"The builders didn't expect an air attack," said Burl slowly, "because of the air disturbances. They did not know we would have a Moon base that could spot their location. Hence they figured that our civilization would remain as it was thirty years ago. We wouldn't have been able to spot the location at that time, because it required outer-space observation. It might have taken us several years of tramping around to locate it."

"And the lack of a strong permanent construction? After all, a concrete and steel-enforced embankment, which any military force on Earth could have put there, would have balked your dynamite attack," probed Haines.

"That means they didn't have the time or the means to make such a construction. They must have had a single ship with the kind of equipment that could lay out a quick base in the shortest time!" said Burl.

"Right!" snapped Haines. "The Sun-tap must have been built by a relatively small team, which probably came in a single explorer ship. The ship was equipped with automatic factory machinery that could turn out an adequate base for an uninhabited planet, an airless moon, and so on—but they didn't have the stuff for a fortified base—and they didn't have the manpower to build it."

"Another indication of that is the thirty-year delay," added Ferrati. "Obviously, they arrived in this solar system from somewhere outside it. We figure that way because otherwise they would have been prepared to do the job on all the planets in the same trip and start operations at once. They must have made some observations of this solar system from a point in space at least as far away as another star. That means not less than four and a half light-years away—Proxima Centauri being the nearest star after our Sun, and four and a half light-years from us. Their observations were imperfect. They found more planets and problems than they had supposed. So they had to make a second trip to get enough supplies to finish their Sun-tap base constructions. It took them thirty years between the first stations and the ones that completed the job.

"And that, too, suggests that only one ship was originally involved here. Of course, maybe they came back with more the second time, but it still looks as if the main force hasn't arrived. And won't, until after the Sun novas."

"Then that means," said Burl quickly, "that we are still dealing with just a small and isolated group?"

"Maybe," said Haines. "Just what constitutes a small group may be hard to say. I 

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