When Jim Lovell and Fred Haise, leaving Jack Swigert to batten down crippled Odys- sey, moved into Aquarius just before mid- night Monday and began powering her up as Apollo 13's lifeboat, the Moon still lay 50,000 miles and 20 hours ahead.
It would be risky to fire the main engine of the Command/Service Module, possibly damaged by an apparent rupture of a high- pressure oxygen tank. The shortest way home--in time--would be to coast on around the Moon and then be pulled automatically back toward Earth.
But on the hybrid trajectory to which the astronauts had maneuvered Sunday evening to facilitate lunar landing in the Fra Mauro Hills, their spacecraft would actually miss the Earth by 250 miles and pass on by, beyond hope of survival.
So a prime concern was to get back on a free-return trajectory that would bring them down in some ocean--almost any ocean-- without need for further major maneuvers.
This would have to be accomplished by firing the LM descent engine--an emergency procedure that, by foresight, had been prac- ticed in space by Apollo 9 and by the present crew in simulations at Kennedy Space Center.
Swigert urged combining the free-return maneuver with an extra push to speed the return journey, and doing it soon to cut the drain on the LM's batteries and cooling water:
SPACECRAFT--The advantage of doing this early is you can do a big burn now in the midcourse and then power the LM down. Otherwise. we got to keep the LM powered up clear till we get around the Moon.
But Flight Director Glynn Lunney held off a decision while trajectory planners ran half a dozen alternatives through their computers.
The fastest return would lead to a splash- down in the Pacific about noon Thursday, west of the originally planned recovery area but within steaming distance of the recovery carrier USS Iwo Jima. But this would take a long burn and leave the descent engine little fuel for later course adjustments that might be required.
Fuel could be saved and the return cut short by dropping the Service Module to reduce the dead weight that had to be maneuvered. But a Lunar Module engine had never been used in space to maneuver just the Lunar and Command Modules. And removing the protection that the Service Module gave the Command Module heat shield might expose the shield to damaging cold on the long voyage home.
The next quickest procedure would be landing early Friday morning in the South Atlantic, in range of U.S. planes--but not of ships. Mission Control chose a two-step abort: an early short burn to reestablish free return, with potential splashdown Friday evening in the Indian Ocean, then a longer burn soon after looping around the Moon to speed the return by 10 hours and shift the target point back to the Mid-Pacific, where the prime recovery forces waited.
The discussion between Spacecraft and Houston continued:
CAPSULE COMMUNICATOR--We've at this time, water critical in the LM. We'd like to use as little as possible. To do this we're going to make a free-return maneuver of 16 feet per second at 61 hours, which is 37 minutes from now. Then we're going to power down the PC, NS (Primary Guidance and Navigation Section), and then at 79 hours we'll go ahead and make another abort maneuver to kick what we got.
SPACECRAFT--Could you give us a little more time?
CAPCOM--Okay, Jim. We'd like to get a suggested time from you.
SC--Let's shoot for an hour if we can. How's that?
CAPCOM--Okay, Tim. How about 61 hours and 30 minutes? That's an hour and five from now.