risky to attempt passing the lab. The light was apt to reflect from their suits, discover their presence to the robots inside. But by describing a circle he could avoid the lighted areas and come up behind the dilapidated tramp freighter.

He glanced upward at the stars, impressing their position on his mind. The constellations were little altered. He found Polaris in the tail of the little dipper. It was not the axis star as it was on Earth, but it served to fix his sense of direction in the impenetrable blackness.

They tripped and stubbed their toes, stumbled into shallow fissures, climbed sharp-edged crests. Sofi, forgetting the radiophone, muttered several well-chosen expletives to herself. They would have done credit to a spaceman. Hen was so shocked, he forgot to reprimand her.

In a few minutes the lights of the igloos reappeared to guide them, the vast black bulk of the tramp freighter screening part of the mining unit. They crept up to the ship, and hugging its shadow, moved noiselessly towards the port. Light from the reduction plant picked them out brightly as they came around the stern.

Hen's stomach contracted. There was a sudden bitter taste in his mouth. He halted so abruptly that Sofi bumped against his shoulder.

The port was open. The gleaming functional mechanism that was R-3 stood complacently in the entrance.

The space ship was being guarded.

The robot caught sight of the humans at the same moment. His reaction, although mechanical, was almost as instantaneous as their instinctive one.

He moved to block the entrance, sent out a call for help.

Hen, guessing his intention, tuned his helmet receiver to the robot's wave length. R-3's mechanical voice rang suddenly inside his helmet.

"... attacking the space ship! Aid! Aid! Father attacking the space ship! Aid!"

Hen switched back to the girl's wave length. "Run," he commanded tersely. "He's calling for help. He'll have the lot of them down on our heads."

Suiting action to words, he took to his heels, plunging for the housing unit.

"Lock ourselves in!" he grunted.

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