed "the Trench." As they listened to the crew in space and the systems controllers in the row behind them struggle with the explosion's aftermath, "we went into the mode of okay, well, can we come back home immediately?" remembers Bostick. The Trench soon calculated that if the crew used the Odyssey's main engine and burned every last drop of fuel, they could turn around and come straight back to Earth, in a procedure known as a direct abort.
But the main engine was in the service module, and who knew what damage had been done to it? It might malfunction: in the worst case, firing it up could result in another explosion and kill the crew instantly. The other option was to let Apollo 13, carried forward by its momentum and the moon's gravity, go around the moon. There, gravity would pull Apollo 13 around the back side of the moon, accelerate it, and sling the spacecraft back toward Earth. This journey would take several days, however, and the lunar module was intended to support only two men for two days--not three men for four. If the crew didn't get home fast, they could run out of power and die.
Kranz says this was his toughest call on Apollo 13. "My team was pretty much split down the middle. Many of my systems controllers wanted to get home in the fastest fashion possible. The trajectory team did not want to execute a direct abort because it had to be executed perfectly. If we didn't get the full maneuver, more than likely we would crash into the moon," he explains, "I was of the frame of mind that said, 'Hey, we don't understand what happened here...and if we execute a direct abort, we're not going to have much time to think about it...We needed to buy some time so that when we did make a move, it would be the proper move.' "
Weighing the concern that the Aquarius wouldn't cut it on a longer return journey, Kranz told Spectrum he had "a lot of confidence in my lunar module team." Apollo 13 was Kranz's fourth mission involving a lunar module. "I knew it was a very substantial spacecraft...I was pretty much betting that this control team could pull me out of the woods once we decided to go around the moon."
Kranz made his decision. The main engine was out. Apollo 13 was going around the moon.
There was, of course, a fly in the ointment. During earlier Apollo missions, the outgoing trajectory of the spacecraft had been selected so that if the service module's main engine failed for any reason, the slingshot effect would aim the command and service module perfectly at Earth, a so-called free-return trajectory. But this trajectory put very tight constraints on the mission timeline, and for Apollo 13, it had been abandoned.
"We were on a non-free-return trajectory. If we did nothing, we'd whip back towards the Earth but miss it by several thousand miles," the Trench's Bostick explains.
As the question of trajectory was being decided a shift change was going on at mission control. When the explosion occurred, Kranz and his controllers--collectively known as the White Team--had been about an hour away from the end of their shift. As was common, most of the next shift--the Black Team, led by Glynn Lunney--had already shown up, so as to be able to take over running the mission seamlessly from their predecessors, and they had been on hand throughout the crisis.
As Kranz's team gathered up to leave mission control, Bostick went to speak to the incoming flight director, Lunney. By good fortune, Kranz and Lunney were perfectly matched to the different phases of the crisis they would be faced with. Kranz was a systems guy--he knew the internals of the spacecraft better than any other flight director, the ideal person to cope with the second-by-second equipment failures and reconfigurations triggered by the explosion. Lunney had come up through the flight dynamics branch, making him ideally suited to get the spacecraft headed in the right direction.