The Herrenknecht tunnel boring machines were the fastest ever built, advertised to excavate at a rate of a kilometre a day. Their real progress was slower, of course, because worn drill bits had to be changed regularly, and dirt had to be removed.

The boring machines used to dig the Chunnel under the English Channel pulled out eight million cubic metres of dirt, which was used to reclaim 90 acres of oceanfront property near Folkestone, creating a public park frequented by children and picnickers. On the Moon, the removed dirt was also thrown into the "sea" – a huge pile at the bottom of Mare Serenitatis.

Under normal circumstances, Big Betsy could have covered two metres well before the falling capsule reached the core, but in this case, it had to turn, and the boring machine did not turn quickly. It tunnelled like an earthworm, the back half gripping the rock while the front half pushed forward, then the front half gripping the rock while the back half pulled back in, spraying plasticised concrete on the new walls to help stabilise the tunnel.

To change direction, it gripped on only one side, its thrusts altering its attitude by several degrees each time. I didn't know how long it would take, or if it would clear the tunnel in time. There was nothing to do but watch Rachel float weightlessly in free fall inside the capsule, counting down the minutes.

Just days before I had left for the moon, six months before Rachel graduated from high school, I had bought her a graduation present: a gold chain with a pendant of a bird flying out of an open cage. It was my joke to her, a symbol of freedom from her life at home, a fatherly invitation to go out and do whatever she wanted with her life. Only I'd never given it to her.

I'd come home late that morning from an exhausting all-night training simulation to find a lawyer waiting. He gave me the divorce papers and my wife's message that she would throw out anything I didn't remove from the house before I left.

The necklace didn't seem so funny anymore. I found Rachel at school and kissed her goodbye. I bunked at Kennedy for my last few days on Earth and never went home again.

"Thirty seconds until imp– … until she passes the core," said André.
The entire team was gathered in the control room, none of them breathing, eyes flicking back and forth between Big Betsy's coordinates and the clock. Ten seconds. Five. Zero.

Nothing. Big Betsy pressed on. The video inside the capsule continued to show Rachel, unharmed. She'd made it through.

I punched the comm for Farside. "Tanager, she's heading your way. Are you ready for the catch?"

Captain Matt Tanager's voice was matter-of-fact, as if this were nothing but a routine drill. "Roger that, Control. The generators are spinning and the magnets are hot. We'll catch her more gently than her own mother."

I thought about Rachel's mother, and hoped for better.

"Just be ready, Farside. I'm counting on you."

"Relax, Papa Bear. She's in good hands."

"Frank!" André called from his terminal.

"What is it?"

"She passed the first sensor."

"And?"

"She was more than two seconds late."

I didn't have to ask why that mattered. Two seconds didn't drop out of a precise gravitational calculation for no reason; something was slowing her down.

"You're sure?"

"I checked it three times."

Then I realised what it must have been.

"The debris."

"What?"

"The dirt dislodged by Big Betsy. In any other tunnel, loose dirt would fall out of harm's way, but in the exact centre of the Moon–"

"–the dirt would fall to the centre of the tunnel," André finished.

"Her capsule ploughed right through it."