Interior view of the Apollo 13 Lunar Module (LM) as the astronauts jerry-rig a system to use the Command Module lithium hydroxide canisters to purge carbon dioxide from the LM. Photo: NASA/JSC

As Apollo 13 is nursed homewards, the astronauts' main concern is that the Lunar spacecraft's air conditioning system is not designed to provide fresh air for the docking tunnel and Command Module as well. On the ground here, hurried experiments have enabled Jim Lovell, Fred Haise and Jack Swigert, by cutting up pieces of hose from spacesuits and using plastic bags and charcoal -- all things available to them -- to devise a way of extracting a little more of the poisonous nitrogen from the air.

A worry here, which is being kept from the crew, is what will happen to the 40lb atomic battery in the Lunar Module which should have been left on the moon to power experiments for at least a year, but which is now being brought back to earth. If it breaks up during re-entry, could the 12 lbs of plutonium 238 inside it create dangerous radioactivity?

The US Atomic Energy Commission insists that the battery was heat-shielded against just such an eventuality as has now occurred. It won't burn up during re-entry, but should emerge unscathed and plunge harmlessly into the Pacific Ocean about 300 miles behind the Command Module.

Meanwhile, preparations for re-entry are hectic for both the crew and Mission Control. For hour after hour instructions are being read up to the astronauts. A few minutes ago Jim Lovell announced that he had only one pad left - after which Houston promised him they would to their best to help to conserve his limited supply of paper.

While there's confidence here that there will be sufficient oxygen and power until splashdown, there are signs that the crew, having to work hard with little sleep in a poor atmosphere, are getting cold and tired. Even Jim Lovell has sounded a little dispirited during recent exchanges about the next vital mid-course correction. But as one NASA official told me here: "They're quite confident they're coming home safely. But of course they've nothing to look forward to; nothing has been accomplished."