Viktor rolled the tractor up to a plastic tube that turned out to be a crude kind of airlock, good enough on this peculiarly benign Mars. He led her through the tube and into a dome. Unzipping their surface suits as they walked, they came to what was evidently a galley, smelling strongly of coffee and alcohol, overlying an earthy stink of body odour and sewage. On a wall-mounted TV an ice hockey game was playing: Russia against Canada.

Viktor said mournfully, ‘TV show. Recorded, stepped across two million worlds and transmitted to us from Gap station. Now no more ice hockey.’

‘Because there’s no more Russia after Yellowstone?’

‘Exactly. We watch same games over and over. Sometimes drunk enough to forget result and bet on scores . . .’

Two more men came bustling in, evidently drawn by the sound of their voices. One was like Viktor, big, dark, maybe fifty; he wore a cosmonaut-type blue jumpsuit with a name tag lettered in Cyrillic and Latin: DJANIBEKOV, S. Viktor introduced him as Sergei. The other, slimmer, blond, maybe under forty – KRILOV, A. – was Alexei, and he wore a grubby white lab coat. These were three men without women, and they stared at her. But Sally met their gazes, Viktor’s too, with a certain look of her own. She had been travelling alone in the Long Earth since she had been a teenager, and was a veteran of such encounters. These three seemed harmless enough.

Once that tricky moment was over, they were fine. Indeed, they fussed over her, like kids eager to please. Sergei’s English was a lot worse than Viktor’s, Alexei’s a lot better. Of course even Sergei’s English was a hell of a lot better than Sally’s Russian, which was non-existent.

They showed her what they called their ‘guest room’, which was one of the tepee-like shacks. She explored the little space, curious. On the floor was a kind of rug made of thick brown-white wool. The tepee’s covering skin felt like ordinary leather, crudely treated, but the Martian wood of the structural frame was so hard and fine-grained it might have been a plastic imitation: this was some adaptation to enhance moisture retention, she imagined.

She returned to the galley. Sergei, gallant but almost wordless, offered her a big baggy sweater evidently knitted of the same wool as the rug. Although it smelled strongly of whoever wore it regularly, she pulled it on; the sweater was cosy in a base that never quite excluded the Martian chill. They fed her a late lunch, of cabbage and beets and even a couple of tiny, wizened apples, which she imagined were a treasure and a great honour to receive. They offered vodka, which she refused, and coffee, or some imitation of coffee, much-stewed, which she accepted.

Before the light faded Alexei insisted on showing her around the rest of the compound. ‘I am the station biologist,’ he said with some pride. ‘Also the nearest we have to a medic, among other things. We must all play multiple roles, in a team as small as this . . .’

There were clear plastic tunnels connecting the domes, so you could get around the base without exposure to the Martian climate, but there were simple self-sealing airlocks that would close up in the event of a pressure breach. Because the whole base was linked up in this way she never escaped the lingering stink of body odour, but at least it was more diluted the further she got from the central quarters. Alexei insisted that Sally carry her oxygen mask loose around her neck at all times, in case of a wall breach. Sally had survived decades alone in the Long Earth; she needed no persuading about such precautions.

Some of the domes were industrial, where compact, crude-looking machinery cracked the Martian atmosphere and water to produce breathable air and fuels such as methane and hydrogen, or processed the rusty dirt to produce iron. Alexei said they were also working on ‘Zubrin kits’, which he said were adapted to generate methane and oxygen in the sparser conditions of more typical versions of Mars, like the Mars of Datum Earth. ‘You must import hydrogen, to such impoverished Marses. But a ton of hydrogen processed with Martian air will give you sixteen tons of methane and oxygen – a good return, you see.’

They walked through the farm domes, which sheltered laboriously tilled fields of potatoes and yams and green beans. The work these Russians had put in was heartbreakingly clear from the quality of the soil they’d managed to create from Martian dirt. ‘Such a challenge, the native dirt is just rusty grit coated with sulphates and perchlorates . . .’ They’d even imported earthworms. But a spindly, yellowed crop was their only reward so far.

Beyond the domes, open to the Martian elements, was a small botanical garden Alexei had established, and he proudly showed Sally his collection of native stock. The cacti were shrivelled and tough-looking, and the trees he’d planted, from seeds collected from adult specimens on the slopes of Arsia Mons, were hardly grown.

He took particular pride in showing her a clump of plants a few feet tall, a kind of ice-cream swirl of yellowish leaves on a base of green leaves. ‘What do you make of this?’

She shrugged. ‘Ugly. But that green looks more Earthlike than Martian.’

‘So it is. It’s a Rheum nobile, a noble rhubarb – or rather a genetically tweaked version. Grows in the Himalayas. Those yellow leaves wrap around a seed-bearing stem within. It’s adapted for altitude, you see, for thin air. The yellow column is a kind of natural greenhouse.’

‘Wow. And here it is growing on Mars.’

He shrugged. ‘One of a suite of plants from Earth that could almost make it on Mars, on this Mars anyhow. And you can eat the stems, yum yum.’

His final surprise, kept in a dome to themselves, was a small herd of alpacas: awkward-looking beasts, imported as embryos from the mountains of South America, scraping at the scrubby grass that grew at their feet. They peered out at the humans, their woolly faces curious and oddly endearing.

‘Ah,’ Sally said. ‘So that’s where you get the wool. And the leather for the tepees.’

‘Indeed. We hope that the descendants of these creatures may some day become adapted to survival in raw Martian conditions, on this Mars at least. Of course we may have to genetically engineer Earth-based grasses for them to feed on.

‘And if alpacas, why not human beings? Today, this particular Mars is like Earth at an altitude of six miles or so. The highest town on Datum Earth is in Peru, at about three miles. Humans cannot live much higher than that, permanently – or rather we cannot. Our children may be different. This Mars is almost within reach, for us, for the alpacas—’

‘For the rhubarb.’

‘Exactly. This was our mission, from the Moscow government. We Russians have always looked to the stars, and the discovery of this near-habitable version of Mars excited our scientists and philosophers greatly. We three were the vanguard; we were sent here to establish how humans might live on this world, as well as to study the life forms already extant here.’

‘The vanguard. More should have followed?’

‘Marsograd should have been a city by now – such was the plan. But your American supervolcano put a stop to that, as to all Russian ambitions. Still, we are here, and we learn much . . .’

Working pretty much single-handed, Alexei Krilov had been able to establish a great deal about the strange life forms of this relatively clement Mars.

‘I have gathered samples from diverse environments, from the deep wet valleys to the flanks of the great volcanoes where life probes at the fringes of space. The cacti have tough, leathery skin which almost perfectly seals in their water stores. The trees have trunks as hard as concrete, and leaves like needles to keep in the moisture. Do not imagine these forms of life are primitive, by the way. They survive in extremely austere environments; they are highly evolved, highly specialized, superbly efficient in their use of mass and energy.