The Passenger Limit

Maya held the paper astronaut above the shoebox airlock and did not put it in.

Soren looked up from the oxygen tank he was making out of two bottle caps and a straw. Rain tapped the kitchen window behind him. On the table, the Mars habitat waited in pieces, silver foil walls, thread ladders, a sleeping bag cut from a sock.

"It fits," Soren said.

"It doesn't," Maya said.

"The rule says one passenger. The astronaut is one passenger. The habitat is one-person scale. I measured. Twice."

Maya turned the little astronaut sideways. Its paper helmet had a face Soren had drawn with one straight mouth and worried eyebrows.

"We forgot the rest of the crew," she said.

Across the kitchen, Soren's dad sprayed the counter with cleaner and wiped it like the counter had personally insulted him. A jar of sourdough starter sat in the sink, still foaming from its escape down the glass.

"No invisible crew in my kitchen," he said. "Invisible crew makes bread rise, then tries to take over the sink."

"That's yeast," Soren said.

"It is currently my enemy," his dad said, and went back to scrubbing.

Soren set down the bottle-cap tank. "If we add crew, we're breaking the rule. No pets. No plants. No extra organisms unless the organism has a mission function. It says that exactly."

He pushed the printed rules toward Maya. She did not read them. She was looking at the astronaut.

"Mission function," she said.

That was when Soren got the uncomfortable feeling he got right before Maya did something that sounded wrong and then became harder and harder to argue with.

He opened his notebook, not to write, but to find the folded page he had taped inside last week. It was from health class, though the teacher had rushed past it because the bell rang. At the top was a drawing of a human body filled with tiny dots.

"I copied the numbers because they seemed fake," Soren said.

Maya came around the table.

Soren read from his own careful handwriting. "Human body, about thirty trillion human cells. Human microbiome, about thirty-eight trillion microbial cells. Bacteria, mostly, plus other tiny things, on skin and in mouth and mostly in the gut."

Maya's eyes went to the paper astronaut's flat belly.

"Same size crew," she said.

"Not size. Number," Soren said. "The microbial cells are much smaller."

"Still."

"Still," Soren said.

His dad paused with the sponge in his hand. "Please do not put thirty-eight trillion anything in that shoebox."

"Too late," Maya said.

Soren turned the page over. There was the part that had bothered him most. "Their collective genome has way more genes than ours. It says about one hundred fifty times as many as the human genome."

Maya looked at him.

"That's not passengers," she said. "That's a library."

Soren had copied the sentence neatly, but he had not known where to put it in his head. The rain ticked faster on the glass.

Maya picked up one of the food packets they had made for the model, tiny rectangles labeled breakfast, lunch, dinner.

"What does the library do?" she asked.

"A lot," Soren said. He flipped through the handout. "Some of it is digestion. We can't break down lots of plant fibers by ourselves. Gut microbes ferment some of them and make short-chain fatty acids. Our cells can use those."

Maya tore the label off the tiny dinner packet.

"Then the menu is wrong," she said.

"It's pretend food."

"The design isn't pretend. Not to us."

Soren looked at the habitat. He had made everything clean and separate. Air went here. Water went here. Waste went here. Food went in, energy came out. A machine you could understand if you labeled every pipe.

But the astronaut was not a pipe machine. The astronaut was carrying a crowded dark place where food became other food because other lives knew recipes the human body did not know.

Soren took the paper astronaut from Maya. He held it carefully by the helmet.

"If the rule says extra organisms need a mission function," he said, "then they qualify."

Maya grinned. "Say it like a space agency."

Soren straightened. "Gut microbial community required for metabolic support."

"Too boring."

"Gut crew makes food finish becoming food."

"Better."

His dad lifted the jar from the sink. The sourdough starter had left sticky streaks down the sides. "For the record," he said, "this gut crew is not invited to finish becoming food on my counter."

"They're already on your counter," Maya said.

He looked at the sponge. Then at his hands. Then at the bread jar.

"I am choosing not to hear that," he said, and carried the jar away.

Maya found the spice tin Soren used for model rocks. It had poppy seeds in one section, black and blue and impossibly many. She shook a few into her palm.

"Microbes," she said.

"Not to scale," Soren said.

"Nothing about this astronaut is to scale. His head is bigger than his bed."

"True."

They slit the paper astronaut open along one side. Soren held the seam apart while Maya dropped in the seeds one pinch at a time. The astronaut's belly puffed slightly. A few seeds slid into the paper legs.

"Mostly in the gut," Soren said.

Maya pinched the legs shut. "Fine. We need a gut."

They cut a crescent from clear tape and stuck it across the middle of the astronaut like a window. Behind it, the seeds pressed against the paper.

Soren wrote a label on a scrap of card for the model wall, not because anyone had asked for labels, but because an unlabeled wonder made his head feel crowded.

One astronaut.

Approximately thirty-eight trillion microbial cells.

Genes in the microbial community, about one hundred fifty times the human set.

Metabolic jobs the astronaut cannot do alone.

He read it out loud once. The words sounded too large for the shoebox. They sounded too large for the kitchen.

Maya took the card and crossed out the first line.

"Hey," Soren said.

She rewrote it underneath.

One astronaut habitat, including the astronaut's habitat.

Soren stared at it.

Maya waited. She was good at waiting when she had already jumped.

"The person is a place," Soren said.

Maya tapped the shoebox floor. "A place going to another place."

Soren moved the sleeping bag farther from the food packets and cut a tiny storage bin from the corner of an egg carton.

"We need fiber food," he said. "For the microbes too."

"Astronaut snacks for the unseen crew."

"Not snacks. Substrate."

"Soren."

"Fine. Snacks with a job."

They made a second row of food packets from green paper. Soren labeled them fiber, then added a smaller note, not digestible by human enzymes alone. Maya made a composting tube from a pen cap, then stopped herself.

"Can we put compost microbes in?"

"Extra organisms," Soren said. "Separate mission function. Different argument. We don't have time."

Maya looked disappointed for one second, then delighted for three. "Next habitat."

The kitchen smelled like cleaner, wet cardboard, and sourdough. Soren's dad came back with the jar, now washed and calm-looking, though bubbles were already gathering again under the lid.

He leaned over the shoebox. "I thought the assignment was one passenger."

"It is," Soren said.

His dad pointed at the seed-filled astronaut. "That looks like a passenger who swallowed a galaxy."

Maya placed the label beside the airlock. "Required equipment."

Soren expected his dad to laugh. Instead, his dad looked at the sourdough jar in his own hands, at the pale mixture lifting itself with bubbles made by living cells too small to see.

"Required equipment," he said quietly.

Then he cleared his throat and pointed the sponge at them. "Do not glue living sourdough into the Mars project. Your mother will ask why the planet smells like pancakes."

"We used seeds," Soren said.

"Excellent engineering choice," his dad said, and returned to the sink.

Maya set the astronaut in the airlock. The paper figure no longer lay flat. It had weight now. Not much, but enough that it tipped forward, as if eager or badly balanced.

Soren adjusted the ladder. Maya adjusted the label. They did not add another bed. They did not add another helmet. The habitat stayed one-person small, but the walls seemed closer now, not because the model had shrunk, because the astronaut had become crowded.

Soren picked up the entry form. Passenger count, it said.

He held the pencil above the blank.

Maya said, "Don't make it tidy."

Soren wrote one, then stopped. After a moment, he added a comma and kept writing until the words had to climb up the side of the box.

Maya lifted the astronaut to the lamp, and the paper belly glowed with specks.