The core temperature began to climb, following the curve laid out in the manuals -

No, it didn’t. The rise was too fast.

Conlig watched with dismay as his numbers drifted away from nominal.

As NERVA lit, the spacecraft shuddered.

Priest was pushed back into his seat with a long, gentle pressure. Perfect. Just like the sims.

Natalie York called up, “You’re looking good here. We’re hawkeyeing your trajectory. You’re right down the center line.”

Priest’s job was to watch the pressure and temperature readouts from the S-NB stage, the NERVA engine, and its big hydrogen tank. Jones was monitoring the attitude indicator with its artificial horizon, ready to take over the steering if the automated systems failed. Dana was calling out their increasing velocity from the DSKY readout. “Thirty thousand feet per second… thirty-three…”

Mike Conlig was aware of a deadly dryness in his mouth. On the loop from the back room, someone was screaming in his ear.

The numbers, white on a green screen, filled his world.

The computers worked constantly to update the numbers, and making sense of them wasn’t easy. He had to check the data-source slots at the top right-hand corner of the screen, to make sure that the sources of his numbers were all still updating him properly, and he had to be sure that he wasn’t diagnosing some problem incorrectly because of a mismatch in numbers of different vintages, fifteen or thirty seconds old.

But he discounted all that. He understood exactly what the silent parade of numbers was telling him. And it wasn’t good. The NERVA core was still overheating.

He tried to increase the flow of hydrogen through the core. That would take away some of the excess heat.

He got no response. In fact, one readout told him that the volume throughput of the hydrogen was actually falling.

Maybe there was a problem with a hydrogen feed line. Or maybe a pump had failed. Or maybe it was his old enemy, cavitation, somewhere in the propellant flow cylinders.

The core’s temperature continued to rise. More screaming in his ear.

Damn, damn. He’d have to abort the burn. And that was probably the end of the mission; he doubted they’d be allowed to go ahead with another engine restart after that.

He sent a command to the engine’s moderator control. He would slow the reaction in the NERVA core, reduce the temperature that way.

He got no response.

If the temperature had gotten high enough, the fuel elements could have distorted, even melted, and it would be impossible to insert the control elements into the core. Was that happening already?

If it was true, there wasn’t anything he could do to retrieve the situation. As he watched his numbers evolve, Conlig felt the first touches of panic.

Priest could clearly see the cones of the volcanoes of Hawaii, upthrusting, broken blisters. Earth receded visibly, as if he were rising in an elevator. The ride was exhilarating.

He felt a surge of elation. The damn nuke works.

It unraveled with astonishing speed.

Conlig watched power surge through the overheating core. After that, the resistance to hydrogen flow through the core sharply increased. Bubbles built up everywhere. The nuclear fuel assemblies were starting to break up. Pressure rose abruptly in the propellant channels, which were also beginning to disintegrate.

The whole structure of the core was collapsing.

The pressure in the reactor began to rise, at more than fifteen atmospheres a second. And, because of the massive temperatures, chemical and exothermic reactions were starting in the core.

And the increased pressure inside the reactor backed up to the pumps, and the pumps’ feedback valves burst. With the pumps disabled, the flow of hydrogen through the core stopped altogether.

The reactor’s main relief valves triggered, venting hydrogen to space. That offered some respite. But the discharge was brief; unable to cope with the enormous pressures and flow rate, the valves themselves were soon destroyed.

And then the massive pressure started working on the structure of the pressure shell itself.

I’ve lost it. I’ve lost the reactor. It had taken seconds for his life to fall apart. He tried to react, to think of something to do, to make a report to Flight. But his mouth was dry, the muscles of his jaw locked.

There was a loud, dull bang, and the Command Module shook: bang, whump, shudder.

Dana, strapped into his center seat, could feel the spacecraft quake under him. Hollow rattles and creaks sounded from around the cabin, a groan of metal as the can around him was stressed; it was a noise oddly like a deep-throated whale song.

The master alarm shrilled in Dana’s headset, a shrill of staccato beeps. Yellow warning lights lit up all over the control panels.

He turned to look at his companions. Jones was staring at the instrument panel, and Priest’s eyes were round. That sure as hell wasn’t routine, whatever it was.

Jones cleared the master alarm.

The feeling of thrust died abruptly. It was like a slow collision in a car; Dana was thrown forward, gently, against his straps.

Jones said, “Jim. The Main A light is on. Check it out.”

Dana looked at his console. A red undervolt light was glowing. Damn. I should have been the first to see it. The Command Module’s systems were Dana’s responsibility.

“Confirm that,” he said. “We’ve got a Main A undervolt.” He was surprised his voice was level. He began to check voltage and current levels; they were showing erratic, inconsistent readings.

He heard pinging and popping noises. It was the sound of metal flexing. The spacecraft was still shuddering. Some damn thing has blown up on us.

Earth was wheeling past the windows. The Service Module thrusters ought to be firing as the spacecraft tried to maintain its orientation. But he couldn’t hear any solenoids thumping.

Jones was talking to Houston. “Natalie, we be a sorry bird up here. We’ve got a problem.” He unbuckled his restraints and floated up to the left-hand window. Dana knew he was following an old pilot’s instinct: at a moment like this, regardless of the telemetry, you needed to take a walk around the bird, to look for leaks and kick the tires, see for himself what was wrong.

Dana glanced out of the window to his right, past Ben Priest.

He saw sparks, chunks of some material, flying up past the Command Module. The material was glowing, red-hot.

Then he could smell something, inside his helmet. It reminded him, oddly, of Hampton: his childhood, the ocean.

Ozone.

Donnelly didn’t even need to hear the specific words. He could feel the event, see it in the changed postures of controllers all over the room, hear it in the sudden urgency of their voices.

Something had fouled up. But at first the cause wasn’t clear; all Donnelly got was a rash of symptoms, monitored by his controllers.

“We’ve got more than a problem.” That was EECOM, in charge of electrical and environmental systems: life support in Apollo-N. He was shouting. “I have CSM EPS high density. Listen up, you guys. Fuel cell 1 and 2 pressure has gone away.” Controller jargon, for fallen to zero. “And I’m losing oxygen tank 1 pressure and temperature.”

Natalie York was talking to the crew. “This is Houston. Repeat that, please.”

“…We’ve got a problem,” Jones said over the air-to-ground. “The NERVA is out, and we’re seeing a Main Bus A undervolt.”

“Roger. Main Bus A. Stand by, Apollo-N; we’re looking at it.”

Guidance said, “We’ve had a hardware restart. We don’t know what it was.”

A hardware restart meant some unusual event had caused the computer to shut itself down and reboot. Donnelly called for confirmation from another controller.

The crew kept reporting the Bus A undervolt.

The electrical power for Apollo-N came from three fuel cells in the Service Module. The current from the cells flowed through the A and B Buses, conduits which fed the rest of the spacecraft’s components. An undervolt alarm meant the spacecraft was losing its electrical power.