ed for the door
to open to empty space.

Instead, there came a sharp voice in his helmet-phones. It was Brown.
"_Radar says there's a rocket on the way up! It's over at what is the
edge of the world from here. Three gravities only. Better not go out!_"

Joe hesitated. Brown still issued no order. But defense against a single
rocket would be a matter of guided missiles--Brown's business--if the
tin can screen didn't handle it. Joe would have no part in it. He
wouldn't be needed. He couldn't help. And there'd be all the elaborate
business of checking to go through again. He said uncomfortably:

"It'll be a long time before it gets here--and three gravities is low!
Maybe it's a defective job. There have been misfires and so on. It won't
take long to try this wagon, anyhow. They're anxious to send up a robot
ship from the Shed and these have to be tested first. Give me ten
minutes."

He heard the Chief grumbling to himself. But one tested space wagon was
better than none.

The airlock doors opened. Huge round valves swung wide. Bright, remote,
swarming stars filled the opening. Joe cracked the control of his
forward liquid-fuel rockets. The lock filled instantly with swirling
fumes. And instantly the tiny space wagon moved. It did not have to lift
from the lock floor. Once the magnetic clamps were released it was free
of the floor. But it did have mass. One brief push of the rockets sent
it floating out of the lock. It was in space. It kept on.

Joe felt a peculiar twinge of panic. Nobody who is accustomed only to
Earth can quite realize at the beginning the conditions of handling
vehicles in space. But Joe cracked the braking rockets. He stopped. He
hung seemingly motionless in space. The Platform was a good half-mile
away.

He tried the gyros, and the space wagon went into swift spinning. He
reversed them and straightened out--almost. The vastness of all creation
seemed still to revolve slowly about him. The monstrous globe which was
Earth moved sedately from above his head to under his feet and continued
the slow revolution. The Platform rotated in a clockwise direction. He
was drifting very slowly away.

"Chief," he said wrily, "you can't do worse than I'm doing, and we're
rushed for time. You might come out. But listen! You don't run your
rockets! On Earth you keep a motor going because when it stops, you do.
But out here you have to use your motor to stop, but not to keep on
going. Get it? When you do come o