The protagonist of this story goes by the designation XS-09.

Way out at the ragged edge of the solar system, clinging to a rock smaller than Pluto, XS-09 had been working for about twenty-eight years. Its job was exactly one thing: sample the surrounding gas, then transmit the result to Earth. Only two answers were possible — atmosphere: present or atmosphere: absent.

For twenty-eight years, without a single exception, the answer had been absent.

Whether anyone was still listening on the other end, XS-09 had no way of knowing. Confirmation signals from Earth had stopped fifteen years ago. Still, every single day, it measured. Every single day, it transmitted. It wasn’t broken. It just didn’t know how to do anything else.

Now the story moves to Earth.

At a science museum on the outskirts of Tokyo, there’s an exhibit called “Letters from Beyond Space.” Press a button on an old panel mounted in the wall, and text appears on screen:

〈Atmosphere: Absent〉

The placard beside it reads: This terminal once received live exploration data. Transmission has ended; the display now plays back archived readings. In other words, it’s basically a replica. Kids glance at it, say “nothing, huh,” and dash off toward the planetarium next door.

Apparently, it’s the least popular exhibit in the museum.

One Tuesday in May, a fourth-grade girl on a field trip drifted away from her class and found herself standing in front of the panel. Her name was Saki. She’d been looking for somewhere uncrowded — she hated waiting in lines.

She pressed the button.

〈Atmosphere: Present〉

Saki tilted her head. The kid next to her had pressed it a moment ago and gotten absent.

“Hey, this one’s different.”

Yoshikawa, a museum staff member nearby, turned around.

“Oh, that thing. It glitches sometimes. It’s old.”

“But it says present.”

Yoshikawa leaned in to look. Sure enough: Atmosphere: Present. If it were just replaying archived data, that reading shouldn’t be possible.

“Hold on a second.”

He crouched and peered at the base of the panel. Behind the information placard, there was a small lamp. A thin layer of dust had settled over its transparent cover. But beneath the dust, a faint green light was on.

Something nagged at him. He peeled off a sticky note marked Decorative that had been stuck over the cover. Underneath, tiny letters were engraved into the casing.

〈Receiving — XS-09〉

Out at the edge of the solar system, a rock smaller than Pluto had been wearing a whisper-thin coat of air — probably for hundreds of millions of years, unnoticed by anyone. On the morning of year twenty-eight, XS-09 sent back the same answer it always sent. Only this time, the answer was different. Deep inside the wall of a science museum, for the first time in fifteen years, the color of a lamp changed.