equally frantic for facts. “What is the job, sir?” he asked quietly.

“You’ve noticed, of course, the suppressed terror of the people back home in the past weeks,” the general said. “They know something big is wrong, that their very lives are being menaced. How they found out I don’t know, because the strictest censorship has been held. Maybe it’s a sort of telepathic hysteria that can’t be censored. At any rate it’s there, and there’s already been trouble from it. The Command at home has been getting crank letters demanding that we tell the people what is wrong. This kind of thing can lead to something bad.”

“Then something is wrong?” Rob ventured, watching the officer expertly avoid a treacherous crack in the frost-riven ground.

The general’s face became haggard, and there was a trace of terror in his own eyes. “There is. Something even worse than the people must suspect.”

Rob shuddered. All of a sudden the minus-200-degree temperature outside his space suit seemed to have penetrated inside. He checked the heater and found that it was all right. No, this was a mental chill.

Next came the inevitable question, “What is this—thing?”

“You and your crew will be sworn to strictest secrecy before you blast off from Luna,” General Forester said. “That pledge of secrecy for you begins at this instant. If the people back home got even an inkling of what the trouble is, there would be widespread panic.”

“You have my word, sir,” Rob said.

That pledge of secrecy for you begins at this instant.

There followed an electric silence for several moments. It was as if the general himself were rallying courage. “There is a giant radioactive cloud approaching the solar system from outer space at a terrific speed. The cloud covers an area roughly as big as Jupiter. Scientists have been plotting its trajectory with electronic instruments for a long time, and there is no doubt but that it will collide with the system if nothing is done about it. Life, of course, would be wiped out completely.”

Rob felt the horror of the statement clear to the marrow of his bones. It left him shaking and numb. The general noticed the effect on him.

“That’s the way it left me when I first heard about it,” he admitted. “If it affects us two, who are reasonably adjusted to the terrors of space, how do you think it 

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