Castor said, "Hold it, Hazel. Time out to scratch. What goes, Pol?"

"'Now is the time for all good men"." He nodded toward their parents.

"Right. I'll do the talking." They moved aft.

Roger Stone looked at them and frowned. "What is it, boys? We're busy."

"Yes, sir. But this seems like a salubrious time to make an announcement."

"Yes?"

"Pol and I vote to go back home.

"Huh?"

"We figure that there's no percentage in taking a chance with Buster."

Pol added, "Sure, he's a brat, but look how much you've got invested in him."

"If he died on us," Castor went on, "it would spoil all the fun."

"And even if he didn't, who wants to clean up after him for weeks on end?"

"Right," agreed Pol. "Nobody likes to play room steward to a sick groundhog."

"And if he did die, the rest of you would blame us for the rest of our lives."

"Longer than that," Pol added.

"Don't worry about that "negat" from Traffic. Hazel and I are working out a skew path that will let us miss the Queen Mary ,with minutes to spare - seconds anyhow. Course it may scare em a little."

"Quiet!" said Captain Stone. "One at a time - Castor, let me get this straight: do I understand that you and your brother are sufficiently concerned about your younger brother's welfare that you want to return to Luna in any case?"

"Yes, sir."

"Even if your mother decides that it is safe for him to continue?"

"Yes, sir. We talked it over. Even if he's looking pretty good now, he was one sick pup and anybody that sick might not make it to Mars. It's a long haul. We don't want to risk it."

Hazel had come aft and listened; now she said, "Nobility ill-becomes you, Cas. You were more convincing with the other routine."

"You butt out of this, Mother. Pol?"

"Cas told you. Shucks, we can make other trips"

Roger Stone looked at his sons. "I must say," he said slowly, "that it is surprising and gratifying to find so much family soli­darity in this aggregation of individualists. Your mother and I will remember it with pride. But I am glad to say that it is unnecessary. We will continue for Mars."

Hazel scowled at him. "Roger, did you bump your head on the take-off? This is no time to take a chance; we take the kid back to Luna. I've talked with the boys and they mean it. So do I."

Castor said, "Dad, if you're afraid of that skew orbit, I'll pilot. I know-"

"Silence!" When he got it he went on as if to himself, "It says right here in the book to give orders, not explanations, and never to let them be argued. So help me, I'm going to run a taut ship if I have to put my own mother in irons." He raised his voice. "All hands! Prepare for maneuvering. Departure for Mars, gravity-well procedure."

Edith Stone said softly to Hazel, "The baby is all right. Mother. I'm sure." Then she turned to her sons. "Castor, Pollux - come here, dears."

"But Dad said -"

"I know. Come here first." She kissed each of them and said, "Now man your stations."

Mead appeared at the hatch, towing Lowell behind her like a toy balloon. He seemed cheerful and his face was cheerfully smeared with chocolate. "What's all the racket about?" she demanded. "You not only woke us; you must be disturbing people three ships behind."

A gravity-well maneuver involves what appears to be a contradiction in the law of conservation of energy. A ship leaving the Moon or a space station for some distant planet can go faster on less fuel by dropping first toward Earth, then performing her principal acceleration while as close to Earth as possible. To be sure, a ship gains kinetic energy (speed) in fall­ing towards Earth, but one would expect that she would lose exactly the same amount of kinetic energy as she coasted away from Earth.

The trick lies in the fact that the reactive mass or 'fuel' is itself mass and as such has potential energy of position when the ship leaves the Moon. The reactive mass used in accelerating near Earth (that is to say, at the bottom of the gravity well) has lost its energy of position by falling down the gravity well. That energy has to go somewhere, and so it does - into the ship, as kinetic energy. The ship ends up going faster for the same force and duration of thrust than she possibly could by departing directly from the Moon or from a space station. The mathematics of this is somewhat baffling - but it works.

Captain Stone put both the boys in the power room for this maneuver and placed Hazel as second pilot. Castor's feelings were hurt but he did not argue, as the last discussion of ship's discipline was still echoing. The pilot has his hands full in this maneuver, leaving it up to the co-pilot to guard the auto-pilot, to be ready to fire manually if need he, and to watch for brennschluss. It is the pilot's duty to juggle his ship on her gyros and flywheel with his eyes glued to a measuring telescope, a 'coelostat', to be utterly sure to the extreme limit of the accuracy of his instruments that his ship is aimed exactly right when the jet fires.

In the passage from Earth to Mars a mistake in angle of one minute of arc, one sixtieth of a degree, will amount to - at the far end - about fifteen thousand miles. Such mistakes must be paid for in reactive mass by maneuvering to correct, or, if the mistake is large enough, it will he paid for tragically and inexor­ably with the lives of captain and crew while the ship plunges endlessly on into the empty depths of space.

Roger Stone had a high opinion of the abilities of his twins, but on this touchy occasion, he wanted the co-pilot backing him up to have the steadiness of age and experience. With Hazel riding the other. couch he could give his whole mind to his delicate task.

To establish a frame of reference against which to aim his ship he had three stars, Spica, Deneb, and Fomalhaut, lined up in his scope, their images brought together by prisms. Mars was still out of sight beyond the bulging breast of Earth, nor would it have helped to aim for Mars; the road to Mars is a long curve, not a straight line. One of the images seemed to drift a trifle away from the others; sweating, he unclutched his gyros and nudged the ship by flywheel. The errant image crept back into position. "Doppler?" he demanded.

"In the groove."

"Time?"

"About a minute. Son, keep your mind on your duck shooting and don't fret."

He wiped his hands on his shirt and did not answer. For some seconds silence obtained, then Hazel said quietly, "Unidentified radar beacon blip on the screen, sir. Robot response and a string of numbers."

"Does it concern us?"

"Closing north and starboard. Possible collision course."

Roger Stone steeled himself not to look at his own screen; a quick glance would tell him nothing that Hazel had not repor­ted. He kept his face glued to the eyeshade of the coelostat. "Evasive maneuver indicated?

"Son, you're as likely to dodge into it as duck away from it. Too late to figure a ballistic."

He forced himself to watch the star images and thought about it. Hazel was right, one did not drive a spaceship by the seat of the pants. At the high speeds and tight curves at the bottom of a gravity well, close up to a planet, an uncalculated maneuver might bring on a collision. Or it might throw them into an un­tenable orbit, one which would never allow them to reach Mars.

But what could it be? Not a spaceship, it was unmanned. Not a meteor, it carried a beacon. Not a bomb rocket, it was too high. He noted that the images were steady and stole a glance, first at his own screen, which told him nothing, and then through the starboard port.