Greg Egan - The Arrows of Time

There was no time for more than the basics, but if they could retain it, it ought to keep them alive. Agata put on her own equipment.

‘Does everyone understand what we need to do with the occulters?’ The protocol she’d written had been copied discreetly from skin to skin, and some of the volunteers would not have received it until they’d reached this assembly point. In a perfect world they would have rehearsed the manoeuvre daily for a stint or two, but at least the jetpacks would handle most of the navigation.

‘Can the machines drill into our bodies?’ a young man asked anxiously.

‘Not intentionally,’ Agata assured him. ‘They’re not that sophisticated; they have no defensive strategies as such. The only danger is if they’re so confused that they mistake you for rock, but if you get out of their way they won’t pursue you.’

Serena passed Agata a helmet. They were aiming not to use the links; this would probably be their last chance to talk until they were back in the mountain again.

‘Happy Travellers’ Day,’ Serena said.

‘Happy Travellers’ Day,’ Agata replied. She put on her helmet and turned towards the maintenance hatch.

A succession of shutters sealed off portions of the final length of the cooling tunnel, opening in sequence to allow air to pass from chamber to chamber at ever lower pressures until it was expelled into the void. The maintenance hatch wasn’t meant to open unless the whole cycle had been stopped and all the chambers had reached the ambient pressure of the mountain’s interior, but Serena’s technician friends had managed to fake the sensor data to convince the hatch that it was safe to operate. The only catch was that it had been too complicated to try to lock it against any real part of the cycle. It would be up to each person exiting to synchronise their access with a time when the shutter below them wasn’t open to the void.

Agata pressed her helmet to the hatch and listened to the sequence of clanks and hisses until the rhythm was embedded in her mind. The last time she’d dealt with machinery in the tunnels it hadn’t ended well, but at least she’d had the timing right.

She slid the hatch open. Air blew in from behind her, but it only took a flicker for the pressure to equalise. She climbed into the tunnel and braced herself against the walls with her hands and feet. Serena closed the hatch behind her.

Agata waited in the dark, mentally composed but still viscerally terrified: there was absolutely nothing about the situation that her body found acceptable. She heard the creaking of the shutters above her and the sibilance of expanding gas drawing nearer.

A span from her head, the rotating disc of the shutter above her finally swung its aperture around to coincide with the tunnel. Agata felt the air rising up across her cooling bag, moving the opposite way to the usual cycle now that she’d wrecked the pressure gradient. But the sensors had been numbed – the anomaly would pass unnoticed.

When the shutter closed there was nothing to feel, and in the perfect darkness no way to be sure that it had happened. But then the exiting wind rustled the fabric on her limbs and starlight entered the tunnel from below. Agata didn’t look down for confirmation; she just brought her limbs together and let herself fall.

Half a dozen strides from the outlet she drew a circle on her chest and the jetpack eased her to a halt, supporting her as she followed the rotation of the Peerless . She sketched an upwards arrow and ascended, until a safety handle set into the rock for maintenance workers came within reach. Wide-field cameras on the slopes monitored the space around the mountain, with their feeds analysed automatically to detect anyone in trouble falling away into the void, but as long as the team remained close to the hull they’d be out of view.

Agata waited for Serena to emerge, and for two more of their companions to join them. They couldn’t stay to watch the whole team exit; there were only four handles, and hovering would waste too much air. Agata gestured to the others and they set off for the base.

The starlit slopes turned beneath them, the pale brown calmstone sliding past ever faster as they moved further from the axis, making their straight-line trajectories seem like giddy spirals. Agata kept watch for sudden changes in the topography ahead; the jetpacks knew the basic shape and motion of the Peerless , but they carried neither detailed surface maps nor proximity sensors. No one had ever intended the wearers to skim the slopes at high speeds, so if she slammed into an unexpected rise she’d only have her failing eyesight to blame.

The jetpack overshot the rim of the base, and the rock below her fell away to be replaced by black emptiness then a sudden, shocking dawn. She could see the blazing lights of the half of the transition circle that the mountain had been blocking, just peeking above the distant horizon. Agata sketched a down-arrow and the jetpack brought her closer to the rock; the dawn ahead vanished, but she still had the other half of the circle behind her. The base was slightly convex, so, however low she dropped, the Peerless wouldn’t hide the entire transition circle, plunging her into complete darkness.

She looked around for the others; she could just make out Serena away to her left and another anti-saboteur to her right. The jetpacks had been programmed to take the volunteers to equally spaced points above the rim, but from here it was up to each of them to choose their path on a sweep in towards the axis.

The rock below her was a blur, moving past at more than a saunter every pause, but if she tried to match its velocity she’d be constantly using her jets to provide the necessary centripetal force – which would empty her air tank long before she got halfway to the axis. In all her training exercises with Tarquinia, in all the manoeuvres she’d performed around the Surveyor , she’d never faced anything like this.

The occulters could be waiting almost anywhere; in their final flight they could cover a lot of distance quickly. She couldn’t assume that they’d all crawled close to their targets. So she had to find a compromise: she had to slow the relative motion of the rock just enough so she’d be sure to notice one of the machines sweeping by, and then try to move closer to the axis as quickly as she could, lessening the drain on the jets.

Agata turned herself around so she was facing the surface at an angle where she caught the reflection of the stars. The grey blur shimmered with colour now, the texture visible if still mysterious. She sketched a double arrow to begin reducing her relative motion; the jetpack complied, though it sent a warning message through her corset, alerting her to the cost.

She waited anxiously, afraid that this unrehearsed strategy would get her nowhere, but then the transition came in an instant: suddenly, the shimmer from the rock was comprehensible. Agata could see the small bumps and concavities, the fine crevices, an endless parade of details rushing by beneath her. The occulters had been clad well enough to blend in with the stone around them from a distance, but she was only a couple of strides from the surface. If she kept her concentration, the machines ought to be unmissable.

The mountain completed each rotation in less than seven lapses; her motion was stretching that out threefold, but rather than hanging back to witness a full turn at every radius she had to trust her companions to cover their own portions of the territory. Agata wished she could have made it a mathematical certainty that no square scant would go unsearched, but before she’d been out here and seen the conditions she’d been in no position to make binding plans. All she could do now was hope that most of the team found their own workable strategies, and between the symmetry of their initial placement and the shared conditions that prompted their individual actions they’d end up executing a combined sweep without too many gaps.