The neat rows of glittering coffins stretched in all three dimensions. Each occupant lay in blissful peace, enshrined in crystal and metal.

All but one.

Darius Bosco glared with bleary eyes at the lid of his pod, fighting a nascent claustrophobia. The computer offered reassuring noises, informing him that life support would have the cabin ready for habitation within an hour.

Bosco loudly informed the computer that life support should have kicked on before the wake-up routine. His only reply was the indifferent thrum of the spaceship.

Hell, but it was tight in that little space. He began to sweat, which didn’t do wonders for the ambiance. He craned his neck and made sense of the readout to the left of his head.

Two years and four months. His next duty cycle wasn’t for another two months. Something must have gone wrong. Terribly wrong, if the ship couldn’t even perform the wake-up routine in the correct sequence.

More sweat. Disaster scenarios ran through his mind. Meltdown. Cladding breach. Irradiation of the passengers. He pounded uselessly on the lid of the coffin. The ship wasn’t patching through any messages from headquarters, which could be good news or bad, depending on the reasoning.

Minimum requirements met at last, the lid slid open with a pneumatic hiss. The frigid air over his sweaty body immediately chilled him. Wreathed in the vapor from his breath, he moved quickly to the reactor control panel and began checking alarm readouts, gauges, and settings. He worked frantically at first, then forced himself to adopt a more methodical approach.

His brow furrowed as he came to the end of his review. One of the heat exchange fins was running a little hot, but only a little. Well within spec. Why was he awake then?

#

He stared through the porthole at the more distant nuclear reactions, reduced by distance to mere twinkling lights. The med unit beeped and he tore away the needle that he had been diligently not looking at. The machine whirred and hummed as it drank in his blood and began its diagnostics panel.

He pulled the flimsy mylar blanket tighter and shivered as he waited. Running the titrations and chromatograms was the fast part. The data had to travel thirty minutes to Earth, spend God knows how long undergoing manual review and report preparation, and then another thirty minutes back.

Humans could do some things better than machines still. Medicine wasn’t one of them, but like realtors and gigolos, the medical profession still managed to find ways to ensure their steady employment in the face of indisputably superior technology.

He could hardly criticize. A nuclear reactor also ran better without human intervention, which was why spacecraft had gradually transitioned from a full-time reactor crew to a single technician who reviewed operations every six months. A single technician supported by a ground crew, of course. A crew equally confused by his early awakening.

The ship hadn’t roused him at all, it turned out. Rather, it was responding to him coming out of his chemical coma on his own. Bosco developed a pretty good idea of what had happened by the time the official report came through.

The cocktail that induced the chemical coma made use of an artificial enzyme that mediated drug transport across the blood-brain barrier. Bosco’s traitorous immune system had produced antibodies against that enzyme. Produced them in prodigious quantities. There were a couple of alternative formulations, developed due to varying responses to the sedatives and common allergen profiles, but all of them relied upon the same biologic as the key delivery mechanism.

He drifted through the rows of sleeping colonists as he considered his predicament. Aside from the aesthetically-pleasing aspect of going to sleep on Earth and waking up on Enceladus, the chemical coma permitted the ship to run without any recreational equipment, minimal sanitation facilities, and dramatically lower calorie and power requirements. There were safety margins, of course, but the hard fact of the matter was that a spacecraft couldn’t afford to plan for every contingency.

The utilitarian metal walls flowed past him until he came to a stop at Dolores’ pod. His wife’s stillness and unnatural pallor caused him to reflexively check the readout for her vitals. She was lovely there, with a few rogue strands of dark hair floating gently around her peaceful face, almost as if she were underwater. He leaned down to kiss the lid before moving on to see Monica.

At seven, she showed promise of her mother’s beauty. She had a lot of her father in her too, though. He watched her sleep for a long moment, thinking. Maybe there would be enough resources for him to make it to Enceladus, but he would be eating into the safety margins for the two people that he cared about, as well as two hundred and thirteen other people on the vessel.

After a sobering inventory of the ship’s supplies, he selected a medical kit and made his way back to the familiar space that housed the reactor controls. He stared, unseeing, at the control panel for a long time before opening the kit. The sharp odors of antiseptics and latex stunned him for a moment, so strong was the contrast with the odorless cavern of the spaceship. He carefully prepared a lethal dose of morphine.

He released it. The acceleration of the ship was too gentle to notice on that time scale. The syringe drifted in front of his face, and he watched the lights glisten off of the wet needle and the clear, deadly liquid within. The sight evoked a complex emotion, a sense of duty blended with raw terror. It was a sour, heavy feeling that swelled in his chest and sat on his guts.

After a time, he recaptured the syringe and taped it to the interior of his sleep pod. It was too early to give up.

#

Bosco tapped at the gauge, scowling, and then checked the digital readout on the SCADA terminal. The starboard fin, one of two structures responsible for radiative cooling of the reactor core, continued to run hot.

He checked the schedule. Efficient travel to Enceladus relied on a slingshot around Jupiter. As the appropriate Jovian alignment was a vicennial event, mankind took advantage by sending a fleet of colony ships each time. He hoped that there might be an active nuke tech on one of the other ships that trailed The Cold Horizon. No luck. The next shift was more than three weeks away.

He turned up the coolant pump on the under-performing radiator before pushing away from the panel to make his rounds again. His wife remained as peaceful and beautiful as ever. His heart ached to have her so near, yet so far.

The idea of waking someone else, either for help or for company, tempted him constantly. He knew that it would merely run his resources down twice as fast, and resolved to rely exclusively on the faceless ground crew.

He ran his fingers gently across the glass over Dolores’ face and then moved on to Monica. The paternal smile faded as he studied her, replaced by a puzzled frown. Instead of the familiar peaceful mask, Monica looked faintly troubled.

The chime of an incoming message pulled him away from his familial vigil. His medical team had, against the odds, conjured good news.

“Good morning, Mr. Bosco,” said the image of Dr. Maya Thompson, “we hope you’re holding up okay out there.”

“Enough small talk, let’s have the point,” Bosco growled, even though—or maybe because—she couldn’t hear him.

“After a review of your medical supplies, we think we have a solution. There is an immunosuppressant in your ship’s stocks that will dampen your overenthusiastic response to the transport glycoprotein.

“It’s high risk in that it will leave you vulnerable to any opportunistic pathogen, but it’s rendered considerably safer by virtue of being four hundred million miles from Earth. There’s some debate here over whether that’s ironic.”

She paused, perhaps to permit him a moment to laugh at her cleverness. “We will be uploading a treatment program to the medical unit shortly. You will receive twice-daily doses until your antibody levels drop, which we expect to happen in around two weeks. Following that, the immunosuppressants will be added to your sleep pod cocktail.”

Some medical details that went over Bosco’s head followed, along with a deluge of disclosure and consent forms. He stuck his thumb on the bottom of each page without reading more than the title.

Al Mikkelson, Terrafirm’s director of Enceladus operations, replaced Dr. Thompson on the screen. “You will no longer be performing your regular duties,” he said. “The medical team feels that regular interruptions and reintroductions of the coma regimen likely contributed to your body’s immune response. Technicians on the other ships will monitor your reactor remotely.”

Bosco growled at the terminal again. The only thing worse than being dependent on a bunch of quacks back on Earth was being useless where he was.

His blood ran cold when he heard a voice from behind him. There was something still worse, it turned out.

“Daddy?”

#

There was a lot of him in the little girl, after all. It seemed that the immune system was part of the deal. They ran the obligatory blood tests anyway, of course.

She looked pale and scared as she sat buckled to the med unit’s exam chair, thin legs swinging nervously below knobby knees as the needle plunged into her arm.

Bosco prepared her a hearty meal of reconstituted pablum in a mylar pouch while they waited for the Earth-side team to tell them what they already knew. She ate it quietly as Dr. Thompson again explained the cause of the problem, and the solution. More forms, this time requiring both of them to press fingers against the screen.

A separate text message came quietly to him only, confirming his fears. The ship didn’t carry enough immunosuppressant for both of them.

He forced himself to look into the deep, hazel pools of her eyes as she searched for comfort.

“What does it mean, Daddy?” she asked. “Are we going to be okay?” She wrapped her arms tightly around her shoulders, hugging herself.

Bosco threw his own arms around her and pulled her close, kissing the top of her head. He closed his eyes, savoring the warmth of her small body against his chest, and took a deep breath. She smelled like gingerbread somehow. “Everything is going to be fine, sweetie.”

It had been so long since he had held her, even if most of that time had been in cold sleep. He forced away tears and spun her towards a porthole. “C’mon. Let’s look at the stars. It’s gorgeous out here.”

They floated together in front of the thick glass, staring at the blazing lights. The stars seen from Earth were a pathetic mockery of the glory in front of them.

They talked quietly about what life on Enceladus would be like. Bosco suddenly fell silent, mid-sentence. His breath caught in his throat as he stared into the night. Something glittered in the fierce starlight, drifting lazily alongside the ship. He pressed his face to the glass, staring, and then threw himself to the bridge, not even hearing Monica’s confused queries.

He flipped the trio of switches that turned on the port-side lights and then sprinted hand-over-hand back to the viewport.

There, sparkling merrily in the naked glare of the LED floodlights, floated a string of silver droplets, pacing the ship. He fumbled for the transceiver, setting up an encrypted message to the nuke team back on Earth.

“What is it?” Monica asked, wide-eyed.

“Death,” he said flatly, not looking away from the sparkling trail.

#

He should not have said that, he told himself for the umpteenth time, as he tried to comfort the terrified girl, to downplay his original statement.

There were many ways that a coolant leak could go, and many of them did not end in death. Nonetheless, it was a terrifying thing to confront in the depths of space with a team of one.

Cursing himself, he gave Monica a mild sedative. Not great parenting by any measure, but he needed to focus on the problem that immediately threatened her life, as well as everyone else on the ship, and most likely the convoy of ships that followed. He strapped her back into the chair to receive her first dose of immunosuppressant before proceeding to the airlock.

He instructed the ship to start waking the captain as he prepped the pressure suit. Another hit to their margins, but it couldn’t be helped. Going walkabout without any local support was strictly against protocol. He sealed the inner door and pumped down the airlock as he waited.

Shortly, Captain Amelia Chen appeared outside the airlock, shivering and rubbing her eyes. “This better be good, Bosco,” she said.

“NaK leak, ma’am,” he said, “I need to assess the situation.”

She sucked air through her teeth and then nodded sharply. Good enough, evidently. “I suppose there’s a reason that you got your daughter up first, too?”

“Naturally, but that can wait. Permission to disembark?”

Shelving her curiosity, Chen stepped to the terminal beside the airlock door and reviewed the remote diagnostics for the spacesuit, confirming that it was holding pressure and that battery and gas tanks were at acceptable levels. She pulled up the EVA checklist and together they ran through radio checks, tether anchoring, and a stress test of the life support system.

Checklist complete, Chen gave her approval and Bosco instructed the door to open. After twice confirming that he really meant it, the airlock hissed open, residual gases rushing past him, trying to tug him out in the process.

Once pressures equalized, Bosco played out his line to the edge of the airlock, and then clipped into one of the anchor points on the edge of the ship before releasing the airlock tether. He described each step to Chen as he went, waiting for her acknowledgment before proceeding.

He kept his gaze on the side of the ship, trying to block out the terrifying nothingness that stretched out to infinity in every other direction. Cautiously, he touched the jets on his maneuvering unit and nudged himself towards the secondary cooling loop heat-exchange array.

The heat exchanger was a miniature labyrinth of thin Hastelloy pipes, arranged on a broad plane and surrounded by a titanium scaffold to allow its installation and service in space without having to touch the delicate tubes. A sodium-potassium eutectic ran through the tubes, pulling waste heat away from the nuclear core and disposing of it radiatively.

A dozen of these units were stacked vertically, creating a roughly cubic structure spanning five meters in each dimension.

He took pictures from various angles with a thermal camera and waited while Chen analyzed them. When she reported back that the image analysis had identified an anomaly on one of the outer assemblies, he sagged with relief. If the leak were on any of the interior panels it would be virtually impossible to service them in the field, one of the countless risks taken in pursuit of the stars.

Following her directions, he pulled himself across the titanium frame until he found it. A tiny bubble of silver metal crowned from the smooth surface of the pipe. It must be the slightest of pinholes to emerge so slowly from the pressurized pipe, most likely the result of an impact with some tiny meteoroid.

The repair was simple in principle, but devilishly challenging when performed with the clumsy gloves. He tried to wipe sweat from his brow, only to laugh and curse as his forearm clanked against his helmet. All he could do was blink rapidly, his eyes stinging as he worked.

He buffed the surface of a thin sheet of Hastelloy foil with an abrasive sponge. He did the same to the pipe surrounding the leak, and then smoothed the foil over it, relying on cold welding to make a seal. He fitted a small clamp over it for strength and torqued it down carefully, mindful not to deform the delicate tube.

Allowing himself to relax at last, he permitted himself to drift to the limit of the short anchor he had clipped to the titanium frame. He blew out a breath, briefly fogging the visor of his helmet, and realized that he was trembling.

Forcing himself to stay focused, he radioed Chen. “Captain? Repair is complete. I’m going to monitor for ten minutes and then head back in.”

No reply. Instinctively, inanely, he looked around, as if he might see what was going on that was taking Chen’s attention. The cold fury of the brilliant starlight, and the depth of the nothing around it seized him as his focus moved away from the ship. A wave of vertigo passed over him, and everything seemed to spin for a minute. He clutched for the scaffold, feeling both relief and embarrassment when he felt the solid metal.

The radio came to life then, providing a further relief from the crushing loneliness. A relief that vanished seconds later, replaced by a frantic panic.

“Sorry, Bosco. I’m here. Your daughter is vomiting, I had to make sure that she wasn’t going to choke; she seems out of it. It’s a real mess down here, I need you to head back in. We’ll have to do a separate EVA to check on the repair later.”

Bosco’s heart thundered in his ears. If Monica couldn’t tolerate the medication, then . . . He couldn’t allow himself to finish the thought. A vision filled his mind, unbidden, of a syringe drifting weightlessly, its deadly payload glittering as it spun and caught the light.

#

The mess was largely resolved by the time that Bosco managed to navigate the airlock and doff his suit. He used that time to force calm onto himself, to wear a relaxed smile for his little girl.

The ship, at least the part of it accessible to a person, was primarily one big room, full of hibernating colonists. It had a cockpit for flight controls, and a small control room for Bosco’s affairs. The medical unit occupied one corner of the main room, next to the lavatory, and Bosco could see his daughter as soon as he exited the airlock.

To his relief, she waved at him. Uncertain though the future was, at that moment she seemed alert and hale. The nausea had passed, but she seemed abashed about it.

“Hi, Dad,” she said. She took a sip from a bulb of water. “Sorry I threw up.”

He hugged her. “Nothing for you to be sorry about, sweetie,” he said. He swallowed his own guilt—surely it was the result of something that he had injected into her body.

Still holding Monica, he turned to Chen. “The repair looked good, but it was such a slow leak that it’s hard to know for sure.” He scowled. “Really slow. We probably would have made it to Enceladus before we ran into trouble. Hard to explain why the fin was running hot. Maybe just a faulty gauge and a coincidence.”

“I don’t believe in coincidences,” Chen said. “I’m going to look at the thermograms again.”

“Sorry, Monica, but I think we need to pull some more blood for the doctors,” Bosco said.

She made a face, but complied. Bosco dug up some freeze-dried ice cream as compensation, and they nestled together on the thin chair and made the medical terminal show them Bugs Bunny while they waited for the blood panel to run.

Chen returned, looking grave, just as Elmer Fudd seemed on the verge of certain victory. Bosco disentangled himself from his daughter and left her to the cartoons while he and the captain moved away to converse privately.

“I should have caught it earlier,” Chen said, “but it looks like there’s a bigger leak on panel three. It’s obscured in most of the images, but I found one decent shot of it.”

She thrust out a tablet, and Bosco squinted at the bright smears of the infrared image. Sure enough, there was a small but brilliant plume of orange issuing from an elbow, deep in the pipework.

“Hell,” he said, “looks like it might be a weld. No way I can patch that, even if I could get to it.”

Chen nodded grimly. “Let’s not jump to any conclusions until we’ve put eyes on it, but I agree that it doesn’t look good.”

Back to the airlock, then. Donning the suit went faster this time, with another pair of hands. The jumpsuit was still slimy with the now-cold sweat of his last excursion, and the clammy feel of it on his arms made him shudder.

He struggled to keep his mind on the task, at first. His mind kept drifting instead back to his daughter, worrying over whether he could protect her even if he could repair the ship. Upon reaching the heat exchanger and getting a good look at it, however, his attention crystallized onto the problem at hand.

A flexible camera snaked into the array to give him a good look at the leak. A hairline crack had formed adjacent to a weld, inside a tight bend in the tubing. NaK seeped through the crack like silvery blood from a wound. Bosco issued a blistering curse upon the quality control team back on Earth.

Accessing the inner panel was nigh impossible, and the geometry of the leak site was equally problematic. He tried anyway, of course, using a probe to ineffectually poke pieces of foil at it until it devolved into an irredeemable mess.

“It’s no good, Captain,” he reported. “I’m going to have to seal off the whole thing.”

He sighed as he pulled himself to hull of the ship, where a manifold permitted him to isolate each section of the heat exchanger. He slowly opened the bypass valves and then closed the isolation valves. He took another set of infrared images and toured the other heat exchanger before returning to the airlock.

Fear for Monica hit him afresh as he finished his task and his brain regained bandwidth for other things. It was all he could do to force himself to properly connect his suit for recharging power and gases before rushing to the privacy of the reactor control room to view the message from the medical team that waited there. He needed to watch it alone first. If it were bad news, he would have to work out how to explain things to Monica.

The doctor looked tired, a condition that Bosco suddenly realized that he shared. Two grueling EVAs and the constant existential stress were taking their toll. Still, there was no chance that he could rest before knowing what Dr. Thompson had to say.

“We don’t have enough information to reach a definite conclusion,” she said. “But I’m cautiously optimistic.”

Endorphins flooded Bosco’s brain at that phrase, and he found that he couldn’t sit quietly. The zero-gravity environment wasn’t conducive to frenetic pacing, so he had to settle for tugging at his hair and breathing rapidly.

“Based on the event chronology provided and the blood work, I believe that the patient’s negative reaction was to the diazepam rather than to the immunosuppressant formulation, or possibly due to an interaction between the two medications.

“At the time of the blood draw, the diazepam had largely cleared the patient’s system while the immunosuppressant blood serum concentration was still over fifty micrograms per milliliter, which is significant. Given that symptoms had resolved by the time of the blood draw, we conclude that the patient reasonably tolerated the immunosuppressant.

“We recommend that you proceed with the next scheduled dosage and observe for other adverse effects. We also recommend against further elective sedation. Please keep us apprised.”

The screen went dark and Bosco let out an involuntary whoop of relief and joy. Things seemed to be turning around at last. He pushed away from the terminal to tell Monica the good news when the reactor control system chimed gently, several times in quick succession. Seconds later, the pager on his hip started to vibrate.

The musical note indicated a non-critical warning that nonetheless required review. Scowling, Bosco pulled up the event log and the trend screens and puzzled through the sequence of events.

Everything was straightforward and predictable for someone with their head in the game. He cursed himself for being surprised. The reactor had been operating at its design capacity, running its normal functions plus life support, heating, and lighting for the whole ship. The heavy use of the medical unit, deep space comms transmitter, and the airlock on top of that didn’t help.

Isolating a section of the heat exchanger had pushed the core temperature above permissible levels, and the system had automatically dropped control rods to compensate, slightly slowing the nuclear chain reaction in the core. Temperatures and pressures had dropped back into spec and everything was running smoothly again.

Everything was fine. Everything, except that power production had dropped by eight percent. Power production and power consumption were now almost at parity. He looked at the battery logs and any residual triumph from Dr. Thompson’s message turned to ash in his throat.

Once a steady line at one hundred percent, the battery level was now fluctuating, falling during periods of high demand and then recovering. Almost recovering. They were, very slowly, losing ground.

They had run out of margin.

#

He talked to Monica first. She didn’t really understand, but she trusted him and seemed happy that he was happy. Her faith in him warmed his heart, while simultaneously terrifying him that it might be misplaced. They shared a hug and then he had to go brief the captain.

She was on the bridge, reviewing the infrared scans. She didn’t look away when he entered.

“Bad news, ma’am.”

“Do you ever have anything else?”

When he didn’t reply, she sighed and turned to look him in the eyes.

“Sorry, Bosco. All right, let’s have it.”

“We’re running at lower power now, and we’re starting to dip into battery reserves. The ship can handle losing a heat exchange module or two, or providing extended life-support for the main cabin, but not both at once.”

Chen’s eyes widened. “How long do we have?”

“There’s not much data yet, and we haven’t done any work towards triaging our power consumption. A rough extrapolation gives us at least nine days before getting to dangerous levels.”

She shook her head. “We need to get everybody back into cold sleep as soon as possible. How long until we can do that?”

“The doctors estimated two weeks.”

She blew out a noisy breath, refusing to meet his gaze now. “Have they worked out how to stretch the drug supply to cover both of you?”

“N . . . Ah, yes ma’am. I have a solution for that.” A glittering syringe danced behind his eyes for a moment, and he felt the corners of his lips involuntarily twitch downwards.

She looked at him again. “You need to rest before anything else. Take four hours of sleep, and then we’ll reconvene to brainstorm solutions.

“I’ll finish going through your images. I haven’t seen any evidence of more leaks. See? At least one of us can generate good news.”

Bosco nodded dumbly. Now that the prospect of sleep was upon him, his brain seemed to have already started the shutdown sequence. He fumbled his way to the lavatory and then collected Monica. Together they huddled in the reactor control room, which was marginally warmer than the main body of the ship, and wrapped themselves in mylar blankets.

Despite the intensity of the situation, sleep came quickly and mercifully. A gentle smile creased his face as he held his daughter and closed his eyes. Surely things could only improve.

#

“Bosco?”

Someone was shaking him. He scowled, scrunching his eyes tighter and muttering something rude and unintelligible in equal measure.

“Sorry, but it’s time to wake up.”

He told the voice what it could go do to itself. Then he cracked an eye, saw that it was the captain, and told her again.

Chen made a distinctly unamused noise in the back of her throat. He slowly disentangled himself from the blankets and the still-sleeping Monica and rubbed his eyes until memory returned. He wished that it hadn’t.

“Ah, hell,” he said at last.

Accepting this as an apology, Chen nodded curtly and got to business. They moved out of the reactor control room so as to not disturb Monica.

Chen shoved a ration pouch at him. “Let’s review our options,” she said. “Assume that we need to sustain main-cabin life support for longer than the battery will hold out.

“Ground control wants to know if we can reactivate the isolated section of heat exchanger. Odds are that it’s been leaking for a long time, what’s a few more days?”

“Yes, probably. Let me check how much reserve coolant we have.” Bosco started hunting through trend screens as they spoke.

“How much energy can we conserve?”

“Not a lot, I’m afraid. It’s not my area of expertise, but I don’t see many options. The ship is designed to run as efficiently as possible to start with. We can turn off some lights, but the power draw there is negligible. Life support systems are critical, naturally. We can turn down the cabin temperature a bit, but it’s already near the limit of what we can handle. In a pinch we could turn off the oxygen and water recyclers and run down the reserves, but that’s a short-term solution.”

“Let’s start with the lights and the cabin temperature then. Anything we can do to increase the reactor output?”

Bosco scowled at that. “Well, we could push the safety margins a bit.”

There was that word again, margins. It seemed like every time he turned around, they were nibbling away at their margins. First their energy margins, and now their safety margins. It would end poorly. And yet, they were headed for a bad end at the moment anyway.

“I could pull the control rods back a bit, run at a slightly higher temperature. We could run the coolant pumps harder and pray that they don’t give out. The automated system is more conservative than the rules permit, and the rules are more restrictive than what we think physics will permit.”

Chen chewed her lip, staring at the floor. “Let’s proceed with the energy conservation plan. Did you determine how much spare coolant we have?”

“Enough to recharge about one-and-a-half heat exchange units. Looking at the level trend, we’ve been losing coolant for two months and could likely handle the same loss rate for another six weeks.”

She nodded. “Easy enough then. Let’s reopen our problem child. We’ll close it again right before we all go back to bed. I’ll get clearance from Earth, plan on suiting up in ninety minutes.”

That was a dismissal. Bosco finished eating and then roused Monica. A dormant anxiety reared its head and gnawed at his gut—it was time for her second dose of medication.

“Do I have to?” Still half-asleep and scared, she turned whiny and shied away from the needle.

He found the cartoons again. What was a few more watts of power draw, after all? This placated her to the point that he was able to administer the shot, and then they sat for a while, watching the show together. She giggled unrestrainedly each time the mouse inflicted some improbable harm on the cat.

Bosco alternated between waiting for any sign of an intolerance to the medication and an unsettling realization that he mostly identified with the cat.

After a few minutes, a crease appeared in the girl’s smooth brow and she turned away from the cartoons to fully regard her father.

“Don’t you need your shot too, Daddy?” she asked.

Bosco grunted. Damn kid was too smart. “Uh, yeah. Thanks for reminding me.”

Sourcing a fresh syringe, he turned his back to her as he filled it with saline. His heart skipped a beat as he looked at it, filled with a clear liquid so similar to the unassuming cylinder still taped to his pod. He turned and forced himself to smile at Monica as he pushed the needle into his shoulder and depressed the plunger.

“There.”

Satisfied, she returned her attention to the show. He watched her for an hour. She grew subdued and lethargic but exhibited no violent reaction to the dose. The fear that gripped like a clenched fist around his heart slowly eased. His eyes slipped towards closed again, but Chen returned before he could fall properly asleep.

“Ground has approved our plan,” she said, “with one small change. You’re to stay here and monitor the reactor, it’s my turn to spacewalk.”

Bosco put up an obligatory protest on the way to the airlock, but his heart wasn’t in it. He would be perfectly happy if he never again had to face that vast emptiness. He helped her into the EVA suit and together they walked through the checklist.

It felt like an interminable wait while Chen navigated to the valve manifold. Bosco didn’t care for the idle time. It allowed him to think about syringes and endings. For the first time, he articulated thoughts about how the number of times he got to hug his daughter might be approaching the single digits. He would never get to speak to his wife again. The sterile, cramped confines of the ship would be the last thing he would ever see.

“Bosco, you there?”

Chen sounded annoyed, and Bosco realized that it was at least the second transmission. He shook himself. “Yes ma’am. Proceed when ready. Open the isolation valves first, and then close the bypass valves. As slowly as you can.”

“Roger. Opening isolation valves.”

Nothing blipped on the SCADA terminal, but then, he didn’t expect it to. The bypass was still the path of least resistance. No need to stress until she closed the second set of valves. It was hard to say how smoothly the delicate system would respond to the slug of colder, denser NaK being mixed into the system on top of the pressure instability reintroduced by the leaky section of pipe.

“Closing bypass valves.”

“Ten-four.”

Bosco held his breath and counted to thirty. His trends held steady.

“Valves closed.”

He watched the screen. Everything looked good for one hopeful minute, until the coolant pressure began to drop. It fell slowly at first. He held his breath as he watched, praying that it would stabilize, maybe even recover. As if in spiteful answer, the loss rate began to accelerate.

“Something’s wrong,” he said, “we’re losing pressure.”

“Give me a second.”

He gave her two tense minutes, gnawing on his knuckles and watching the pressure steadily tick down.

They were already down over a hundred psi, which was well over five percent of the initial pressure. Warnings were beginning to flash yellow on the event log when Chen’s panicked voice came piping through the radio.

“It didn’t like all this pressure cycling. The cracked pipe is spraying coolant everywhere now. I have to isolate it again.”

“Roger.” He said a few other words, but he didn’t broadcast them.

He watched the trend screen anxiously as the pressure loss blew past two hundred psi, then three.

The pressure suddenly stopped dropping and rose precipitously.

“Yes!” Bosco shouted, throwing his fists into the air in triumph. Then he saw that flow had dropped to zero.

Red splashed across the status screen. The pager at his hip began vibrating insistently and the spacecraft shuddered. An alarm began to wail.

“Ah, hell,” he said, arms still in the air.

#

“What’s happening?” Chen asked, her composure slipping. “The whole ship jerked!”

“You closed the isolation valves before opening the bypass. The pump deadheaded and then shut down.”

Somewhere, a little girl was shrieking in heart-rending terror, but Bosco had to tune it out.

He scanned the trend log. Everything had happened so fast. “Core temperature spiked. The system compensated by venting the pressure relief valves on the primary coolant loop and initiating an emergency shutdown.

“The relief valves probably pushed the ship around a bit, but nothing serious. I’ll restart the core after you open the bypass valves and we should be in business. You can head back in to work on plan B.”

He prepared to withdraw control rods, but he couldn’t bear the crying anymore. They could run on battery power for a few more minutes. What was one more hit to their margins?

He comforted Monica, savoring the minutes that he got to spend with her even as he stressed over his absence from the control panel. After calming down, she joined him in the control room. Shortly, she forgot the terror and began enjoying the null-gravity environment, swooping about the room in a way that quickly annoyed her father.

For his part, Bosco worked with an ever-deepening scowl. He pushed away from the terminal and met Chen at the airlock just as she exited it. She could tell from his face that their troubles were an ongoing matter.

“The core pressure keeps dropping. I need to go put eyes on it.”

“What are you thinking?”

“The only thing that makes sense is that one of the PRVs is stuck open, continuously venting. The control system instructed them all to close, but this wouldn’t be the first time that one of these has failed.”

“That sounds bad.”

“It is. The core can’t operate that way, which means we have no power, while the discharge is simultaneously costing us irreplaceable water and pushing us off-course. We need to move fast.” He donned his suit and vented the airlock to space rather than pumping it down first.

The core lurked on what Bosco considered the underside of the ship. Having a nuclear reaction so close to the colonists might terrify some, but spaceflight was never for the faint of heart.

From the exterior of the craft, the only visible part of the reactor was the bioshield, a great expanse of metal with a smattering of pipework and conduit playing over it.

He expected to see a jet of white steam when he rounded the curve of the ship. When he saw nothing, he was at first relieved, but then concerned that the cause of the problem remained unsolved.

He stood there, puzzling for a moment. After a time, it occurred to him that a spout of superheated steam might look different in space, with the vacuum immediately boiling it away to an invisible vapor. He took an infrared picture and squinted at the tiny screen.

The plume that he expected shone vividly there, in shades of brilliant orange rather than white.

He activated his radio. “Found it.”

A single failed pressure relief valve spelled doom for the entire colony ship. Although planning for every contingency was not in a spaceship designer’s charter, they did make an allowance in this case. The vent valves consisted of a length of pipe that thrust out of the surface of the reactor like a stalagmite. An automated solenoid-actuated valve, buried in the shielding, governed each pipe. The saving grace was an accompanying quarter-turn manual valve at the tip of each jutting length of pipe, permitting a technician to isolate it.

Bosco moved slowly towards the offending valve. Being so close to the reactor always unnerved him. He was about fifteen feet away when the quiet clicking finally penetrated his consciousness. His radiation monitor, the one inside his shielded spacesuit, was registering a significant gamma field.

He retreated and reported back to Chen, trying to keep his voice from shaking.

They shared a moment of silence. They both knew what needed to happen, but Chen didn’t want to order it. Bosco took a deep breath.

“All right, I’m going in. It should only take a few minutes.”

The hard part was making a rapid approach without overshooting. It took more than a few minutes to get in range, and a few minutes more to fail to turn the valve. The terrifying staccato of the dosimeter kept him company the whole time.

“It’s galled!” He tried to get a wrench on it, but only succeeded in snapping off the handle of the valve. He retreated, cursing. “I’m coming back to the airlock. I need some more gear.”

Thirty minutes later, he returned to the crest of the spacecraft and stared at the deadly valve, which looked as innocent as ever. He felt faintly queasy, whether from the radiation or the task ahead, he couldn’t tell.

The offending stub of pipe looked innocent there in the poor light. He stared at it for a long time from a safe distance. He had fancied that he had made peace with death—it was only a few short days away in the best case, housed in a tiny cylinder of clear glass.

The immediacy of it overwhelmed him all the same.

“I can’t do it,” he said, but he didn’t activate the radio. Closing his eyes, he played back his last day on Earth, when he and Dolores had taken Monica to the park to enjoy the sun and birdsong one last time. The laughing face of his little girl filled his mind and his heart.

Clenching his teeth, he moved forward. They had one chance left. He was a dead man either way, the least he could do was give everyone else a shot.

He imagined that he could hear the escaping steam as he approached the pipe. The chattering of the dosimeter at his hip would have drowned it out even had there been an atmosphere to transmit the sound.

The taste of metal filled his mouth as he set about arranging his gear and anchors. The last hope of The Cold Horizon and all its inhabitants was a length of steel pipe about three feet long and four inches in diameter, with a flange on one end and a valve on the other.

At more than twice the diameter and a foot taller, it fit neatly over the hateful vent pipe. The escaping steam continuing to shoot through the open valve at the top of the repair pipe. He attempted to report back to Chen, but the radio proved unable to handle the radiation. He was on his own.

An ion beam made three quick tack welds between the flange and the bioshield to hold it in place. It took most of an hour to drill the six holes into the hard steel, and another thirty minutes to drive the bolts home. The clumsy gloves fought him every step of the way.

He was starting the final weld around the base to guarantee a complete seal when the radiation suddenly blinded him. At first, he thought that the ship lights had gone out, perhaps due to a catastrophic power failure. When he realized that there was no starlight and no suit lights, he felt a mad terror like nothing he had ever known. His whole universe was utter darkness, his only company the continuous, indifferent chatter of the radiation monitor.

Bosco screamed. Then he screamed again. He thrashed and kicked, felt himself spinning wildly. He pivoted on his short tether, which was clipped a few feet away, crashed into the ship, and bounced.

Already queasy from the intense radiation, his stomach threatened a total revolt. He seized on this feeling, almost grateful to have something real to focus on, and clawed his way back to sanity. He tried the radio again, failed to make contact again. And then, just as suddenly as it went, his vision returned.

Sobbing, he hauled on the tether until he returned to the surface of the ship and, hands shaking like a maraca player on a roller coaster, went back to welding. There was nothing else he could do, except pray, which he also did.

Somehow, miraculously, he finished the weld without destroying his suit with the ion beam or drowning in his own vomit. Only one thing left to do.

The big, multi-turn valve moved reluctantly, but move it did. It took so many rotations to fully close that Bosco began to fear that it was just spinning freely, not closing anything, but at long last it came to a halt and refused to move any further.

Exhausted, he unclipped his local tether and began reeling in the main line, pulling himself towards home. He left the torque wrench and the welder where they were, sitting on a mag strip on the surface of the bioshield.

He stopped once, after the clicking of the dosimeter mostly died down, to take a final infrared image of the repair. Then, as fast as he was able, he hauled himself back to the airlock.

His mind was reduced to repeating a simple mantra over and over. Get back to the airlock. Get back to the airlock. Get back to the airlock.

Airlock achieved, a new mantra took over. Must decontaminate. Must decontaminate. Must decontaminate.

He managed to close the airlock behind him before the contents of his stomach erupted violently inside his helmet. Oblivion took him.

#

Bosco gradually became aware that someone was holding his hand. Turning his throbbing head to the right, he saw his wife.

“Dolores?” He tried to sit up, but straps held him tightly to the chair. “You can't be here!” His protest, intended as a shout, came in a hoarse whisper.

“I had to see you.” She smiled at him, eyes wet and bright. “They told me how brave you were.”

“Our margins . . .”

Somehow, she laughed. “I’ve heard all about them. You’ve been out for nineteen hours, muttering that word over and over in your sleep.”

He turned his head to the left. The skin of his neck felt tight and hot, like a bad sunburn. “Where is Monica?”

Chen moved into view. “It seems that someone finally informed the medical crew about our power shortage. They’re regular miracle workers down there. She was already responding to the immunosuppressant, and they were able to temporarily boost the dosage of her coma cocktail enough to overcome any lingering resistance. She’s asleep already.”

Bosco shook his head, a lump of relief in his throat. Still, he wished he could have held her one last time. “And the reactor?”

“They roused Keith Simmons of the Dreamcatcher, the next ship behind us. He’s finalizing the startup sequence still, but I’m told that everything looks good. We have the recycling systems off to conserve power, but the reactor should be back up and all of us asleep again within the next six hours. We made it.”

This was it then. All to bed, with one last private task for Bosco when everyone else was asleep. He looked at Dolores, glad that she was there while simultaneously wishing she were still comatose. Should he say good-bye? Did she understand the situation? His eyes found Chen.

“One more miracle from Dr. Thompson,” Chen said. “You absorbed four sieverts of radiation while saving us all. An enormous dose, but very survivable with modern medicine. Your immune system is shot though. You’re not going to be producing any more antibodies until you get a bone marrow transplant.”

Bosco didn’t even realize all the little muscles in his face were tense until he felt them relax. His eyes widened in wonder and an astonished laugh erupted from his lips. He wasn’t going to die out here in the lonely emptiness. He was going to see his daughter again, hold his wife. He couldn’t process all the emotions at once.

Gently, slowly, Dolores and Chen disconnected his various IVs and unstrapped him from the chair.

“C’mon,” Dolores said, “You need some rest. I’ll see you on Enceladus.”

Copyright © 2023 by Brad Zeiger

Brad Zeiger is nominally a chemist, an engineer, or a writer, depending on which dubious source you care to reference. He lives in northwestern Oregon, squandering his time in the local semiconductor industry. He has several self-published novels on Amazon, but offers the caveat that they are substantially different in style from this work. "The Insomniac," which won the 2023 Jim Baen Memorial Short Story Award Contest, represents his first professionally-published piece of fiction, and he is deeply grateful to Baen for that opportunity.