"So, with the shield up, men can stay on the planet when it is less than three diameters from its sun. No heat reaches it. No neutrons. No radiations at all. It doesn't heat up. And that's the answer. For three months in every four-year revolution, they have to keep the shield up all the time. For three months more, they keep it up intermittently, flashing it on for fractions of a second at a time, just enough to temper the amount of heat they get.
"They live on great platforms of uranium glass, domed in. When they go out mining they wear shielded space-suits and work in shielded machines. The whole trick was worked out about five hundred years ago, they say, and the last space-line went out of existence, because they could use a matter-transmitter for all but six of our months of that planet's year."
"And did you find out how it's done?" asked Dona.
"Hardly," said Kim. "The planet belongs to the king of Pharos Three. Even five hundred years ago the governments of all the planets were quite tight corporations. Naturally Pharos wouldn't let the secret get out. There are other planets so close to their primaries that they're radioactive. If the secret were to be disclosed there'd be competition. There'd be other plutonium mines in operation. So he's managed to keep it to himself. But we've got to find out the trick."
There was silence. Kim began to check over the pictures the cameras had taken and developed. He shook his head. Then he stared at a photograph which showed the blue-white dwarf itself. His face looked suddenly very drawn and tired.
"Kim," said Dona presently. "It's stupid of me, but I don't see how you're going to learn the secret."
Kim put the picture on the enlarger, for examination in a greater size.
"They made the shield to keep things out," he said wearily. "Radiation, charged particles, neutrons—everything. The planet simply can't be reached, not even by matter-transmitters, when the shield is up. But by the same token nothing can leave the planet either. It can't even be spotted from space, because the light of the sun isn't reflected. It's deflected to a right-angled course. You might pick it up if it formed a right-angled triangle with you and the sun, or you might spot it in transit across the sun's disk. But that's all."
"Yes."
"The shield was a special job," said Kim. "For a special purpose. It was not a weapon. But there were all those planets that could be grabbed if only Ades were knocked out. So why shouldn't King Pharos sneak a force-field generator on to Ades? When the field went on, Ades would be invisible and unreachable from outside. And the outside would be unreachable from it. Space-ships couldn't get through the field. Matter-transmitters couldn't operate through it. If a few technicians were sneaked to Ades as supposed exiles and promised adequate reward, don't you think they'd hide out somewhere and turn on that field, and leave it on until the folk on Ades had starved or gone mad?"