Pushing Ice

by Alastair Reynolds

  “Sorry — just trying to make helpful suggestions.”

  “It was helpful,” Pagis said, with something like contrition, “it’s just that we’ve already gone through all the obvious stuff.”

  “Keep trying,” Bella said. “At least in half an hour we’ll know if the sensitivity is the issue. That’ll help, won’t it?”

  “I guess,” Pagis said, unenthusiastically.

  Bella let her get on with her work. The next thirty minutes oozed past, the passage of time made slower by the regular notifications that probe and ship were still in contact, but with the signal becoming slowly fainter. The falling away of the signal strength was exactly as predicted, with no loss in detection efficiency due to some antenna fault.

  Bella reminded herself that she still had a ship to run, and that Janus was still sitting there waiting to be examined. Her inbox contained a dozen messages from Nick Thale, each of which — as she skimmed them quickly — contained updating summaries of the latest remote-sensing operations. By contrast to the uplink, the equipment under Nick’s guidance was all working normally. Thale’s most recent message requested Bella’s formal permission to launch a free-flier on a pseudo-orbit that would take it around the far side of Janus, observing the as yet unseen “bow” face.

  Bella authorised it without hesitation. The technical aspects of the mission had already been covered, and the flier would not be approaching any closer to Janus than Rockhopper had already come. There would be no additional risk.

  With five minutes left until Fletterick’s free-flier had reached its terminal velocity, Bella decided she could no longer stand the wait. She called Pagis again and asked her to meet her at the puppet booth. Jens Fletterick was still in the couch, barely moving. Every now and then he whispered some arcane command to his machine. Timelag was now appreciable.

  “Here’s something odd,” said Hinks, holding a Ziploc plastic bag in which she had gathered the free-flier’s spare processor boards. “That star-tracker glitch Jens tried to pin on me?”

  Bella blinked back to an hour earlier. “Yes,” she said, with an ominous sense of premonition.

  “We’ve got a similar problem with the flier you just authorised Nick Thale to launch. I never went anywhere near the star-tracker boards on that machine.”

  “Doesn’t make any sense.”

  Hinks nodded. “Add it to the pile.”

  “Wait,” Bella said. “We have to clear this up. One star-tracker failure I can understand, but two, in completely isolated machines?”

  Hinks looked at Bella with a dawning comprehension. “You think the two things might be connected?”

  “I don’t know what I —” But Bella halted, looking at Jens Fletterick. He’d flipped up his visor.

  “The free-flier has reached terminal velocity,” Fletterick said. “All systems are functioning normally, including the uplink antenna.”

  Bella looked to Pagis for confirmation. Pagis had a stiffened flexy across her forearm, hectic with sketchy, hand-annotated diagrams in primary colours. “Still reading you,” she said. “Signal’s on the nose, too: it’s exactly where it should be. Doppler’s flattened out now that the machine isn’t accelerating.”

  “And this represents the strength of the uplink signal from Earth, if they were sending?” Bella asked.

  “Within a few per cent of the modulated average.”

  “Then our system must be good,” Bella said.

  Pagis nodded meekly. “We’ll continue to collect data as the free-flier falls away from us at terminal velocity, but I don’t think it’s going to tell us much we don’t already know.”

  “Keep listening anyway.”

  “Is it me,” Hinks said, “or is this beginning to make no sense at all?”

  “It’s not you,” Bella said.

  TEN

  Jens Fletterick’s hands moved in exaggerated arcs, like a shadow-boxer. Bella, Hinks and Pagis stared at him, mesmerised. He kept that up for another minute, his gestures gradually becoming slow and resigned, until he stopped moving completely. He lay still for another minute, breathing shallowly. At the end of that minute he flipped up the opaque mask of the immersion headset and unbuckled himself from the couch.

  “It’s gone,” he said.

  “Gone?” Bella asked.

  “The link is dead. I can’t talk to the machine any more.”

  “But you were nowhere near the limit of radio communications,” Hinks said. “Was there a falling off of signal strength?”

  “Nothing,” he said. “It just disappeared. One moment I was there, looking back at Rockhopper. I could still see Janus. And then I just wasn’t there any more.”

  “As if someone cut the puppet strings,” Bella said.

  “No,” he said, correcting her with gentle firmness. “That’s not quite how it felt. There was a moment… a transition.” For once, this usually precise man struggled for words. “It was as if the strings became stretched, pulled, until they snapped. But not cut. Not cut at all.”

  Hinks knelt down next to the couch. “This looks funny,” she said, scratching a finger across one of her flexy read-outs. “Look at the Doppler on your telemetry.”

  Fletterick removed the heavy immersion gear. Still gloved, he took the flexy from her. “It should have been flat,” he said.

  “It was, after you stopped burning fuel. That is, right until the end. Then something funny happened.”

  “Show me,” Bella said.

  The free-flier had accelerated away from Rockhopper, firing its nuclear rocket until it had run out of fuel. The radio signals sent back by the free-flier had become red-shifted as its recessional velocity increased. That was entirely expected, as was the flattening of the red-shift curve from the moment that the fuel ran out and the free-flier coasted away at constant velocity. And it should have stayed like that all the way until loss of radio contact.

  But it hadn’t.

  In the last six seconds before Fletterick lost contact with the free-flier, the Doppler curve had begun to rise again. The rise was sharp, as well, the slope of the curve steeper than it had been during the hour-long boost phase.

  With a mere six seconds of data to go on, Bella could only estimate the surge in acceleration that the free-flier had experienced, but she judged that the slope was around five times steeper — which meant that the free-flier had experienced a boost in acceleration of five gees before radio contact was interrupted.

  “That isn’t possible,” she said, shaking her head in flat denial. “There has to be a mistake, a misreading.”

  “It’s all right there,” Hinks said.

  “Then give me an explanation for it. Could there have been a re-ignition of the motor at five gees?”

  Fletterick chose to answer. “No. It was programmed to burn until the fuel was completely exhausted. And even if there’d been a small amount of fuel left in the system — which there wasn’t — there’s no reason why the motor would ramp up to five gees unless we specified that in the burn sequence. Which we didn’t.”

  “An explosion, then,” Bella said. “Something uncontained. A detonation of fuel vapour violent enough to provide some impetus to the free-flier.”

  “If there had been an explosion,” Fletterick said, “I think we’d have seen some of the telemetry channels drop out. Unless it was a very selective explosion that managed not to damage any mission-critical systems, yet still hurl the free-flier away from us at five gees along exactly the same vector it was already following.”

  Bella smiled at him. She loved sarcasm, especially from engineers.

  “Oh, wait,” Hinks said, scowling at one of the flexy readouts. “This is really odd. This really makes no sense.”

  “What now?” Bella asked.

  “You see this telemetry channel?” Hinks indicated one of the boxed graphs containing a display of some system parameter against time. “That’s data from the free-flier’s onboard accelerometer. It’s like an inertial compass. But look how it starts: f
lat at one gee for one hour. Then it does a delta function to zero gees: that’s the engine shutting down. All okay so far. Then it holds at zero for another twenty-five minutes, which is the time the free-flier spent in cruise phase.”

  “And then it climbs to five gees,” Bella said.

  “No — that’s my point. It stays flat at zero, right until the last data packet.”

  “That’s… that’s weird,” Bella said. “Let me get this straight: the Doppler telemetry says the free-flier went shooting off at five gees during the last six seconds of transmission.”

  “Correct,” Hinks said.

  “While the on-board accelerometer says nothing happened.”

  “Right.”

  “Then one or both channels must be incorrect. So maybe my putative explosion really did happen, and knocked out the accelerometer.”

  “Well, no,” Hinks said patiently, “that’s not what we’d see if that had happened. We’d get zero packets on that channel. Whereas the packets we received from the accelerometer were all well formed.”

  “According to the accelerometer,” Fletterick said, “the free-flier didn’t feel that five-gee surge at all.”

  “But it did accelerate,” Bella said.

  “According to the telemetry.”

  “So which is right?”

  “They’re both right,” Svetlana said. She had just appeared in the puppet booth. Bella had not given her permission to leave her quarters — indeed, her appearance here was a clear violation of the agreed terms of her confinement, which permitted her limited access to ShipNet if she agreed to treat her room as a locked and guarded prison cell. But at that moment Bella felt no inclination to punish her.

  “You have an explanation?” she asked.

  “I have one,” Svetlana said, “but you’re not going to like it.”

  “Just tell me what you think is going on,” Bella said.

  “I want Belinda to try something for me first. It shouldn’t take her long.”

  “I’m listening,” Pagis said.

  “Point the uplink dish back in the direction of Earth, if you haven’t already done so.”

  “It’s done,” Pagis said, shaking her head, “and there still isn’t a signal.”

  “No, but I think I know where you can find one. You need to shift the bandpass well out of the frequency range you’re searching.”

  “We’ve allowed for the Doppler effect.”

  “Just try it. Look on the low-frequency side, as if you’d underestimated the degree of red shift.”

  “I don’t see —” Pagis began.

  Svetlana cut her off impatiently. “Just do it, all right? Start at the nominal frequency and slide the bandpass into the red. Tell me when you hit a signal.”

  It took less time than Bella had expected. Pagis entered commands into her flexy, talking directly to the uplink antenna.

  Within a few minutes Bella saw her frown and open her mouth in a silent, “What?”

  “You’ve found your uplink signal, haven’t you?” Svetlana said. “Earth is still on the air. They always were: you were just looking off-frequency.”

  “This isn’t possible,” Pagis said. “I’ve had to apply half as much red shift again.”

  “That can’t be right,” Bella said, but she could tell from Svetlana’s expression — fearful and triumphant at the same time — that there had been no mistake.

  “It’s right,” she said.

  “Svieta, what’s happening?”

  Svetlana coughed and looked at everyone in the little gathering. “What’s happening is that we’re moving faster than we thought we were,” she said. “That’s why the Doppler shift was wrong. You weren’t allowing for enough motion difference between us and Earth.”

  “We know how fast we’re moving,” Bella said.

  “No, we don’t. We think we do, but that’s only because we’ve made a terrible mistake.” Svetlana paused; she had their absolute attention. “It wasn’t the free-flier that accelerated away at five gees. It was us. We’re the thing that’s accelerating.”

  “At five gees? We’re standing still, Svieta. We’re not even moving as quickly as we expected to be.”

  “No,” she said, with a resigned calm. “We’re moving much, much faster than we were.”

  “All this since Fletterick lost his signal?”

  “No. We’ve been accelerating for a lot longer than that, at least as far back as the time we first lost the Earth signal, and probably for several hours before that.”

  “How can you know this?”

  “Only thing that fits the data. You’re also having problems with star-trackers. Fine — that’s exactly what I’d expect if we’d suddenly picked up a lot of speed.”

  “Explain,” Bella said. Her mouth felt dry.

  “The trackers are set up to recognise bright stars in fixed constellations. They’re programmed to ignore stars that don’t fall at exactly the right angular separations from each other.

  The problem is that now the stars have moved relative to each other, so they can’t find the matches they expect. It’s called aberration: an apparent displacement in the positions due to speed.“

  “I don’t get it,” Hinks said. “What has speed got to do with where the stars are?”

  Bella was afraid that Svetlana was going to lash out at the robotics technician for not knowing basic astrogation theory, but instead she seemed drained of all fury.

  “It’s like this. You’re driving at night, in snow. There’s no wind, yet the snow seems to be falling horizontally, heading towards your windshield from the direction you’re driving — even though you know it’s really falling vertically. Well, the same thing’s happening with starlight, only to a much smaller degree. Trouble is, it’s still enough to throw the trackers.”

  “And the trackers don’t know that?” Hinks asked.

  “No, they do know it, and they’re programmed to correct their expected stellar positions to allow for aberration. But to do that properly they need to know how fast they’re moving.”

  “The free-fliers hitch a ride with Rockhopper,” Saul Regis said, speaking for the first time since Svetlana’s arrival. “They assume that Rockhopper knows how fast it’s moving, so they query Rockhopper to keep their kinematic parameter file up to date.”

  “In other words, they ask the ship how much of a correction to allow for, and the ship tells them,” Svetlana said. “But this time the ship got it wrong.”

  “We can check this as well,” Bella said. “It won’t be difficult. But it still doesn’t answer my basic question: what the hell is happening?”

  * * *

  Bella called Svetlana and Craig Schrope to her office. Before Schrope could lodge an objection to Svetlana being there, Bella said, “These are exceptional circumstances, which is why I’m turning a blind eye to Svetlana’s presence. She’s already solved the uplink problem, and I believe she has an explanation for the star-tracker errors as well.”

  Schrope’s pen glittered in his hand like a twirling six-shooter. “Let’s hear it.”

  “It looks as if Janus is dragging us with it,” Svetlana said. “We’re caught in some kind of slipstream.”

  Schrope pulled a face. “It’s moving through vacuum. You don’t get slipstreams in vacuum.”

  Svetlana kept her composure. “There’s a lot here we don’t understand. Adding one more thing to the list doesn’t strike me as the worst crime imaginable.”

  Schrope responded with a noncommittal shrug.

  “Explain what you think is happening,” Bella said, “then what you think we should do about it.”

  “I think we should reverse, and reverse fast. We should be doing it already, not sitting around discussing it.”

  “I still need to hear your argument,” Bella said patiently. “If I’m swayed by it, I promise I’ll act with all due swiftness.”

  Svetlana leaned forward. “I’ll tell you, but you have to act as soon as I’m done. Every second we spend —”<
br />
  “Just tell us,” Schrope said.

  “Janus never slowed its rate of acceleration. Our only point of reference was the laser we were shining on Janus, and suddenly we were closing the distance too quickly. So we throttled back our engine in response. By the time we reached the initial study position, we thought we were nearly in free fall. But we weren’t. We were still accelerating.”

  “Then why didn’t we feel it?” Schrope asked.

  “Because we’re in an accelerated reference frame that just happens to feel inertial. I have no idea what this implies. Janus must be doing something weird to space-time, and we’re caught up in that weirdness.”

  Bella fingered her shark’s-teeth necklace. “So what happened with the free-flier?”

  “My best guess is that we let it drop far enough behind us to fall out of the slipstream,” Svetlana said. “It went from being caught in this accelerating field to not accelerating at all. We read that as the free-flier suddenly accelerating for no reason — but it was us, all along.”

  “But five gees — that’s ridiculous. Janus was never accelerating that hard.”

  “Something’s changed, in that case. When Janus left Saturn it was shedding ice, just as we’d expect if it were a physical object experiencing stresses due to its own acceleration. But at some point the ice-shedding stopped: we saw that in the images. We just didn’t think about what it meant.”

  “Which was?”

  “Janus must have switched over to a different drive mechanism. Maybe it used one drive to leave the solar system, something relatively slow and primitive by Spican standards but which wouldn’t do too much harm to the neighbourhood. But now it’s a long way from the Sun. It’s engaged something altogether more powerful: something capable of accelerating an entire moon at five gees.”

  “And we’re stuck in the wake,” Bella said.

  Svetlana nodded. “It’s been at least a day. We’ll have a better idea of how fast we’re moving once we have precise numbers on the extra Doppler component. But I’ll give you a good guess: we’ve been sustaining five gees ever since we lost contact with Earth, probably longer. We were moving at three per cent of the speed of light this time yesterday. Now you’d better make that four-and-a-half per cent, maybe even five.”