The Two Slow Ships

"They were slowing down," Maya said. "Both of them. That's the part nobody said last night."

Soren had the back panel off the old space heater, unscrewing it slowly so the screws went into the tin can, not the gravel. The depot smelled like rust and warm dust. Somebody's radio played two aisles over.

"The Pioneers," he said. "Ten and eleven."

"Both of them. Same amount. Sunward." She held up two fingers pointed at her own chest. "Something was tugging them back toward the Sun. Tiny. But real. For thirty years nobody could say what."

"The documentary said they checked everything."

"The documentary said they checked everything and then went to a commercial." Maya crouched by the heater. "That's not the same as knowing."

Soren lifted the panel away. Inside was the element, a coil, and behind the coil a curved sheet of metal, bright on one side, dull and scorched on the other.

"Why is it shiny on the front and burnt on the back," Maya said. It wasn't really a question. She reached in and touched the shiny side, then pulled her hand back and touched the dull side. "Warmer here. The dull side's way warmer."

"The shiny part throws the heat forward," Soren said. "Into the room. That's the whole point of the reflector. It doesn't want to warm the wall behind it."

"So the heat comes off unevenly." Maya turned the panel in her hands. "Lots off the burnt side. Almost none off the shiny side."

"Because the shiny side sends it the other way."

They both looked at the reflector for a second. The radio changed songs.

"Okay, weird thought," Maya said. "Does heat push?"

"Light does. A little. We did that with the foil and the lamp." Soren set his screwdriver down. "The spinning vane thing."

"Heat is light. Just the kind you can't see." She said it fast, the way she said things she was still catching up to. "Infrared. It's light. If it leaves, it pushes."

Soren picked up the reflector and held it flat on his palm, dull side down. "If this were floating in space," he said slowly, "and it kept glowing heat off the burnt side, and almost nothing off the shiny side, then it would get pushed."

"Which way."

"Toward the shiny side. Away from where the heat's leaving." He tilted his hand. "A little push. All the time. As long as it stayed warm."

Maya went quiet. She picked a bolt out of the tin can and rolled it between her fingers.

"The Pioneers had a generator," she said. "A hot one. Nuclear. It ran for decades because that's the whole point, it just keeps being warm."

"Right."

"And the ship isn't the same all the way around. One side faces the antenna dish. One side faces open space." She was building it in the air with her hands now, a little lopsided shape. "So the heat wouldn't come off even. It'd come off more on one side."

Soren was already shaking his head, not to say no, to say wait. "But the dish points at Earth. Always. That's how they talk to it. So the dish is always on the Sun side, more or less, because Earth is over by the Sun."

"So if the heat bounces off the back of the dish," Maya said, "and leaves the ship on the dish side, the push goes the other way. Away from the dish."

"Sunward," Soren said.

The word just sat there between them.

"Sunward," Maya said again. "That's the direction. That's the exact direction it was slowing."

Soren put the reflector down very carefully, like it might get away from him. "That can't be enough force. It's heat. It's nothing. We're talking about a whole spacecraft going faster than anything."

"How much did it slow, though. The number."

"The show said it was tiny. Something like, if you ran it for a year, the ship ended up a few thousand kilometers behind where the math said." He frowned. "Which sounds huge. But over a whole year, across billions of kilometers, that's basically"—

"Nothing," Maya finished. "That's a nothing-sized push. Heat off a warm ship is a nothing-sized push." She looked at him. "They match. The size of the mystery matches the size of a warm thing glowing in the dark."

Soren didn't answer right away. He picked his notebook up off the gravel and opened it on his knee and drew the ship, the dish, an arrow for the heat going one way and a smaller arrow for the ship drifting the other. He held it out so she could see.

"So there was never anything out there pulling it," Maya said.

"No."

"It was the ship. Pushing itself. With its own leftover warmth." She laughed, a short surprised one. "For thirty years the smartest people alive thought space was doing something strange, and it was the ship the whole time. Warm on one side."

"They had to check everything else first," Soren said. "You can't just guess it's the boring answer. You have to earn the boring answer."

"It's not boring, though." Maya took the notebook and turned it so the arrows pointed up. "A machine so far away we can barely hear it. And it's warm. It's still warm out there. And being warm is enough to move it. Every ship we ever send is going to do this. A little. Forever. Just from being warm."

Soren took the reflector back and turned it over, shiny to dull, dull to shiny. "You'd have to build for it," he said. "If you wanted to go really far and really straight. You'd have to figure out which way your own heat leaks."

"Every warm thing leaks a direction," Maya said.

She put her hand flat above the heater's element, not touching, just in the rising column of it, and felt the push of it against her palm, the plain everyday version of the thing that had fooled the whole world for thirty years.

Soren reached up and put his hand in the warmth next to hers. The old coil ticked as it cooled. Above their two hands the hot air kept climbing, invisible, going somewhere, pushing the whole time.