und, like sheep on a hillside, but
it just doesn't work that way." Johnny peered at the screen. "Of course,
to an astronomer the Belt is just loaded ... hundreds of thousands of
chunks, all sizes from five hundred miles in diameter on down. But
actually, those chunks are all tens of thousands of miles apart, and the
Belt _looks_ just as empty as the space between Mars and Earth."

"Well, I don't see how we're ever going to find one particular rock,"
Tom said, watching the screen gloomily.

"It's not too hard. Every asteroid has its own orbit around the sun, and
everyone that's been registered as a claim has the orbit charted. The
one we want isn't where it was when your Dad's body was found ... it's
been travelling in its orbit ever since. But by figuring in the fourth
dimension, we can locate it."

Tom blinked. "Fourth dimension?"

"Time," Johnny Coombs said. "If we just used the three linear
dimensions ... length, width and depth ... we'd end up at the place where
the asteroid _was_, but that wouldn't help us much because it's been
moving in orbit ever since the Patrol Ship last pinpointed it. So we
figure in a fourth dimension ... the time that's passed since it was last
spotted ... and we can chart a collision course with it, figure out just
where _we'll_ have to be to meet it."

It was the first time that the idea of time as a "dimension" had ever
made sense to Tom. They talked some more, until Johnny started bringing
in fifth and sixth dimensions, and problems of irrational space and
hyperspace, and got even himself confused.

"Anyway," Tom said, "I'm glad we've got a computer aboard."

"And a navigator," Johnny added. "Don't sell your brother short."

"Fat chance of that. Greg would never stand for it."

Johnny frowned. "You lads don't like each other very much, do you?" he
said.

Tom was silent for a moment. Then he looked away. "We get along, I
guess."

"Maybe. But sometimes just gettin' along isn't enough. Especially when
there's trouble. Give it a thought, when you've got a minute or two...."

Later, the three of them went over the computer results together. Johnny
and Greg fed the navigation data into the ship's drive mechanism,
checking and rechecking speeds and inclination angles. Already the
Dutchman's orbital speed was matching the speed of Roger Hunter's
asteroid ... but the orbit had to be tracked so that they would arrive
at the exact point in space to make contact. Tom was assigned to t