and overhead was a tube mounted on a
swivel joint that looked like a small telescope, but which was actually
an astrogation prism for taking sights on the celestial bodies in space.

Roger concentrated on the first problem.

" ... you are now in the northwest quadrant of Mars, chart M, area
twenty-eight. You have been notified by the control deck that it has
been necessary to jettison three quarters of your fuel supply. For the
last five hundred and seventy-nine seconds you have been blasting at
one-quarter space speed. The four main drive rockets were cut out at
thirty-second intervals. Making adjustment for degree of slip on each
successive rocket cutout, find present position by using cross-fix with
Regulus as your starboard fix, Alpha Centauri as your port fix."

Suddenly a bell began to ring in front of Roger. Without hesitation he
adjusted a dial that brought the radar scanner into focus. When the
screen remained blank, he made a second adjustment, and then a third and
fourth, until the bright white flash of a meteor was seen on the
scanner. He quickly grabbed two knobs, one in each hand, and twisted
them to move two thin, plotting lines, one horizontal and one vertical,
across the surface of the scanner. Setting the vertical line, he
fingered a tabulating machine with his right hand, as he adjusted the
second line with his left, thus cross-fixing the meteor. Then he turned
his whole attention to the tabulator, ripped off the answer with
lightning moves of his fingers and began talking rapidly into the
microphone.

"Radar bridge to control deck! Alien body bearing zero-one-five,
one-point-seven degrees over plane of the ecliptic. On intersecting
orbit. Change course two degrees, hold for fifteen seconds, then resume
original heading. Will compensate for change nearer destination!"

Roger watched the scanner a moment longer. When the rumbling blast of
the steering jets sounded in the chamber and the meteor flash shifted on
the scanner screen, he returned to the problem in his hand.

Seven minutes later he turned to Strong and handed him the answer.

"Present position by dead reckoning is northwest quadrant of Mars,
chart O, area thirty-nine, sir," he announced confidently.

[Illustration: "_I was unable to get a sight on Alpha Centauri_"]

Strong tried to mask his surprise, but a lifted eyebrow gave him away.
"And how did you arrive at this conclusion, Manning?"

"I was unable to get a sight on Alpha Cent