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 André 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."