‘Well . . . don’t,’ Tarquinia replied irritably.

‘I’m sorry,’ Agata repeated.

The point of the exercise had been to try to use her cooling bag’s air tank as an improvised jet. She’d understood perfectly what the prerequisites for a successful burst would be: a tight grip on the tank and a thrust aimed straight at her centre of mass. But she’d held the tank wrongly, or slipped, or panicked.

She wasn’t actually going to die of hyperthermia; she had two small emergency canisters strapped to her belt. She managed to detach one and connect it to the bag’s inlet without mishap. The cool rush of air felt unearned, but then, if the Surveyor broke apart halfway through the mission, casting them all out into the void, she doubted that a few dextrous manoeuvres with her air tank would be enough to save her.

Agata saw a figure approaching out of the mountain’s shadow, a little closer each time it spun into view. It was easy for Tarquinia: she had one of the new jetpacks strapped to her body. The six nozzles were all controlled by photonics, their thrust automatically balanced so they imposed no torque at all. Bulky as the things were, Agata decided that she was going to wear her own pack for the entire twelve years of the mission, rendering these dispiriting training exercises redundant.

Tarquinia collided with her roughly, grabbing hold of Agata’s arm. Agata’s gut twitched; she was tumbling in a completely different plane now, and the sudden shift in the flow of stars across her vision was more wrenching than the impact. Tarquinia seized Agata’s other shoulder and embraced her, pulling the two of them into equal intimacy with the slab of equipment that covered Tarquinia’s chest. Then she must have told the jetpack to kill their rotation: the torque itself was imperceptible, but it looked as if someone had slammed a giant brake against the spinning black bowl that held the stars.

When the sky had ceased turning, Tarquinia released her grip and hooked Agata’s belt to the front part of the jetpack.

‘Are you all right?’ she asked, less brusque than usual.

‘Yes.’ Agata realised that she’d been shivering.

‘I know it isn’t easy, but you have to reach the point where things like this are just instinctive.’

‘I understand.’ Agata gazed past her into the dark hemisphere that had once held the orthogonal stars. They were still out there, in the blackness, but her own eyes were now emitting the light she no longer received from them. ‘I don’t think I can do this,’ she said. ‘I think I made a mistake.’

Tarquinia interposed her helmet into the view. ‘You want those light-deflection measurements, don’t you? I thought the fate of the cosmos hung on those fractions of an arc-flicker.’

‘You’re the astronomer. You could do a better job at that than I could.’