An object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

The pretending was the worst part of it.

He's almost positive Beth feels the same way. He can see the way her pupils dilate and cheeks flush when they talk or her hand grazes his by accident; the way her body language reflects his and their bodies gravitate closer during conversations before one or both of them hastily move back and look away.

As a biologist, he understands the processes involved: the hormones pumping into their systems trying to push along sexual reproduction. Humans are wired to crave the endorphins released by sex, the oxytoxin that creates emotional bonds. Physical attraction combined with established compatibility- he can identify the brain structures involved; quote whole sections of papers on exactly why he feels the way he does.

And yet they can't bring this feedback loop to a close because they're off-limits to each other until the mission is over.

It's been gnawing at him but he's disciplined and dedicated: he won't jeopardize their mission or working relationship by jumping over the clearly defined lines, especially without an explicit indication of interest from her. And while his feelings are hardly healthy, given the close quarters and dangers of an extended space journey, they're not entirely unexpected- she's beautiful, smart, and unattached. Still, higher reasoning overrules baser instincts in almost all instances, and all those in the Space program strive to hold themselves to an impeccable code of conduct.

Not that it's foolproof. But it's not just infamous examples of NASA astronauts going AWOL after failed romances putting him off: a physician doesn't sleep with his patients. Period.

On top of that, Commander Lewis had pulled the men aside and explicitly banned hitting on Johanssen. Orders on top of rules on top of common sense.

The only thing to do is to keep a firm lid on his feelings and keep their interactions friendly and professional until the mission is over.

Still, it's impossible not to notice how much they like each other. They're all close now that they've been working and living together for so long, but it pleases him immensely that she prefers his company even over Watney's. Beth's the most introverted of all of them and usually holes up in her quarters to unwind, but she also likes to plop down next to him with her laptop or a digital novel when he reads in the rec room. It's worth the ache in his gut, sitting at a careful distance and yet close enough to smell her shampoo, chatting intermittently and reading or coding in companionable silence.

Catching the way she looks at him before she blushes and looks deliberately back to her screen also strengthens his resolve.

The message seems clear: I want you, too. But not yet.

He respects that. And if he only gets through a couple papers on nights when she's more talkative, he's happier for it anyway. He loves how her eyes light up when she describes the programs she's creating, an artist in a mathematical medium that he can only marvel at. And he adores the telltale flush in her cheeks when she asks about more personal matters or opens up about herself; the way her nose crinkles when she's sharing some story she finds embarrassing. Even thinking about her raises his pulse and has him calculating the time until he'll have the chance to see her again.

Christopher Beck, you've got it bad, he mentally chastises himself.

But he can't help but compare it to courtships from the past couple centuries that were limited to polite conversation and lingering looks, complete with crewmate chaperones to quash any inappropriate behavior. Martinez and Watney especially delight in crashing "the nerd corner", and it's impossible to be annoyed with their hilarious pestering, even if he suspects they're both getting a kick out of cockblocking (or more accurately talkblocking) them.

The journey home wasn't the same.

They all struggled to process Watney's loss. Beth curled into herself even harder; Martinez stopped cracking jokes and spent a lot of time in prayer, the scientists buried themselves in their work. They morosely divvied up the tasks that should have been his and dragged themselves through the motions a day at a time. When the sharpest edge of grief had eased, they slowly started to slip back towards their old patterns, although the lingering feeling of wrongness remained. Part of their team had been ripped out, and it was impossible to move on in the same ship he had occupied with them.

And then everything changed again. Watney was alive.

And then suddenly they were the best bet not just to keep him alive, but to bring him home.

It's not even a question really, even if it adds more than a year and a half to the mission. And it's great to feel energized and hopeful again, driven and focused instead of merely waiting out the remainder of the journey. Beth's been tackling her new assignment with an unholy relish: hacking the Hermessystems isn't a chore –it's fun.And the scientists can continue working on their experiments, getting long-term data instead of extrapolating from a few months of results, and they've also made requests for materials to conduct new experiments with the bonus space time. The positives absolutely outweigh the drawbacks, even if they're likely never going to be allowed back in space again.

But it was only supposed to be another month or so of this agonizing arms-length holding pattern before he was finally free to find out one way or another if Beth was interested in a relationship. Watney's letter only confirms that he's been as obvious as the moon. At least his professional conduct hasn't suffered enough for Lewis to call him out, but he still feels exposed and more than a little ridiculous. Yet another lovesick astronaut, just when public interest in the Ares mission is at an all time high.

So Chris decides to search for a way to reduce the intensity of his feelings, although most of the advice he digs up online is not exactly applicable. The recommendations to refer her to another physician make him sigh heavily and keep scrolling. Neither can he change jobs, date more appropriate women (hah!) or develop a new social group over the additional nineteen months they'll be on the Hermes.

But he can try to create some emotional distance by socializing more with Martinez, Lewis and Vogel. And he can email back women, the way prisoners find romance and connection while locked up. (As analogies go, their situations are sadly not that unsimilar, and unlike Beth, some have made their interest in him abundantly clear.) The thought of purposely spending less time with Beth hurts, but with so much additional mission time, it might be for the best.

Maybe this is the push he needs to accept that it's just not meant to be.

First things first though: the Taiyeng Shen still has to make it to them. Or they're all doomed.

Well, most of them.

And that, it turns out, is what changes everything.

There's a tentative knock on Chris' door, and he looks up, surprised. The only time any of the crew would disturb him in his quarters is if they're ill and in need of treatment.

"Come in!" he calls immediately.

Johanssen steps in and shuts the door behind her, and he takes one look at the way she's hugging herself miserably and knows.

"…Lewis just told you."

She gives him a stricken look. "You knew?!"

It's hard to look her in the eye when she looks like he's just betrayed her. "We all agreed. It was the only logical choice to make."

She opens her mouth to snap at him but nothing comes out, and she tries to speak for another moment before she ends up pressing her hand to her mouth in anguish.

He crosses the room in a couple steps, pulling her close, and she buries her face in his chest and clings.

"Beth- it's only the absolute worst-case contingency plan," he says, holding her tight. "You know Martinez is an amazing pilot. And if it still can't dock, Vogel and I have it covered- we will unpack that thing by hand if necessary. Lots of smart and talented people on the job. Including you on Sys-Op."

She keeps her face hidden in his chest for a minute, forcing herself to control her breathing until she can finally speak, but the words still come out gravelly and bitter.

"…And if… it explodes like the Iris?"

"Come on. You really think NASA will skip any inspections this time?" he asks lightly.

She shakes her head, but they both know that so many things could go wrong. So many things have already gone wrong.

She swallows thickly. "Lewis said if it was life critical we'd all die, not- "

She chokes on the words and shakes her head.

"Well... I guess after that she figured out a way where we don't all have to. One survivor is better than none, right?" he says coaxingly.

"How can you say that?" she demands, horrified.

He exhales and gives her a hapless look. "Come on, Beth. You have to know how I feel about you. Even if it wasn't the only decent choice—which it is- I'd… still choose you over me any day."

Which is apparently a totally unacceptable reason, because she pushes him away.

"No, damn it! No!" She holds up her hand and shakes her head, refusing to be comforted until she's explained why she's acting so unreasonable.

"It's not supposed to happen this way!" she protests. "We've done everything right and been so stupidly patient, just waiting and waiting… And if you have to die in two days, I can't… I just… I need you to know-" Her breath hitches as she grips his sweatshirt in her hands, looking pleading into his eyes.

But he's already choking up with stunned elation, and he smiles ruefully.

"… that I'm crazy about you," he finishes for her, stroking her hair back. She nods and laughs with relief before she stretches up on her toes and kisses him.

His brain stops working for probably a solid minute. It's as if every nerve in his body is firing with joyous triumph, hyperattuned to the taste and feel of her mouth against his and smell of her skin, how she's arching her back under his hands and pressing herself against him, trying to get closer.

But when she breaks off the kiss and pulls off her hoodie, she bowls him over all over again.

"God, Beth," he groans, staring. "We shouldn't-"

She lifts her chin and inhales, firmly back in control of her emotions. "Fine. Tell me you don't want me and I'll go."

He gives a humorless laugh. "I- Can't. Do. That," he admits. He arches a brow in frank admiration before he grins teasingly. "One little mutiny and now you can't stop breaking rules?"

That gets a sly smile out of her, and she twitches her shoulders lightly. "Watney said Lewis ordered you all not to hit on me. But she never gave me any such orders."

She smiles, pleased with herself, before she kisses him again and slides her hands up his back, rubbing herself against the now-obvious bulge in his groin.

"Help me stop thinking about it?" she whispers between kisses, and he searches the liquid yearning in her eyes before he caves with a groan, picking her up and carrying her to his bunk.

"Don't have condoms, but you have an IUD and we don't have any STDs," he gasps between kisses, striving to ignore the nagging guilt over knowing her medical history.

"Perfect," she pants, pulling his sweatshirt off impatiently. Any lingering reluctance shatters when she moans as their bodies first come into contact.

Consequences be damned. They're both consenting adults and he doesn't want to think anymore either.

Chris stirs reluctantly when his alarm goes off. It's unusual- his body clock usually has him awake before it even goes off.

But he's immediately aware of the warm body he's spooned around, and he nuzzles Beth's neck and inhales her scent greedily as he cuddles close, recalling exactly why he feels so damn good even if he's also tired as hell. Last night had been incredible- kissing and talking and laughing and making love over and over until they'd both been too worn out to resist sleep any longer. It's hard not to be humbled by the memory of how passionately they'd responded to each other, how intensely they'd connected after so many months of resisting. He wishes she hadn't been so motivated by desperation, but he's too grateful that she's finally here in his arms to regret it.

He just hopes that she feels the same way. There's no going back now.

But it's not like they can spend the morning in bed together discussing how they're going to approach this. There are a thousand things that need to be done while transmission range is negligible and re-supply imminent: no days off for any of the crew until they're well back en route to Mars. Which means he needs to haul his exhausted ass out of bed, wipe the jubilant smile off his face, and get down to the gym as if it were just a normal morning.

He shifts and sits up to turn off the alarm and Beth inhales and stiffens, disoriented for a moment before she turns.

"Hey you," she mumbles, reaching for him with a sleepy smile, and his heart flips as he pulls her close. He'd been a little worried he would see regret in her eyes, but all he sees is trust and remembered pleasure.

"Morning," he replies, and he relishes that first lazy kiss, wishing reality wasn't such a pressing concern. "Sorry to wake you. I know how you like to sleep, and neither of us got much last night. I can reset the alarm for you, if you want."

She frowns and her eyes crack open again. "You woke me up at-" she peers at the clock, "0600 but not for morning sex?"

He groans and kisses her again, striving to ignore his body's automatic response and how badly he wants to go along with it. "Ugh, I wish. But I usually head to the gym first thing. Lewis is going to notice if I don't show, since she's also usually in there early."

She pouts a little, wrapping her leg around him possessively.

"Can't you give yourself a doctor's note?"

He chuckles. "Because that would be totally ethical."

"Shut up, Beck," she grumbles, shoving him half-heartedly. She sighs and tucks her head against his shoulder, worry shadowing her eyes again as she traces the lines of the muscles on his chest.

"Hey." He strokes her hair back tenderly. "If everything goes according to plan, we're all going to get a couple days off in a week or so. And then… I'm hoping you're still game for morning sex..."

She has to smile. "Deal."

He can tell she's still dwelling on the grisly alternative anyway. "Beth. Try not to think about it. It's not likely we'll have to go through with it- she just didn't want to spring it on you."

She sighs and closes her eyes. "I know. And I understand why Lewis wanted me to be mentally prepared. Why she asked me to re-enable Remote Override on the Hermes. It's just… we work so hard to get the odds of failure down to negligible. And yet somehow we seem to keep hitting that 0.01."

He considers. "Well, if I remember my stats, that doesn't make it any more likely that we'll hit it this time."

She sighs and grimaces. "The numbers and multiple back up plans always helped me feel in control, but... not this time."

He gives her a squeeze, grateful that he can reinforce his words with physical comfort. "Well, spectacularly shitty luck led to some pretty awful consequences for one of our closest friends. But remember what you said about Murphy's Law when we were in training?"

She smiles and tilts her head, surprised and flattered. "Anecdotal Fallacy. You remember that?"

He chuckles. "You were so pissed about the simulations- it was kind of adorable."

"They were all so statistically improbable! They were really trying to see how we would do under stress, I know, but still. Assholes."

He gives her an expectant look, raising his brows and she rolls her eyes. "Okay, okay. I know. I'm psyching myself out."

"Well, a statistical anomaly of a storm set off a chain reaction of crap for us. And it's only human to focus on the negative- loss aversion helps our species survive, most of the time. But... we're astronauts."

She quirks her lips and smiles. "And exploring space is needlessly dangerous. So we plan ahead meticulously and focus on the work… because it's also pretty fucking cool."

"Which reminds me..." He looks at the clock again reluctantly and swings his legs off the bunk. "I really gotta go, or Lewis is going to get suspicious. Don't want to find out what being keelhauled feels like."

She yawns, grabbing her hoodie and panties back off the ground as he gets dressed. "And risk damaging our perfectly good hull? The Commander's way too practical for that. She'll just shove you out the airlock."

He huffs a laugh and shakes his head, relieved that she seems to have relegated the worst-case scenario back to abstract. "Ah- true."

Before he goes, he looks back to where she's sitting on his bed, dressed only in her hoodie and hugging her knees, her eyes thoughtful and expression as calmly introspective as usual. It sucks to have to leave her, even if he knows it'll probably suck more to be officially reprimanded.

"I'm not sorry," he declares softly. Her eyes meet his before she smiles, cheeks flushing.

"Chris? Wait-"

She hops out of bed, quickly running over.

The kiss she gives him eases the ache in his gut, as does her own declaration.

"Neither am I."

A/N: Way late to the party, but I can't resist filling in gaps in the canon, and this catalyst to make them step over the lines from crewmates to lovers made the most sense to me?