Letting Go by David Walton

The Hole was two metres wide, stretching from Mare Serenitatis straight down to the core. Big Betsy was chewing one long tunnel through lunar rock, nearly 7,000km from one side of the Moon to the other.

I never wanted Rachel to go into space. Space was my passion, but for my daughter I wanted a normal American life: Barbie dolls and pony rides, make-up and boys, the senior prom. A good college, a good career, marriage, kids.

Only I wasn't there, most of the time, to see it happen. When she played Mary in her Sunday School pageant, I was on the ISS.

When she graduated high school, I was on the Moon, already starting the Gravity Train project. But there must be some truth to the argument for nature over nurture, because despite my long absences she took after me instead of her mother. Joined the academy, earned her pin and followed me to the Moon.

The morning of the accident, we were all ready to celebrate. We gathered in the control room, Rachel holding my hand, Commander Andre Gretzsin, Jr. beside her, and the rest of the crew pressed in close behind.

On screen, a Herrenknecht tunnel boring machine named Big Betsy churned through the lunar rock at the bottom of The Hole, as it had been doing without stop for almost eight years.

Today was different, though. Today, Big Betsy would finally reach the end of her task. No one dared speak. The only sound was the bass growl of the machine, transmitted back to us through the video feed.

The Hole was two metres wide and more than 3,000 km deep, stretching from Mare Serenitatis straight down to the core. On the other side of the Moon, starting at Farside Station and plunging nearly as far, was the dig known affectionately as The Other Hole. The boring machine on that side had been redirected the day before, to get it out of the way.

As we watched, the last bit of rock tumbled forward in a loose spray as Big Betsy connected the two holes into one long tunnel, the longest ever dug, nearly 7,000 km from one side of the Moon to the other.

We broke into cheers. On another video display, the team at Farside danced and hugged each other. It was finished. We'd actually done it.

Four hundred years ago, Robert Hooke proposed the perfect transportation system to Isaac Newton: a straight tunnel through the Earth from any two points on its surface. A frictionless sled dropped down the hole at one end would arrive at the other side 42 minutes later with perfect conservation of energy. For Hooke and Newton, it was a thought experiment - a puzzle on which to apply the new laws of geometry and gravitation.

On the Moon, however, with modern drilling techniques and no atmosphere to cause friction, their idea had become a reality. In a few months, when the train capsules were completed and put into service, a 1,000 pounds of helium- 3 would be scooped up each day from the vast deposits around Farside and transported by gravity train to the near side that always faces the Earth.

"So there it is," said Rachel. "A really, really deep hole."

I snorted. "That's all we hear from the media." The project had required almost twice the original budget, and most of their news coverage revolved around how much money the government was sinking into a very deep hole.

"One of those helium-3 capsules will provide enough fusion energy to power New York for a year. We're going to solve the world's energy crisis, and all they can talk about is the Guinness Book of World Records."

Rachel squeezed my hand. "When I was little, I was always out digging holes in the backyard." I smiled, but it gave me a pang for her to mention her childhood. The childhood I had missed.

"Digging a hole to China?"

"Digging a hole to anywhere, as long as it wasn't home."

"You wanted to get away that badly?"

"Mostly I just wanted to be with you."

"You and Mom-" I began, but she put a finger on my mouth.

"Let it go, Dad. We were too different. Not your fault."

Her hair was cut short, a concession to the hardships of space, and her face showed the fluid swelling typical of long stints in low gravity. She was my height, slender, strong, and independent. She was different from her mother, all right. But she wasn't what I had wanted her to be.

"Dad?"

Her tone worried me. "What's wrong?"

"Nothing. I wanted to pile on some more good news."

Something was wrong. Rachel was never hesitant.

"Andre and I are going to be married."

I pressed my lips together. She knew my opinion of astronauts and marriage. That's why she chose that moment to tell me.

"What happened to 'single forever'?"

"Changed my mind."

I glanced around the control room and spotted Andre at his terminal, watching us.

"What do you want, my blessing or something?"

"I just want you to be happy for us."

"I probably won't make it to the wedding."

"Sure you will. We'll have it as soon as we get home."

"I might have to stay here longer than you, make sure everything keeps running."

"Then we'll wait. You have to come back to Earth eventually."

I shook my head. Against all odds, she'd beaten her mother's influence and made a career for herself. Now she wanted to throw it away for domestic life?

But she knew what I thought; I didn't have to say it. Back to work. Big

Betsy had to be redirected to drill into the side of the tunnel, where she would be left forever - it wasn't worth the energy to pull her back to the surface. The other boring machine was already snug in its own little grave, 10 m into the rock.

Andre must have snuck up behind me, because I heard him say, "I'll take good care of her, Frank."

I wanted to hit him, but I kept my voice soft. "You're on the short list for Mars, Andre. She's not. What are you going to do, leave her in a Florida apartment with a baby in her belly while you go away for seven years?"

Andre looked angry, but he didn't snap back.

Rachel said, "Dad..."

"Forget it. It's your life. I have work and so do you."

I sat down and ignored them. Eventually they left.

An hour later, I watched her onscreen as she worked inside Capsule A. Our command centre partially encircled The Hole in a rough U-shape, inside which were the oxygen tanks for the complex, fuel and extra cutting blades for the tunnel boring machines, construction materials, and in the centre, the capsule itself. It was held in place above the chasm of The Hole by the electromagnetic capture system, but for safety, a series of bolts prevented accidental release.

Rachel was inside, suited up, spraying the capsule with insulator before its debut drop the next day. She barely fitted; just shy of two metres wide, the capsule had been designed for cargo, not human passengers.

I watched her work, confident, at ease in a spacesuit, skilled at her task. I had been unfair. Andre was a talented spacer, a third-generation astronaut whose grandfather had been on the Mir. He was stable, trustworthy, a great commander. As a child he had lost his own father in a training accident when he was young; he knew the risks of space.

My reaction had more to do with guilt about my own failed marriage than about him, but I still couldn't see past it. A life in space and a family just didn't fit together. Even so, I owed her an apology. I reached for the comm. The video feed dropped to static, and I felt a deep vibration in the floor.

The moment froze, like a shuttle when the last booster drops away and the battering five-g ascent becomes instantly silent and perfectly still. I ran to the windows, my body sluggish, underwater. My eyes met Andre's across the room, and we both knew. An explosion. Disaster.

We looked out and saw the impossible: a fire on the Moon. The capsule platform was engulfed in flames. (It was months before we found out what had happened - lunar dust had fouled a valve, causing pressure to build up. The resulting explosion doused the platform with burning fuel and at the same time pierced an oxygen tank, providing the fire with a steady supply of fresh oxygen to keep it alight. At the time, all we knew was that Rachel was in trouble.)

Crew members packed the airlock, frantically suiting up for a rescue attempt, but I could see they would be too late. I rushed back to the comm.

"Rachel? Are you there?"

"Roger that, Control," came her calm voice, just as she'd been trained to react in a crisis. "The temperature is rising fast in here. Can I get out the hatch?"

"Negative. Egress is completely blocked."

"Can they put it out?"

The two men who had reached the fire with extinguishers backed away, unable to get any closer.

"Not in time."

My mind raced, trying to keep the horror out so I could think clearly. It wasn't my daughter; it was a problem to be solved. And then the solution was obvious.

"Drop her!"

Andre turned from the window to stare at me.

"Come on, help me. Release the bolts and drop the capsule."

Andre shook his head. "Big Betsy hasn't cleared the tunnel yet. She'll die."

"She's dying right now; that fire's going to cook her before anyone can stop it. It's her only chance. Do it!"

We kicked chairs out of the way and grabbed our consoles. It didn't take long. The bolts retracted and the capsule, released, plummeted into the Hole.

Andre grabbed the comm, thumbed the global override, and bellowed into

it. "I need every hand back to Control right now. The capsule is in the Hole.

Repeat, everyone back to the control room at once."

He turned back to me. "Big Betsy has 20 minutes to get out of the way."

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 Andre.

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!" Andre 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," Andre finished.

"Her capsule ploughed right through it."

We were silent for a moment. We both knew a miss would be fatal. If the capsule didn't reach high enough for Farside to catch it, it would oscillate back and forth over a matter of days, slowing gradually and finally coming to a rest at the core of the Moon. Rachel would run out of oxygen long before then.

"Will she make it?" I asked.

Andre didn't answer. He loaded an analysis tool and started typing furiously. I couldn't wait. I grabbed a pen and a safety manual and started writing equations on the back. The math was complex, but it was also the basis for the project I'd been working on for eight years. The catching mechanism was designed to be extended down into the tunnel to account for slight variations in velocity; the question was, could it extend far enough to catch Rachel?

We reached the answer at almost the same moment and looked at each other without speaking, both praying that we were wrong.

Finally, I said, "She'll fall 10 m short."

Andre nodded.

The comm beeped. I punched it and growled, "What is it?"

"Dad?"

"Rachel! Are you safe? Everything OK?"

"I'm fine, Dad. My air is good, my temp is good. Farside's going to catch me, right?"

I didn't hesitate. If there was anything I knew as an astronaut, it was that people do better when they have all the information.

"Negative. Repeat, negative. You hit some friction at the core. You won't reach the magnets." I coughed, then said, "We're going to catch you on this side."

She knew as well as I did that the loose dirt would cut her velocity even more on the return trip. Her response was quiet and simple. "How?"

I shook my head. "I don't know."

Rachel's capsule fell short exactly as our calculations predicted and started the long fall back toward the core.

"If there were only some way to speed her up..." Andre tapped a pencil on the screen in nervous frustration.

"Forget about it. No rockets, no means of propulsion, nothing to push out the back."

Propulsion meant energy, and there was no more energy in the system.

"Could we fire the laser at the capsule, cut a hole in the top?"

"What, and have her jump out?"

Andre shrugged. "If she jumped just at the top of its rise, in the moment the capsule was completely still, she could prop herself up in the tunnel with her legs and arms until we could pull her out."

"Hold herself up in a sheer concrete tunnel over a several-thousand kilometre drop?"

"If it's her only chance, it's better than nothing."

I held up my hands. "I'm sorry. But it won't work. The tunnel is gravitationally straight, but it's not actually straight, remember? It has to account for mascons and mountains. You can't reach her with a laser until she's too close to do any good."

We argued for several more minutes, but finally settled on the only possibility. We had no more time to talk if we wanted time to prepare. The only remaining disagreement was who would go down The Hole.

"There's no question, Andre. You have to work the numbers. They need to be perfect - correct to the centimetre. This isn't something you want me doing on the back of an envelope. If it isn't right ... well, we'll only get one chance at this."

Andre nodded. "I'll get it right."

"You do that."

They attached the cable to my spacesuit, the same cable and winch we'd used to lower Big Betsy into the initial hole. It was easily thick enough to handle the strain. I stood, fully suited, at the edge of The Hole. There was no time for second thoughts. I checked my connections and my air, then dove down into blackness.

The cable slowed my descent while above me, another member of the team kept a precise measurement of the length of the cable. At 200 m, they slowed me to a stop. On the surface, Andre was measuring how much the core debris had slowed Rachel's capsule on her second pass. He would make precise measurements of her ascent and adjust his calculations accordingly.

I felt the cable vibrate as it was pulled a metre or so higher, then lowered by tiny increments, then raised again.

It was dark. I turned on my torch and pointed it down the shaft. Living in space had accustomed me to enormous distances, but the sight of that endless tunnel set my heart pounding. I checked my watch, which glowed a faint green. Three minutes.

The cable continued to hum, adjusting my position by small amounts.

Why did it take so much adjustment? Was her acceleration still changing? I thought about hailing Andre on the comm to ask him what the problem was, but rejected the idea. There was nothing I could do to help now.

Less than two minutes left. I couldn't see it yet, but the capsule was coming. I hefted the hook in my right hand, knowing I would only get one chance. If Andre's calculations were off, I would get no chance at all - either the capsule would fall short, plummeting back toward the core before I could reach it, or else ... or else it would rise too high, and two people would die instead of just one.

Where was it? Why couldn't I see it? Had it already reached its peak and fallen back down? I didn't dare look at my watch again, lest the capsule appear in that split second.

Suddenly, there it was, flying up toward me.

The lack of reference points made it hard to gauge its speed; one moment it looked too slow, others like it would plough right into me.

I shrank back, adrenaline pumping, unable to dodge it or move any higher. Then, just as I braced for the impact, it stopped - just centimetres away. I gaped at it, frozen with shock, and almost didn't move fast enough. I swung out desperately with the hook and wrapped it under a bar on the top of the capsule, just in time.

It shrieked and held, jerking the cable with a force that set it thrumming like a bass fiddle. But it did not fall.

When the hatch finally opened and Rachel stumbled out, she ran straight into Andre's arms. My hands were shaking. During the crisis, there had been no time to be afraid for her.

Now that it was over, the emotions flooded me. At that moment, life seemed so fragile, so fleeting. I watched Rachel and Andre hold each other and wondered if they could make a success at marriage. Rachel wasn't her mother, and Andre wasn't me; maybe they could. Even if they couldn't, what good would I do by standing in the way?

I removed my spacesuit and retreated to my sleeping quarters - a closet barely large enough for a narrow bed. I rummaged in my trunk and finally pulled from the bottom a gold pendant on a chain - a bird with wings spread wise, flying free from a cage. I had kept it long enough.