Her head emerged through the helmet ring. Stone was grinning at her. “Welcome back.” He pulled her HUT down, jamming it so that it rubbed against her shoulders, and guided the metal waist rings of the two halves of the suit to mate and click together.

Then she helped Stone don his suit.

York and Stone had already been inside the cramped airlock for two hours. Challenger’s atmosphere was pressurized to 70 percent of Earth’s sea level, with a mix of nitrogen and oxygen, but to stay flexible their suits would contain oxygen only, at just a quarter of sea-level pressure. So York and Stone had had to prebreathe pure oxygen to purge the nitrogen from their blood.

It was a tedious ritual. And EVAs on Mars could last only three or four hours, at most. Apollo backpacks had been capable of supporting seven hours of surface working. But Mars’s gravity was twice as strong as the Moon’s, and Mars suits had to be proportionately lighter, and could therefore only sustain much briefer EVAs. There would also have to be a long tidy-up period after each EVA: the suits would have to be vacuumed clean of Mars dust, which was highly oxidizing and would play hell with their lungs if they let it into Challenger.

The brief EVAs, with the surrounding preparation and cleanups and anticontamination swabbing, were going to occupy most of each exhausting, frustrating day on Mars.

York fixed on her Snoopy flight helmet, and over the top of that Stone lifted her hard helmet, with its visor, and twisted it into place against the seal at her neck.

The last pieces were her gloves; these were close-fitting and snapped onto rings at her wrists.

Stone flicked a switch on her chest panel, and she heard the soft, familiar hum of pumps and fans in the backpack, the whoosh of oxygen across her face. He rapped sharply on the top of the helmet and held up a gloved thumb before her clear faceplate. She nodded out at him and smiled.

She held up her arm; there was a reflector plate stitched into her cuff, allowing her to see the panel on the front of her chest which gave her a readout of oxygen, carbon dioxide and pressure levels, and various malfunction warnings. She could see her oxygen pressure level stabilizing.

Stone tested out the radio link. “Hi, Natalie. Able Baker Charlie…” His voice sounded soft and tinny, echoed by muffled sound carried through the thick glass of her faceplate.

She checked the small plastic tubes protruding from her helmet’s inner surface; she sipped out little slugs of water and orange juice. The OJ was okay, but the water was too warm. It didn’t matter. She pushed her suit’s internal pressure up to maximum, briefly, to test for leaks. She fixed her little spiral-bound EVA checklist to her cuff.