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"Yeah—I know," answered Astro, "but I've still got to be able to do it. If anything happened to you two guys and I didn't know how to get you home, then what?"

Tom hesitated. Astro was right. Each member of the unit had to depend on the other in any emergency. And if one of them failed…? Tom saw why the ground manuals were so important now.

"Look," offered Tom. "Suppose we go over the whole thing again together. Maybe you're fouled up on the basic concept."

Tom grabbed a chair, hitched it close to the desk and pulled Astro down beside him. He opened the book and began studying the problem.

"Now look—you have twenty-two tons of fuel—and considering the position of your ship in space—"

As the two boys, their shoulders hunched over the table, began reviewing the table of ratios, across the quadrangle in the examination hall Roger Manning stood in a replica of a rocket ship's radar bridge and faced Captain Strong.

"Cadet Manning reporting for manual examination, sir." Roger brought up his arm in a crisp salute to Captain Strong, who returned it casually.

"Stand easy, Manning," replied Strong. "Do you recognize this room?"

"Yes, sir. It's a mock-up of a radar bridge."

"A workable mock-up, cadet!" Strong was vaguely irritated by Roger's nonchalance in accepting a situation that Tom had marveled at. "You will take your manuals here!"

"Yes, sir."

"On these tests you will be timed for both efficiency and speed and you'll use all the tables, charts and astrogation equipment that you'd find in a spaceship. Your problems are purely mathematical. There are no decisions to make. Just use your head."

Strong handed Roger several sheets of paper containing written problems. Roger shuffled them around in his fingers, giving each a quick glance.

"You may begin any time you are ready, Manning," said Strong.

"I'm ready now, sir," replied Roger calmly. He turned to the swivel chair located between the huge communications board, the adjustable chart table and the astrogation prism. Directly in front of him was the huge radar scanner, and to one side 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