It was the end of the second day of this tentative companionship before she spoke meaningfully to him.

She pointed at the band of zodiacal light. ‘Asteroids, right?’

‘Yeah. You can see something similar at home – I mean, in the skies of the Datum. But there’s more asteroids here. A whole extra band, actually, between the orbits of Venus and Mars.’

She thought that over. ‘Oh. The wreckage of Dead Earth, the planet Bellos smashed.’

‘That’s it. But it’s not wasted. We’re out there already, in little dinky rocket ships, mining those fragments of planet for water, hydrocarbons, even iron from what used to be the Earth’s core. Easily accessible. And we’re manufacturing rocket fuel. Eventually, the plan is, we’ll be independent of materials brought over from the stepwise worlds altogether. Some people are planning to live out there, on the asteroids themselves. Mind you, others find it kind of morbid that we’re feeding off the ruins of a devastated world.’

Sally shrugged. ‘I think I lost my capacity for sentimentality a long time ago. Ever since I came across evidence of a few Donnerparty disasters out in the reaches of the Long Earth. Butchered human bones. This is just another kind of cannibalism, I guess.’

She said this with such flat finality that Frank had to look away, shuddering.

‘Come on, Frank. You’re tougher than that. Whatever it takes to survive, right?’

‘Sure. So.’ He forced a smile. ‘How are you finding the trip so far?’

She thought that over. ‘Surprising, if I’m honest.’

‘Surprising?’

Loosely strapped into her chair, she touched the hull wall. ‘For a start, this is kind of a bigger rocketship than I imagined we’d need.’

‘Well, the technology’s incredible. We’re driven by a stream of tiny fusion bombs, pellets of deuterium and hydrogen that are made to detonate by a laser barrage, hundreds of bomblets every second going off behind a big pusher plate. We plan to use it in stacks to assemble more ambitious missions further out, to Venus, even Jupiter maybe—’

‘Slow down, Apollo 13, you’re hyperventilating.’

‘Sorry. This stuff’s been my life’s work. My boyhood dreams before that.’

‘My problem is I don’t see why you need a rocket at all. I thought the Gap saved you from all that.’

Frank said, ‘Well, to get to Mars from the Datum, you’d have to climb out of Earth’s gravity well first. That’s why you need a Saturn V, even to get to the moon. Using the Gap we don’t need a Saturn V to get away from Earth. But we do still need a rocket for the Mars transfer.

‘You have Earth and Mars following their circular orbits around the sun, OK? Even when you’re in free space at Earth’s orbit, you need a boost from some rocket or other to add at least seven thousand miles per hour to your speed to get up into an elliptical transfer orbit, as we call it. You coast all the way to Mars’s orbit, and then you need another squirt, six thousand miles an hour this time, to slow down at Mars. And then you land. The whole thing is reversed when you come home. Actually our ICF rocket will provide a lot more push than that minimum.’

‘I guess that’s all logical enough.’

‘Except,’ called Willis Linsay, ‘that you skipped over the real mysteries behind all this.’

Frank turned to see Willis swimming up the fireman’s pole that ran along the axis of the hab module. He asked, ‘Which are?’

‘How does conservation of momentum work between the stepwise worlds? Or indeed conservation of mass? Sally, if you step from Earth A into Earth B, sixty-some kilograms of mass suddenly disappear from A and appear in B. How come? Mass, like momentum and energy, is supposed to be conserved. These are basic principles of physics – without which, incidentally, this firecracker of a rocketship wouldn’t work at all.’

‘True enough,’ Frank said. ‘So what’s the answer?’

Sally said, ‘Mellanier—’

‘That fraud!’

‘– would say that the conservation principles work across the worlds, not just in one world. Earths A and B share their mass and momentum, so nothing overall is lost or gained.’

‘Whereas others,’ Willis said heavily, and Frank suspected he was talking about himself, ‘argue rather more convincingly that such principles can only work one world at a time. And if you step to world B, you borrow a little momentum from that world – it slows its rotation just a little – and you borrow some mass-energy from its gravity field.’

Frank said, ‘Surely you could devise tests to establish which is which.’

Willis shrugged. ‘The effects are too small. Some day it will be done. But the latter is the more appealing idea, don’t you think? That a destination world somehow welcomes you as you step in, by giving of itself. And of course you give back when you step away.’

Sally said sourly, ‘If you like your scientific hypotheses to come loaded with emotional freight – yes, I guess that would be an appealing idea . . .’

Frank could sense a lifetime of fencing going on under the surface of this quasi-technical conversation. They didn’t even share the same accent. Willis was pretty broad Wyoming, which must have caused him to be underestimated by snobbier academics, while the daughter had a much more neutral accent, as if she’d deliberately distanced herself from her origins, from her father. Frank didn’t detect any real animosity between father and daughter. They were too vivid for that, the pair of them, much too real personalities in their own right to have that kind of negative relationship. But it certainly wasn’t all positive. They were two powerful people, with a shared past, yes, respectful, but wary of each other.

‘By the way,’ Sally asked now, ‘which way’s Mars?’

Frank glanced out of the window, thought for a second, then pointed over his shoulder. ‘Thataway. It won’t show up as more than a spark for, oh, weeks yet. Then we’ll see it hanging there like an orange. It has big features, you know, the giant mountains, the canyons, visible from far away – well, you saw the images back in the auditorium. And on this Mars there’s oceans, and the green of life.’

Sally glanced at her father. ‘And is that the point of this mission? To figure out why Mars, this Mars, is warm and wet and alive?’

‘Oh, no,’ Willis said dismissively. ‘That’s trivial.’

Frank raised his eyebrows. ‘Life on Mars is trivial? Tell that to Percival Lowell. So if life on Mars is just part of the scenery—’

‘It’s life on the Long Mars that I’m after. Life, and mind, and what it might – what it must have achieved.’

Sally faced him. ‘To what end, Dad? What are you looking for – some kind of technology, like a new Stepper box? And what will you do then? Just turn it loose? Mellanier once compared you to Daedalus, the father of Icarus, the boy who used his father’s invention to fly too high and upset the gods. And that’s you all over, isn’t it? Tinker, tinker for the sake of it, caring not one whit about the consequences. The Daedalus of your age.’

Willis rubbed his chin. ‘But Daedalus is supposed to have invented the saw, the axe and the gimlet among other goodies. That’s not such a bad charge sheet, is it? And as for—’ An alarm sounded softly. ‘Ah. That’s my latest experiment. Excuse me . . .’ Stiff but oddly graceful, he swam through the microgravity to the fireman’s pole and pulled himself away to his lab area.

Frank eyed Sally. ‘You OK?’

She didn’t reply. For a long while she sat silent, withdrawn, quite unreadable to Frank.

Then she said, ‘So what could go wrong, Frank? With the Galileo. I know they put us through some of the emergency procedures. But that mostly involved us climbing into air bags and floating around helplessly, while you saved the day.’

Frank shrugged. ‘I think Al and the others figured that was all you would sit still for. And Willis even less. So we fed you the most basic stuff.’