construction, with gun handy.

Fifty tons of such a light metal as lithium is a pretty large volume of the stuff. Albrekt assumed that Carrel's shield was to be a square or disc of the metal, rather thick to absorb the radiation, which would be interposed between the By Jove! and Jupiter. When work began, after several days of planning, it became apparent that the construction task was something more than cutting out and fastening together chunks of lithium.

Instead of working inside the ship, the crew moved a furnace to the outside of the cargo hull and anchored it down. The Earthmen wore spacesuits, of course, but Qoqol did not, as Martians do not breathe, but extract oxygen from solid matter and store enough of it to last several hours at a time.

To Albrekt's surprise, they next hauled out some of the big packages which were plastic domes for use on Titan. At extra-terrestrial bases, these hemispherical domes were inflated to form huge air bubbles in which humans could live.

"Plastic?" said Albrekt through his helmet radio. "I thought you were going to use lithium."

"We are," replied Carrel's voice. "We'll fasten some of these domes together to form an airtight sphere, then inflate it from the oxygen supply. It won't take much pressure, and we can recover the oxygen later with the ship's compressor.

"Before we recover the oxygen, we'll charge the plastic sphere electrically, so it'll stay rigid. Then we'll vaporize the lithium in the boiler and spray it over half the plastic sphere. We'll blacken the plastic and melt it with solar heat, returning it to the boiler by charging the boiler. I'm afraid we're going to ruin a few of the plastic domes, but that's not important now."

"Spray the lithium? Fifty tons of it?"

"Wait and see," Carrel said. "This will be a bigger shield than you expected."

Later, at mealtime, Carrel brought a worry to the surface of Albrekt's mind which the Flanjo agent had been trying to keep suppressed.

"That was a pretty rash business, jetting all the fuel," said Carrel. "What do we do if we're off orbit?"

"It seems to me I've mentioned before that some very good spacemen plotted this orbit," replied Albrekt.

"The best orbits sometimes require minor corrections, when they're this long," said 

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