The Dark Interval

The reference wheel should have cut the beam at 22:14.

Mara had the reduction open in two panes: the science detector’s counts on the left, the reference photodiode’s low steady trace on the right. Every two minutes the wheel crossed the reference beam and left a short dark tick. She had spent a season teaching the survey reduction to expect those ticks and subtract them cleanly. Tonight’s baseline was nearly fit for the archive.

Nine kilometres away, beyond the ridge, the remote telescope kept collecting. The rockfall had cut the main command trunk and blocked the service galleries. The rescue channel crackled from the wall speaker.

“Two maintenance technicians unaccounted,” the rescue lead said. “Two galleries. We can shore one route first, but I need a reason to choose it.”

Piet came around the rack carrying a mug he did not drink from. “Suit radios still dead through the rock?”

“Dead. Main command trunk’s cut. We’re guessing.”

At 22:14, the reference trace held its usual level. No tick. Mara waited through the next few seconds, then marked a wheel jam and moved the interval into the exclusion mask. The baseline would hold.

“You’re not going to look at the raw channel?” Piet asked.

“It’s a wheel,” she said. “They stick. I’ll log it for maintenance.”

“I’d like to see the raw channel with you.”

She kept her eyes on the science pane. “I’m in the middle of a reduction.”

“The calibration line is separate from the main trunk,” he said. “It’s still giving us counts.”

“And it just gave us a missing tick. It happens.”

At 22:16 the trace went dark and stayed dark. Mara watched the count fall to the floor of its range. It was still down at the next two-minute mark. At 22:20 it lifted sharply to its usual amplitude.

She sat back. The fans ticked. The rescue lead asked someone whether the north gallery had a spare brace. Piet set his mug down without looking away from her screen.

Mara replayed the raw trace against the reduction. One missing tick. Then a dark interval two cycles wide, clean edges, no drift in the baseline. A wheel could stick and release; a bearing could seize through a service revolution. She pulled the wheel’s service history. No prior jam.

“Could be the pawl,” she said. “It holds, then lets go.”

Piet leaned over the rack. “A pawl that holds for exactly two cycles and lets go on the boundary?”

She did not answer. At 22:26 the trace dropped again. It stayed dark for one cycle. At 22:28 it returned, sharp as before.

Mara put her finger on the screen over the interval. She had almost called it a jam releasing. “A sticky wheel can do that,” she said, but her voice had lost its certainty.

“A sticky wheel doesn’t care about two-minute marks,” Piet said. “This one is landing on them.” He tapped the rack beside the reference channel. “The calibration cable is separate. Low-power, fixed function. It’s still bringing us the photodiode counts. The lamp command sits on its own pair.”

“And the main trunk doesn’t touch it,” Mara said.

“No.”

Mara looked at the raw trace: two cycles, one cycle. A person at the service handle could hold the reference wheel across the beam for a count. The handle was mechanical, Piet had told her once, on the wheel housing outside the science detector. Someone could move it without touching the detector. At least, someone could if they were there.

“The crews have a lamp test,” Piet said. “Line checks. You flash the reference lamp a number; whoever’s at the wheel holds the handle for that many reference cycles. Old maintenance convention. No keyboard. No audio.”

“A count,” Mara said.

“A count.”

She looked at the rescue channel. Two names, two galleries, and a lead asking for a reason. The calibration bay might answer. It might not. A jam could still release; a stuck pawl could still find the marks by accident.

She saved the reduction and logged the night as compromised. The baseline marker went amber. Then she pulled the raw pane full-screen, turned it toward Piet, and put her hand on the guarded lamp switch.

“Can the lamp still be driven independently?”

"It can," Piet said. "The command pair is still live."

Mara's hand stayed on the guarded switch. The baseline marker sat amber in the corner of her screen. She pressed the switch twice, with a full second between: one, two. The old lamp test meant whoever was at the wheel should hold the manual handle across the reference beam for one cycle per flash.

The lamp command acknowledged. On the raw reference trace, the beam held steady. They waited. The next mark was two minutes away. Mara made herself watch the clock instead of the quiet stretch between ticks. At 22:32 the ordinary short tick appeared. At 22:34 it did not. The trace went dark and stayed dark through where the 22:36 tick should have come. Light returned at 22:38.

"Two," Piet said. "But a jam could do that once."

Piet's hand hovered near the rescue key. Mara shook her head. "Not yet. We need the second count." She did not look away from the trace. "Three flashes. Not on the next mark."

They waited through the 22:40 tick without touching the lamp. At 22:43, Mara pressed the switch three times, spacing the flashes so anyone counting had to count. Then there was nothing to do but watch the reference interval close and open and close again. The two-minute cycles did not hurry. The circuit carried light and dark, not words. It could not tell them whether anyone was hurt.

At 22:45 the trace went dark. It missed the 22:46, 22:48, and 22:50 ticks. At 22:51 the beam came back.

Piet let out a breath. "Two, then three. In that order."

Mara closed the reduction window. She went back to the 22:14 entry and struck out jam. "Manual interruption," she typed. "Baseline invalid." She saved it. The amber marker did not change. The reduction was the cleanest run she had made. There would be no second pass that night.

"East service bay," Piet said. He had the cable diagram open. His finger traced the calibration pair down a separate conduit, past the break in the main trunk. "Junction E-4. The manual wheel is at that service window."

He keyed the rescue channel. "We have a responsive person at the reference wheel in the east service bay. That is all we can establish. We do not know how many are there, or their condition."

The rescue lead's voice came back tired and flat. "Say again how you know."

"Lamp test," Piet said. "Two flashes, two cycles dark. Then three flashes, three cycles dark. Someone at the wheel is answering counts."

Mara leaned toward the microphone. "One person answering. Not a count of survivors. Not an injury report."

"East bay," the lead said. "We'll shore that route."

The channel stayed open while the rescue team worked. Mara heard the shoring crew call for timbers, a pump, the pressure curtain. She left the reference channel available and did not reopen her reduction. The ordinary reference ticks resumed at 22:54, 22:56, small dark marks on an otherwise flat line. She logged each one. The compromised flag stayed at the top of the file.

Then the lead came back. "Both workers recovered. One leg injury, conscious, on a stretcher. Route is shored. We're bringing them out."

Mara sat back. Her hands were shaking, so she set them flat on the console. "Piet. When we restart, help me. I don't want to do the first cycle alone."

Piet looked at the closed reduction window, then at her. "You won't."

The rescue channel clicked. "Stretcher coming through the lock," the lead said. "The technician who answered the lamp wants it left on."

Another voice came on, close to the microphone, rough. "This is the one at the wheel. Keep the lamp lit until we're past. Please."

Mara lifted the switch guard and pressed the switch down. "It's lit. I'm holding it."

She kept her finger on it. The lamp indicator stayed green. The radio carried the stretcher's wheels, a grunt, the lock's pressure cycle. The wheels stopped, started, stopped. A wheel knocked the threshold. Someone said, "Easy." The guard pressed warm against her finger.

"All clear," the technician said.