‘Yes. All those years ago, at the UN-China conference at Obelisk.’

‘No matter what we go through, Mars, it seems, endures.’

Quintus Fabius faced her. ‘Maybe Mars has not yet changed very much, Academician. But it will shortly. Look up there.’ He pointed to a slice of dark sky, beyond Mars’s western limb.

Where hung a single brilliant star.

‘Ceres,’ Penny whispered.

‘Höd, yes.’

‘How close is it? That thing looks almost large enough to show a disc.’

Stef said, ‘Penny, we haven’t been troubling you with updates during the voyage. We hoped you’d sleep through it—’

‘Oh, shut up, you fusspot.’

Quintus said, ‘Höd is larger than the width of Venus, as seen from Earth. So the Arab observers assure me.’

Penny tried to work that out. ‘Then it must be – what, a few million kilometres out?’

‘Rather less,’ Stef said. ‘The asteroid has undergone episodes of immense thrust. We suspect Earthshine has ordered the use of significant chunks of the body’s own material to use as reaction mass. The observers on the Malleus have computed the new trajectory.’

Penny could see the conclusion of all that in her sister’s expression. ‘My God.’

Stef took a deep breath. ‘Ceres is going to impact Mars. That’s finally confirmed. It’s probably what Earthshine intended all along.’

Quintus looked furious, as if this was some personal betrayal. ‘But why?’

‘We’ve no idea,’ Stef said. ‘Not yet.’

Penny looked at Stef. ‘How long?’

‘The Arabs estimate twelve hours. No more.’

‘As little as that? Very well. That’s the time we have remaining to stop Earthshine.’

Quintus nodded grimly. ‘Of course we must. This great act, this hurling of cosmic masses, can be intended to do nothing but harm. It may even start a war. We have to stop him. But we will face resistance.’

‘Then,’ Penny said drily, ‘I’m glad I’m on a ship full of Roman legionaries. Let’s work out our plan.’

But as the soldiers began to discuss tactics and fallbacks, a clock in Penny’s head began a dreadful countdown.

Twelve hours, and counting.

CHAPTER 27

To Stef’s relief, Penny submitted to Michael’s insistence that she needed rest.

‘And make sure she straps down again,’ the centurion called as she was led from the bridge. ‘We may have some more hard acceleration to undergo before the day is done.’

‘As you wish, Centurion.’

The rest of them inspected Quintus’s images of the layout of Earthshine’s latest base on the ground, at Terra Cimmeria. They were large-scale photographs, grainy wet-chemistry productions like all Roman or Brikanti imagery, but good enough, Stef thought, to get a sense of the layout. She saw three broad clusters of facilities, grouped close together. Further out the ground was marked by swathes of scorching, places where the ground had melted altogether: the relics of multiple landings of kernel-drive rockets.

‘So, Colonel Kalinski,’ Quintus said. ‘We have been scouting this area for some time – for years, as Earthshine has developed his operation. But I welcome your input now. This is the location where you say that the Xin had their Martian capital in your world.’

‘Slap in the heart of the highland we called the Terra Cimmeria, yes.’

‘Which was no doubt why Earthshine chose it,’ the ColU said from Chu’s backpack, ‘because of that resonance. Everything Earthshine does will be shaped by an awareness of competing realities. And it is also, no doubt, why the site of a city that was called Obelisk for its greatest single building should be marked here by – point for me, Chu Yuen, left and down – that.’ The slave seemed to work well with the master he carried; his finger stabbed down on the image of one of the three clusters of domed buildings.

Stef peered down. ‘I see a sharp stripe on the ground. Wait – where is the sun? That’s a shadow, of something very tall—’

‘A tree,’ the ColU said. ‘Not an obelisk. A tree. Encouraged to grow to some four hundred metres, which is three times the maximum theoretical height on Terra. A tree’s height is limited by the need to lift water to its uppermost leaves—’

‘But on Mars, with its one-third gravity,’ Stef said, ‘you can grow as tall as this. It must have been force-grown.’

‘Yes. Earthshine has been established on Mars for some years, but not that long. Force-grown and encased in some kind of enclosure to retain air and moisture. We don’t have good enough images to determine the species yet. An impressive stunt.’

Beth leaned closer to see. Beth and Mardina had been quiet since Penny’s brief visit to the bridge. Only Ari had been quieter, Stef thought; the druidh had not spoken a word.

Now Beth asked, ‘But why would Earthshine grow a tree on Mars? It doesn’t seem to fit.’

‘It’s for his allies,’ said Kerys grimly. The nauarchus had also been quiet during this voyage on a Roman ship, Stef had observed, but she had watched and listened, evidently filing everything away. Now she pointed to another shadow traced on the Martian ground, in a second compound some distance to the north of the tree. ‘That is a ship – a ship of the Brikanti Navy, called the Celyn. Earthshine has at least one ship’s company’s worth of support on the ground with him, and most of them drawn from Brikanti ranks.’ She glared at Quintus, defiant. ‘We don’t have time for blame games. This monster, this Earthshine, was after all found, fortuitously, by a Brikanti ship – my ship, all those years ago. How I wish now we had simply thrown the boxes that sustain him out into the Skull of Ymir! Even if we had preserved the rest of you.’

‘Thanks,’ Beth said drily.

‘It was natural that as he began to lay out his schemes for the exploitation of other worlds, he would gather support from the Brikanti government at first. We believed we could control the situation – control him.’

‘Well, you were wrong,’ Quintus said.

‘It began with his subversion of the crews of the ships we sent out to support him. He persuaded them to betray their nation – to follow dreams of greed and power, under him. That is what we believe happened. But they are Brikanti.’

‘Ah,’ Stef said. ‘I’ve been reading up on this during the journey home. To the druidh, in the Brikanti tradition, the tree is a sacred symbol.’

Ari spoke now. ‘Whatever other projects they are pursuing, they will have relished the chance to nurture what may still be the only tree on Mars, and certainly the greatest – greater than any on Earth. Even Christians would respond to the symbol. You Romans nailed Christ to a wooden cross, and His blood nurtured the roots of the World Tree Yggdrasil, which—’

‘Yes, yes,’ Quintus said impatiently. ‘Hardly the time for a theology lesson, druidh. So – the holy tree. And around it, as you see, a series of domed habitats that we believe are residences for Earthshine’s human supporters, or most of them, along with workshops, stores. To the north, and a reasonably safe distance away from the tree, you see the Celyn standing, and accompanying support facilities for a kernel craft. Room for others to land too, and we have seen craft shuttling between Mars and Höd in the last few years.’

‘Relief crews,’ Kerys said. ‘There are teams working up on Höd, manning the kernel banks there. They seem to be swapped every month or so.’

Quintus said, ‘And we believe that Earthshine himself, or at least the gadgets that support him, must be here.’ And he pointed to the third complex of buildings.

Stef leaned down to see better, silently cursing ageing eyes. ‘More domes. But the heart of it is that tilted rectangular slab.’

‘A reinforced bunker,’ Quintus said. ‘A familiar design. Hardened against our ground-based weapons, hardened even against any rock pushed from orbit short of anything massive enough to destroy the whole site altogether. No doubt Earthshine is down in a hole deeper still.’