A Fall of Moondust

“For those of you who don't know what Selene looks like, here's a one-in-twenty scale model.” He lifted the model from the table, and turned it slowly in front of the camera. “She's just like a bus, or a small aircraft; the only thing unique is her propulsion system, which employs these wide-bladed, variable-pitch fans.

“Our great problem, of course, is the dust. If you've never seen it, you can't imagine what it's like. Any ideas you may have about sand or other materials on Earth won't apply here; this stuff is more like a liquid. Here's a sample of it.”

Lawrence picked up a tall vertical cylinder, the lower third of which was filled with an amorphous gray substance. He tilted it, and the stuff began to flow. It moved more quickly than syrup, more slowly than water, and it took a few seconds for its surface to become horizontal again after it had been disturbed. No one could ever have guessed, by looking at it, that it was not a fluid.

“This cylinder is sealed,” explained Lawrence , “with a vacuum inside, so the dust is showing its normal behavior. In air, it's quite different; it's much stickier, and behaves rather like very fine sand or talcum powder. I'd better warn you-it's impossible to make a synthetic sample that has the properties of the real thing. It takes a few billion years of desiccation to produce the genuine article. If you want to do some experimenting, we'll ship you as much dust as you like; heaven knows, we can spare it.

“A few other points. Selene is three kilometers from the nearest solid land—the Mountains of Inaccessibility. There may be several hundred meters of dust beneath her, though we're not sure of that. Nor can we be quite sure that there will be no more cave-ins, though the geologists think it's very unlikely.

“The only way we can reach the site is by dust-ski. We've two units, and another one is being shipped round from Farside. They can carry or tow up to five tons of equipment; the largest single item we could put on a sledge would be about two tons. So we can't bring any really heavy gear to the site.

“Well, that's the position. We have ninety hours. Any suggestion? I've some ideas of my own, but I'd like to hear yours first.”

There was a long silence while the members of the committee, scattered over a volume of space almost four hundred thousand kilometers across, brought their various talents to bear on the problem. Then the Chief Engineer, Farside, spoke from somewhere in the neighborhood of Joliot-Curie.

“It's my hunch that we can't do anything effective in ninety hours; we'll have to build special equipment, and that always takes time. So—we have to get an air line down to Selene. Where's her umbilical connection?”

“Behind the main entrance, at the rear. I don't see how you can get a line there and couple it up, fifteen meters down. Besides, everything will be clogged with dust.”

“I've a better idea,” someone interjected. “Drive a pipe down through the roof.”

“You'll need two pipes,” pointed out another speaker. “One to pump in oxygen, the other to suck out the foul air.”

“That means using a complete air purifier. And we won't even need it if we can get those people out inside the ninety hours.”

“Too big a gamble. Once the air supply is secure, we can take our time, and the ninety-hour deadline won't worry us.”

“I accept that point,” said Lawrence . “In fact, I've several men working on those lines right now. The next question is: Do we try to raise the cruiser with everyone inside, or do we get the passengers out individually? Remember, there's only one space suit aboard her.”

“Could we sink a shaft to the door, and couple it to the air lock?” asked one of the scientists.

“Same problem as with the air hose. Even worse, in fact, since the coupling would be so much bigger.”

“What about a cofferdam large enough to go round the whole cruiser? We could sink it round her, then dig out the dust.”

“You'd need tons of piles and shorings. And don't forget, the dam would have to be sealed off at the bottom. Otherwise the dust would flow back into it, just as fast as we took it out of the top.”

“Can you pump the stuff?” asked someone else.

“Yes, with the right kind of impeller. But you can't suck it, of course. It has to be lifted. A normal pump just cavitates.”

“This dust,” grumbled the Port Roris Assistant Engineer, “has the worst properties of solids and liquids, with none of their advantages. It won't flow when you want it to; it won't stay put when you want it to.”

“Can I make a point?” said Father Ferraro, speaking from Plato. “This word 'dust' is highly misleading. What we have here is a substance that can't exist on Earth, so there's no name for it in our language. The last speaker was quite correct; sometimes you have to think of it as a nonwetting liquid, rather like mercury, but much lighter. At other times, it's a flowing solid, like pitch—except that it moves much more rapidly, of course.”

“Any way it can be stabilized?” someone asked.