Once more she felt cool, stale air wash over her face.

Ares, assembled, was a slim, fragile pencil of metal. It was a big, bright object, and it would be easily visible from Earth, as a naked-eye star passing over Cape Canaveral.

Stone said, “Go for ET hydrogen pressurization.”

“Confirm.”

York began closing switches that would raise the temperature inside the booster’s two great External Tanks. Liquid hydrogen would boil and evaporate, and the resulting gas would force liquid propellant through the feed pipes and into the combustion chambers of the MS-II.

York was a geologist, and that was why she was going to Mars. But a crew was only three people. So, if you expected to fly in space, you had to expect to study up on a lot of mundane crap that was necessary just to keep the spacecraft and booster working.

And Natalie York’s specialty was the External Tanks.

She knew enough to give expert papers on External Tanks to the industry. In fact, she had given a paper on them, God help her.

“One minute,” Gershon said.

York glanced at the window to her right. She was over the west Atlantic, and it was early morning down there; she could see boats on the Gulf, ribbons of land laid out like a cartoon map.

TOI was Transfer Orbit Injection: it meant departure from Earth orbit, the start of the long transit to Mars. That was a key moment in the mission — in her life, in fact.

But a day and a half here, orbiting Earth, wasn’t enough.

She had tried to fix some of the more memorable scenes of Earth in her head. Night over Africa: the fires of nomad encampments, spread across the desert. Thunderstorms over New Zealand: lightning like flashbulbs, exploding under cottony layers of cloud, discharges sparking each other in great chain reactions covering the country.

November 6, 1986. That was the day when Ares was due to return to Earth orbit. Mission day 539. Then I’ll be back; I’ll be seeing you again. A bright Sunday morning, with my sample crates full of bits of Mars.

“Ares, you are go for the burn,” Crippen said.

Stone set the “master arm” switch to ON, and York could see him checking over the rest of the instrument panel. Guidance control was set to primary; thrust control was on automatic; the craft was in the correct attitude; the engine gimbals were enabled, so that the nozzles could swivel like eyeballs in their sockets to direct the craft.

Eight seconds before ignition, York felt a push at her back. Ullage: small rockets firing around the base of the stack, settling the propellants before the main burn.

The commit code, “99:40,” started flashing up on the small computer screen before Stone. Are you sure you want to do this?

There was a small button marked PROCEED under the screen. Stone reached out a gloved finger, and pressed the button.

Gershon counted down: “Five. Four…”