eyed up.

But he talked technology. He examined the inner skin and its lining
before going beyond the temporary entrance. The plating of the Platform
was actually double. The outer layer was a meteor-bumper against which
particles of cosmic dust would strike and explode without damage to the
inner skin. They could even penetrate it without causing a leak of air.
Inside the inner skin there was a layer of glass wool for heat
insulation. Inside the glass wool was a layer of material serving
exactly the function of the coating of a bulletproof gasoline tank. No
meteor under a quarter-inch size could hope to make a puncture, even at
the forty-five-mile-per-second speed that is the theoretical maximum for
meteors. And if one did, the selfsealing stuff would stop the leak
immediately. Joe could explain the protection of the metal skins. He
did.

"When a missile travels fast enough," he said absorbedly, "it stops
acquiring extra puncturing ability. Over a mile a second, impact can't
be transmitted from front to rear. The back end of the thing that hits
has arrived at the hit place before the shock of arrival can travel back
to it. It's like a train in a collision which doesn't stop all at once.
A meteor hitting the Platform will telescope on itself like the cars of
a railroad train that hits another at full speed."

Sally listened enigmatically.

"So," said Joe, "the punching effect isn't there. A meteor hitting the
Platform won't punch. It'll explode. Part of it will turn to
vapor--metallic vapor if it's metal, and rocky vapor if it's stone.
It'll blow a crater in the metal plate. It'll blow away as much weight
of the skin as it weighs itself. Mass for mass. So that weight for
weight, pea soup would be just as effective armor against meteors as
hardened steel."

Sally said: "Dear me! You must read the newspapers!"

"The odds figure out, the odds are even that the Platform won't get an
actual meteor puncture in the first twenty thousand years it's floating
round the Earth."

"Twenty thousand two seventy, Joe," said Sally. She was trying to tease
him, but her face showed a little of the strain. "I read the magazine
articles too. In fact I sometimes show the tame article writers around,
when they're cleared to see the Platform."

Joe winced a little. Then he grinned wryly.

"That cuts me down to size, eh?"

She smiled at him. But they both felt queer. They went on into the
interior of the huge space ship.

"Lots o