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.