The older crew members of the Linden Moon – flight engineers, commanders, even security personnel – woke to the soft, smooth notes of classical music from distant times and places. Beyond these airwaves, behind the doors separating them from the dense but navigable maze of platforms and ladders, were the weakening cries of delight which the younger, more experimental crew always indulged in every “night”. Out of the guiding light of any nearby star or rotating planet, local timekeeping defaulted to a standard 24-hour cycle agreed upon by physical and biological systems. “In the end, nobody is forced to go to bed or wake up”, Kellam Hazel wrote in her personal journal, minimally attentive from the outside din. “Whenever a phase shift needs to happen, it happens”.

Kellam, the inquisitive fox who felt an affinity for every minute shift in the ship’s culture and atmosphere, slid her paw on the pen which had wrote her all the way from the selection test for this mission, itself an experiment in using a black hole for a gravitational slingshot manoeuvre. It had been done many times around less massive bodies to overcome the power limitations of on-board rockets and access objects as diverse as mineral-rich asteroids and spectacular comets. In contrast, the ultimate wielder of gravity had only been exploited once. “Six months ago,” she recalled, turning to a previous page.

The objective there was simply to make a U-turn and collect data on the process. While the crew returned in one piece, many complications arose: they flew a bit too close and spun around twice, firing emergency boosters on the second loop to prevent “eternal damnation” within the event horizon. The exhaust thus emitted burst into pretty colours falling inwards, unacceptable within the strictly economising philosophy of deep space travel. Some foxes were admonished upon return for underestimating the risks.

“Closest approach to black hole Lavandie is within three hours local time”, first officer Alvin Malthuse announced at breakfast. He made it clear that observers would perceive their venture more slowly because of their current speed and the gravitational field around the black hole contributing to time dilation. Most of the crew could remember solving relativity problems in training with only calculator beside, a tiring and error-prone activity now conducted by admittedly simple linear algebra programs. Despite this convenience, understanding the concepts was still vital to keeping interplanetary networks in split-second synchrony or even pinpointing a location. Even after five years as an astronaut, Kellam still found pleasure in assembling different yet consistent timelines of the same events, depending on the observer.

The lightless star of the show had already been felt by the Linden Moon’s external accelerometers. As for internals, much of the mission so far involved spinning its roughly cylindrical body around its centre to generate an appreciable gravity, the living quarters occupying a sector of its boundary. Such a design was absent from the previous black hole slingshot mission, resulting in piled-up bodies, inaccessible locations and an overall lack of control when it mattered. Now they would simply stop spinning and let the black hole supply gravity around periapsis, as if they were piloting a normal aeroplane. It was quite a wasteful idea – the central axle was weightless and mechanical – but in the absence of a theory of gravitons, that was the best the ground engineers could do.

Coffee and other stimulants were not recommended for staying alert in space, since they compromised motor coordination for long enough after the initial high to negatively interfere with operating the complex, interdependent onboard systems. Contemplating the planned trajectory in front of her, Kellam took an appreciation of the higher dangers specific to this mission – there was a window the ship had to pass through within one minute of a specific time to ensure safe passage, a situation shared with satellites probing the mass distribution of planets. Then a bead of sweat fell onto the plan, which she wiped off. Keep running, keep running… echoed the voices in her head, fighting the treadmill’s whirring and zizzing.

Lavandie’s surroundings were captured by three rings of cameras, each tuned for a different range of wavelengths, attached to the non-rotating superstructure. For now, it was a pretty view: the companion star which had helped to discover Lavandie itself contributed to an accretion disc the crew were approaching edge-on, with blueshifted X-rays thrown off in its collapse. Behind the event horizon, an otherwise dim starscape lensed into redshifted streaks of light. Of course, the attainment of such extremes implied the entire spectrum’s existence, a sea of white in the visible portion. Yet all three colours were dimming against a growing black circle. Blue, white and red, that’s what space looks like outside black.

A necklace bearing a portrait of another vixen, second officer Schiaparelli, jingled up and down, though its motions were somewhat attenuated. Kellam had kept in contact with her since the start of this mission, dining with her, drawing scenes of their home planet Sidritti together, bonding in the serene weightless interior when mechanical problems arose. Schiaparelli was of lower rank though; this limited the chief relativistic navigator’s opportunities, and when the overhead speakers rang to the effect of “please take your positions” she felt her heartstrings tugged a little.

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“Stopping general rotation now?” Alvin asked the captain of the Linden Moon, Darien Nenndil. His fur brushed the button panels, but this was fine since they were on autopilot, whose controls lay elsewhere on the deck.

“Kellam, how’s the local field?” Darien polled.

“Twenty minutes until Lavandie overtakes artificial gravity. We’re clear.”

“Beginning lateral thruster fire.” The keyed operations displayed on Alvin’s screen were confirmed, both by himself and Darien, and the thrusters boomed.

Stellar black holes had too strong a variation in their gravitational field to be safely used for slingshots. Lavandie had twenty thousand solar masses and (like the majority of black holes in the universe) a high angular momentum, generating an ergosphere indicating which direction should be followed. Contrary to popular depictions, modelling the ergosphere by an oblate spheroid was only accurate for slowly spinning black holes; here it was shaped more like a red blood cell whose “poles” were clamped together by the event horizon, giving extra equatorial distance to manipulate the ship. The Penrose process enabled extracting energy from such an object, but something had to be sacrificed inside the ergosphere for this and the mission did not call for that. Instead, this was merely a simple fly-by at a reasonable distance where gravity remained at a tolerable level.

Between the black hole and the straightening spaceship there was a moment where Kellam reconnected with the subtleties of space. Her greyish-red fur rippled in patches, caught between two domains, while her internal organs loosened up, relieving some measure of stress. “Those were the days… when spaceflight was just an extreme variation of air travel,” the words playing into nearby headsets, “when those pioneers were having so much fun to themselves.”

“What do you say of this, dear?” Schiaparelli tossed into the air.

“We may be having our moment, but what was a novelty then is bread-and-butter now. Don’t think of me, think of what’s coming next.” With Lavandie’s gravity settling in as designed, the three spatial dimensions were locally reduced to two.

Despite most of its mission operating under some kind of gravity, there were still periods when the Linden Moon had to operate without it. Besides the transition across Lavandie, there were also the post-launch and pre-landing schedules where rotation had to be lowered. Terrestrial amenities, albeit with many modifications to accommodate weightlessness, included a dining hall, a kitchen, toilets and a table tennis room; the tragic poem Aniara, about the eponymous spaceship losing similar amenities and ultimately itself, was read daily by Kellam and Schiaparelli. Fluids were always kept in containers and drunk with straws, whereas gases were strictly controlled and treated where unavoidable to provide cabin oxygen.

Seen through the forward cupola, many more background stars curved across the boundary of an intruding photon sphere – the black of the black hole reaching out to one-and-a-half Schwarzschild radii. The accretion disc grew into a streaming curtain of diffuse matter on the right. Its electromagnetic emissions were not very harmful, being redshifted to lower energies, but they showered the spaceship in their radiation and interfered with its radio antennae. Even if messages could be sent out, they would themselves fly in the wrong direction or fall towards the singularity and dissipate into Hawking radiation. Thus the crew continued, alone and chasing across the hostile environment of a colour-inverted planet, not unlike the collaborative forest hunts so ingrained in their species’ culture. Was it this memory that compelled Kellam, among innumerable others, to reach for “the final frontier” only to discover many more? It never crossed her mind.

“Threading the window,” she announced. It lay just beyond the accretion disc at periapsis. Features of the curtain differentiated themselves to the crew approaching at ten degrees, at first by variations in intensity, then splitting of colours. To minimise potential damage to the Linden Moon from the hot, swirling matter, a thinner region was desired; Alvin was responsible for the fine corrections towards that goal. Levers were clutched, breathing steady, eyes wide open…

Bare metal struck the gases, buffeting the spaceship’s forward motion. For around five seconds the crew endured, at once surrendered to exotic beauty and trepid at the possibility of something failing somewhere through. The embrace with Lavandie was at the tips of their tongues, and neither did they know how much time passed for external observers, at least before their return to flat space.

Just as they began to comprehend it all, the relative darkness returned. Without a word, Kellam and Schiaparelli yanked the main rocket’s accelerators, zipping through the window towards their ultimate destination. While the spatial window was narrow enough, the temporal window was closer to the proverbial needle’s eye – one second off and they would be flung in the opposite direction, or maybe beneath the event horizon. Their eyes stared into each other’s, unconcerned about the complexities they had overcome together. The pupils within dilated.

“You two vixens, get back to work!” Darien interrupted. Having left Lavandie’s gravitational well, the Linden Moon remained to be re-spun for its outwards journey, a task as simple as reversing the insertion process using the opposite set of lateral thrusters. There were no definite spaceways this far out from civilisation, but Kellam hugged herself believing she had just exited one.

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Much jubilation filled the air at the press conference on Benvo. “More than a success,” Alvin boasted, pointing to the spectacular images captured inside the accretion disk and beyond the black hole itself. Radio messages from Sidritti were expressing relief after waiting for more of their time than expected; they received replies of understanding that reiterated the counterintuitiveness of general relativity, even to more experienced space personnel.

The two foxes sitting to Alvin’s left blushed simultaneously. “As a matter of fact,” began Schiaparelli, speaking to the cameras, “we’ve perhaps dedicated a little too much time to our duties. For now we can celebrate the proving of a novel, powerful transportation device. Kellam.” A collective gasp broke the silence. “Will you marry me?”

Against blinding flashes, the recipient smiled. “Why… yes.”

There was a kiss, broadcast across the vacuums, their flight of the black hole.