The Arrows of Time

‘If it comes loose that’s not the end of the world,’ Tarquinia stressed. ‘It can always fly back and reattach itself.’

Agata said, ‘Try the walking mode.’

Tarquinia tapped the remote. The four bits remained fixed in the stone, but the plate on which the drills were mounted began rotating on the end of its arm – or rather, the arm began rotating around the plate, swinging the entire occulter forward, carrying it from Agata’s end of the slab towards Ramiro’s.

When this repositioning was complete, the four drills at the end of the second arm pushed up against the stone and began biting into it. The new quartet managed to gain purchase with only the first set bracing them; there was no need to start up the air jets again. Then the first four went into reverse and disengaged from the stone, and the whole process began to repeat itself.

Agata watched anxiously as the machine whirred and clanked its way down the slope from her shoulder to Ramiro’s. If the slab was unrealistically smooth, at least they’d made sure that it wasn’t gravitationally level.

When the occulter had come within a span of Ramiro’s body, Tarquinia used the remote again. The craft freed itself from the slab and flew away to alight on the cabin floor. Ramiro looked to Agata, and they carefully put the slab down together.

‘Not bad,’ Tarquinia declared.

Ramiro said, ‘No. But we still need to decide what happens when the surface is uneven.’

Tarquinia had already reached her position on that. ‘It should detour around the problem if it can, or drop away and fly past it if it can’t. That makes it purely a question of navigation.’

‘And a question of air,’ Ramiro corrected her.

‘Whatever we do,’ Tarquinia replied, ‘there’ll always be a chance of the air running out. Letting the arms tilt so they can conform to the surface won’t guarantee anything – and it’s one more joint that can jam, two more actuators that can leak, plus six more sensors to make the idea work at all.’

Ramiro turned to Agata. ‘It looks as if it’s your vote.’

‘Can we model the air use for different scenarios?’ she wondered. ‘Take a guess about the roughness of the mountainside, and see what the chances are that we can get these things from the dock to the antipode with air still in the tank, for each design?’

Ramiro said, ‘I can try, if you want to help me with the model. “Roughness” isn’t the easiest thing to quantify, but you’re the expert on curvature.’

Agata sat beside him and they spent the next three bells working through the problem. In the end, with some plausible assumptions, there was a chance of about five in a gross that the current version of the occulter would run out of air before it had completed its task. With a new model where the arms could fold together or bend apart – allowing it to keep its grip in bumpier regions – that fell to three in a gross.

Ramiro said, ‘That’s for a single machine. But even if we build half a dozen spares, we can’t afford too many failures.’

Tarquinia had been doing calculations of her own. ‘You need to add in the chance of the modification itself leading to a failure. I get two in a gross for that.’ Ramiro looked sceptical, but when he went through her numbers he couldn’t fault them.

‘Five in a gross… versus five in a gross.’ Agata couldn’t see how to break the tie. None of these numbers were precise, but without more information she couldn’t make the uncertainties any clearer.

She looked across the cabin at the occulter. Their encounter with the Hurtler and their bomb-removal project offered plausible excuses for all sorts of items ending up in the void, or lost in the dust of Esilio – but she was already afraid that their depleted stores might attract suspicion. Ramiro’s proposed changes would require dozens more proximity sensors – spares that ought to have been packed away neatly in a single large box. Why would they have taken that box out of the storeroom for safekeeping, but then never brought it back?

‘I’m voting with Tarquinia,’ she said. ‘What we’ve got now is physically robust – and we’ll already be hard pressed to build and test the whole swarm before we arrive. This isn’t the time to start making things more complicated.’

For a moment Ramiro looked poised to respond with a further plea, but then he was silent.

Tarquinia said, ‘It’s good to have that settled. Everyone should get some sleep now, and tomorrow we’ll go into production.’

‘Do you want some fresh pictures for your wall?’ Azelio asked, offering Agata a sheaf of papers.

She took them from him. The first drawing showed the mountain coming into view through the window of the Surveyor, with Luisa and Lorenzo standing on the summit waving, very much not to scale. In the second, they’d thrown out a docking rope to the craft and were reeling it in by hand. ‘These are great,’ she said. ‘Thank you.’

Azelio lingered in the doorway. ‘Come in for a while,’ she suggested. He followed her into the cabin. There was only one chair, so she sat on the edge of the desk.