The Rolling Stones

by Robert A. Heinlein

  “You mind yours,” Hazel answered.

  Castor gave his father a sly grin. “You picked the easy one, eh, Dad?”

  His father looked at him. “Is it too hard for you? Do you want to swap?”

  “Oh, no, sir! I can do it.”

  “Then get on with it—and bear in mind you are a crew member in space.”

  “Aye aye, sir.”

  He had in fact “picked the easy one”; the basic tack-around-Earth-for-Mars problem had been solved by the big computers of Luna Pilot Station before they blasted off. To be sure, Luna Pilot’s answer would have to be revised to fit the inevitable errors, or deviations from flight plan, that would show up when they reached perigee rounding Earth—they might be too high, too low, too fast, too slow, or headed somewhat differently from the theoretical curve which had been computed for them. In fact they could be sure to be wrong in all three factors; the tiniest of errors at blast-off had a quarter of a million miles in which to multiply.

  But nothing could be done to compute the corrections for those errors for the next fifteen or twenty hours; the deviations had to be allowed to grow before they could be measured accurately.

  But the blast back to shape an ellipsoid home to Luna was a brand-new, unpremeditated problem. Captain Stone had not refused it out of laziness; he intended to do both problems but had kept his intention to himself. In the meantime he had another worry; strung out behind him were several more ships, all headed for Mars. For the next several days there would be frequent departures from the Moon, all ships taking advantage of the one favorable period in every twenty-six months when the passage to Mars was relatively “cheap”, i.e., when the minimum-fuel ellipse tangent to both planets’ orbits would actually make rendezvous with Mars rather than arrive foolishly at some totally untenanted part of Mars’ orbit. Except for military vessels and super-expensive passenger ships, all traffic for Mars left at this one time.

  During the four-day period bracketing the ideal instant of departure, ships leaving Leyport paid a fancy premium for the privilege over and above the standard service fee. Only a large ship could afford such a fee; the saving in cost of single-H reactive mass had to be greater than the fee. The Rolling Stone had departed just before the premium charge went into effect; consequently she had trailing her like beads on a string a round dozen of ships, all headed down to Earth, to tack around her toward Mars.

  If the Rolling Stone vectored back and shaped course for Luna rather than Mars, there was a possibility of traffic trouble.

  Collisions between spaceships are almost unheard of; space is very large and ships are very tiny. But they are possible, particularly when many ships are doing much the same thing at the same time and the same region of space. Spacemen won’t forget the Rising Star and the patrol vessel Trygve Lie—four hundred and seven dead, no survivors.

  Ships for Mars would be departing Luna for the next three days and more; the Rolling Stone, in rounding Earth and heading back to Luna (toward where Luna would be on her arrival) would cut diagonally across their paths. Besides these hazards, there were Earth’s three radio satellites and her satellite space station; each ship’s flight plan, as approved by Luna Pilot Station, took into consideration these four orbits, but the possible emergency maneuver of the Rolling Stone had had no such safety check. Roger Stone mentally chewed his nails at the possibility that Traffic Control might refuse permission for the Rolling Stone to change its approved flight plan—which they would do if there was the slightest possibility of collision, sick child or no.

  And Captain Stone would ignore their refusal, risk collision and take his child home—there to lose his pilot’s license certainly and to face a stiff sentence from the Admiralty court possibly.

  Besides the space station and the radio satellites there were the robot atom-bomb peace rockets of the Patrol, circling the Earth from pole to pole, but it was most unlikely that the Rolling Stone’s path would intersect one of their orbits; they moved just outside the atmosphere, lower than a spaceship was allowed to go other than in landing, whereas in order to tack the Rolling Stone would necessarily go inside the orbits of the radio satellites and that of the space station—wait a minute! Roger Stone thought over that last idea. Would it be possible to match in with the space station instead of going back to Luna?

  If he could, he could get Lowell back to weight a couple of days sooner—in the spinning part of the space station!

  The ballistic computer was not in use; Castor and Hazel were still in the tedious process of setting up their problems. Captain Stone moved to it and started making a rough set-up directly on the computer itself, ignoring the niceties of ballistics, simply asking the machine, “Can this, or can this not, be done?”

  Half an hour later he gave up, let his shoulders sag. Oh, yes, he could match in with the space station’s orbit—but at best only at a point almost a hundred degrees away from the station. Even the most lavish expenditure of reaction mass would not permit him to reach the station itself.

  He cleared the computer almost violently. Hazel glanced toward him. “What’s eating you, son?”

  “I thought we might make port at the station. We can’t.”

  “I could have told you that.”

  He did not answer but went aft. Lowell, he found, was as sick as ever.

  Earth was shouldering into the starboard port, great and round and lovely; they were approaching her rapidly, less than ten hours from the critical point at which they must maneuver, one way or the other. Hazel’s emergency flight plan, checked and rechecked by the Captain, had been radioed to Traffic Control. They were all resigned to a return to Luna; nevertheless Pollux was, with the help of Quito Pilot, Ecuador, checking their deviations from their original flight plan and setting up the problem of preparing a final ballistic for Mars.

  Dr. Stone came into the control room, poised near the hatch, caught her husband’s eye and beckoned him to come with her. He floated after her into their stateroom. “What is it?” he asked. “Is Lowell worse?”

  “No, he’s better.”

  “Eh?”

  “Dear, I don’t think he was spacesick at all.”

  “What’s that?”

  “Oh, a little bit, perhaps. But I think his symptoms were largely allergy; I think he is sensitive to the sedative.”

  “Huh? I never heard of anyone being sensitive to that stuff before.”

  “Neither have I, but there can always be a first time. I withdrew the drug several hours ago since it did not seem to help him. His symptoms eased off gradually and his pulse is better now.”

  “Is he okay? Is it safe to go on to Mars?”

  “Too early to say. I’d like to keep him under observation another day or two.”

  “But—Edith, you know that’s impossible! I’ve got to maneuver.” He was wretched from strain and lack of sleep; neither had slept since blast-off more than twenty-four hours earlier.

  “Yes, I know. Give me thirty minutes warning before you must have an answer. I’ll decide then.”

  “Okay. I’m sorry I snapped at you.”

  “Dear Roger!”

  Before they were ready to “round the corner” on their swing past Earth the child was much better. His mother kept him under a light hypnotic for several hours; when he woke from it he demanded food. She tried letting him have a few mouthfuls of custard; he choked on the first bite but that was simply mechanical trouble with no-gravity—on the second bite he learned how to swallow and kept it down.

  He kept several more down and was still insisting that he was starved when she made him stop. Then he demanded to be untied from the couch. His mother gave in on this but sent for Meade to keep him under control and in the bunkroom. She pulled herself forward and found her husband. Hazel and Castor were at the computer; Castor was reading off to her a problem program while she punched the keys; Pollux was taking a doppler reading on Earth. Edith drew Roger Stone away from them and whispered, “Dear, I guess we can relax. He has eaten—and h
e didn’t get sick.”

  “Are you sure? I wouldn’t want to take even a slight chance.”

  She shrugged. “How can I be sure? I’m a doctor, not a fortune-teller.”

  “What’s your decision?”

  She frowned. “I would say to go on to Mars.”

  “It’s just as well.” He sighed. “Traffic turned down my alternate flight plan. I was just coming back to tell you.”

  “Then we have no choice.”

  “You know better than that. I’d rather tell it to the judge than read the burial service. But I have one more card up my sleeve.”

  She looked her query; he went on, “The War God is less than ten thousand miles behind us. If necessary, by using our mass margin, in less than a week I could match with her and you and the baby could transfer. She’s a ‘tumbling pigeon’ since they refitted her—anything from Luna-surface to a full gravity.”

  “I hadn’t thought of that. Well, I don’t think it will be necessary but it’s a comfort to know that there is help close by.” She frowned. “I would not like to leave you and the children to shift for yourselves—and besides, it’s risky to use your margin; you may need it badly when we approach Mars.”

  “Not if we handle the ship properly. Don’t you worry; Hazel and I will get it there if we have to get out and push.”

  Pollux had stopped what he was doing and had been trying to overhear his parents’ conversation. He was unsuccessful; they had had too many years’ practice in keeping the kids from hearing. But he could see their intent expressions and the occasional frowns; he signaled his twin.

  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.”

  Meade 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.”

  CHAPTER SEVEN

  IN THE GRAVITY WELL

  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 toward 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 be, 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 be 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 Fomolhaut, 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.

  “I
n 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.

  Good heavens! he could see it!

  A great gleaming star against the black of space…growing—growing!

  “Mind your scope, son,” said Hazel. “Nineteen seconds.”

  He put his eye back to the scope; the images were steady. Hazel continued, “It seems to be drawing ahead slightly.”

  He had to look. As he did so something flashed up and obscured the starboard port and at once was visible in the portside port—visible but shrinking rapidly. Stone had a momentary impression of a winged torpedo shape.

  “Whew!” Hazel sighed. “They went that-a-way, podnuh!” She added briskly, “All hands, brace for acceleration—five seconds!”