"

"That's something we know about our opponents, anyway," said Burl, "They
must have physical limitations enough like ours to rule out places where
we couldn't move, either."

Boulton showed no further effects from his experience. In time, the
_Magellan_ drew near Jupiter. Callisto, its fifth satellite outward,
moved about the mighty planet at a distance of 1,170,000 miles. It was a
large satellite as they go, 3,220 miles in diameter, larger, in fact,
than Mercury. But, as Russ explained, it was a queer place in its own
fashion.

For despite its size, Callisto was apparently not a solid body as we
think of it. Its density totaled only a little more than that of water,
its mass half that of the Earth's Moon--a notoriously porous body.

They bore down on Callisto, matching their speed to its, and swung close
to its surface. It had almost no atmosphere, just a thin layer of the
heavier gases. It was a belted world, without clearly defined continents
or surface markings. Its equatorial zone was one vast, featureless belt
of darkish-gray. Its temperate zones were white, with patches of yellow
here and there. But its poles were gray again.

"The satellite's like a huge ball of thin mud that's never hardened,"
said Burl as they studied the strange terrain.

"The equator's the softest--it seems to be a river of muddy water,
hundreds of miles wide--only it can't be water. Probably semisolidified
gases holding dust and grains of matter in suspension," said Russ. "The
temperate zones are the same stuff, only colder, and therefore more
stable. A thin crust of frozen gases over a planet-wide ocean of
semiliquid substance."

"The Sun-tap station's on the southern pole," said Burl. "That must be
solid."

It was. The poles of Callisto were actually two continent-sized islands
of shell. Dry, mudlike stuff, hard as rock, floating on the endless seas
of the semiliquid planet.

The station, a ringed setup quite like the one on Earth, stood in the
geographic center of the south pole. The _Magellan_ hovered over it
while a landing party went down in the four-man rocket plane.

Clyde, Haines and Burl were the landing party. This time, only Burl
entered the station after a hole had been blasted in the outer shell.
While the redheaded astronomer took samples and made observations,
Haines kept watch. Nobody knew what type of defense awaited them here.

Burl found the controls easily enough. He was afraid that he might have
lost hi