Leanne had three hours of oxygen remaining. The display on her wrist had been announcing the drop-off every five minutes, callously reading her strained vital signs. Now, she jabbed her finger onto the controls and turned the volume down.

Her breathing filled her helmet. With every bit of air escaping her lips, she imagined the CO2 scrubbers working, the internal fans whirring to spread her dwindling life support. She had to concentrate on that. If she tried to take in everything around her, her mind would collapse in on itself.

Darkness loomed like a living presence on each side. It was only through the automatic guidance system on her suit that she knew whether she faced up or down. She seemed to be doing both at once. Her torch bled through the shadows and found what could be a ceiling or a walkway. She half-floated below it, clinging to grates when they appeared beside her gloved hands or pushing herself along through the zero-gee. She had no idea how far the drop stretched beneath her dangling feet.

From the external appearance of the ship, she reckoned she was in the long central hub. It stretched from stem to stern, split into compartments, with the great centrifuge arcing around it. Endymion station had a small, experimental centrifuge in one of its labs – a glorified hamster's wheel for simulating gravitational forces. It was nothing like what Leanne had seen of the Chimera's elaborate structure. Even in its slumber, Leanne felt the terrifying power of the unknown.

The unknown, cut off from all comms and with her companion... With her companion dead. Leanne could not make herself think it or believe it. She expected at any moment for her radio to crackle and for Cliff's voice to come through it. What she would have given to hear him humming that inane song about coming home again.

She closed her eyes for a moment and tried to ground herself. First: find the command centre of the ship. Second: try to restore power, even if it was auxiliary, backup, anything to get the juices flowing. Third: pull this hulk of metal into a stable orbit. If she didn't do that, oxygen deprivation and unknown shadows would be the last of her worries.

She kept moving, chasing the arc of her torchlight, until her foot nudged against something. She willed herself to let go of the handholds, pushing away so she up-ended. Or maybe this was the right way.

She found a small door, closed with a manual portal lock. An experimental spin of it made the latches click hopefully. It didn't take much more effort to pull open. Her light bled through the tight gap.

She thought back to her days at the BASE's Aquatic Training Centre. She had learned to manoeuvre in and out of modules flooded with water. It was the best way to simulate zero-gravity movements, and to prepare astronauts for the dreaded possibility of a sea-landing going wrong. She had been there with Cliff, side-by-side in the submersed tanks. It had always been two minimum in the training. Two minimum. Never one. Never alone. Not like now.

Leanne forced the thought from her head. This mission had to continue. Cliff would have said that. And if she did not survive, there would be no one to inform his wife and children about what had happened. She would not allow the bureaucratic BASE teams do so: a standard, unemotional speech about astronauts giving their life to the spirit of adventure and the future of humanity. She wanted to be there, to hold Joanne's hand and grieve alongside her.

Who would be told if Leanne perished? No one waited for her back home. Wherever home was now.

"Goddammit, Ziegler, pull yourself together," she hissed to herself.

Holding her breath, she twisted her body to get through the gap of the ajar door. Her oxygen cylinders scraped against the rim of the portal.

Her hands brushed against the floor, or the ceiling, of another module. She rotated her torch and found herself in a long round corridor, stretching off beyond the reach of her light. Apart from her breathing, utter silence loomed. She almost wished there would be a noise – anything – to break this void. Almost as if to prove to herself that she was actually here, and this wasn't all some elaborate illusion.

She felt for handholds nearby. Little by little, she pulled herself along the corridor. Other doors lined the curved wall, leading to unknown parts. But the only one she cared about right now was the hatch at the end of the hall. From what she could recall, the emergency airlock she had come through had been in the second of the ship's three large modules. She assumed the foremost sector was the command centre.

She pushed up the lock of the portal, relieved to hear it open. This one slid upwards, straight into the ceiling. She ducked through and felt her breath leave her.

She stared directly down at the dusty surface of Mars. The connecting tube she had just entered to was utterly transparent, the polycarbonate so clean that she felt nothing separated her from the vacuum. The dark splotches that decorated the Tharsis region sat right below her feet, as though she had left marks on the brown-red landscape. The enormous shield volcanoes were just gentle rises from up here, surrounded by scattered pockmarks on the rolling dome.

Also down there, she knew, lurked the great Martian guns, trained on the anomalous ship. Nerio had informed the BASE that any investigatory team would have ten days. Then they would scatter this vessel into oblivion.

Leanne slowly turned, not daring to look down again. She kept her eyes ahead on the next hatch. To her relief, there was a metal platform not far ahead. She pulled herself onto it and clung to the handhold by the door. A keycard slot occupied the space next to it.

"Good afternoon," a female voice suddenly said. Leanne just about stopped herself from yelling. She strained around, but she was still alone in the tunnel.

"Who's there?"

"My name is Valkyrie," the unseen tone continued. It had a soft transatlantic accent. "Please state your name."

Leanne swallowed past her dry throat. "My name is Leanne Ziegler, out of Wells station, originally from Helios. I have been sent by the United Worlds Federation Body for Air and Space Exploration to investigate the nature of this anomalous ship."

There was a long pause. Leanne continued. "I request entry to the cockpit. If unchecked, this ship will soon decay into Mars and burn up."

Whatever intelligence she was talking to, it must have been swayed by that, computer or not. But Valkyrie simply said, "Only authorised personnel are allowed access to the cockpit."

"I have a warrant. I have it somewhere..." It was stored in one of her exosuit pockets. The BASE had drafted a document in the event that she and Cliff had entered the ship and found it already manned. Cliff, oh god. He had been the commander; he should have been here.

"Are you an employee of the BASE?" Valkyrie asked.

"Yes. Leanne Ziegler. I have worked for them for ten years. I have been stationed on Helios for three years in total, and I had a tour on Endymion, and for the past year, I have been at Wells station. I was here when this ship...appeared." That was the only way she could describe it. "You have entered into Martian airspace, and Nerio colony requests information on your purpose and mission."

"This ship is registered to the BASE."

"No. No, you are mistaken. I have been sent to investigate it as an anomalous vessel."

"I assure you, I am not mistaken."

Leanne shook her head. She couldn't stand here and play cat-and-mouse with a computer. She had work to do beyond this door. "Please, I request entry to the cockpit. Course correction manoeuvres have to be performed."

Again, Valkyrie paused. Leanne could see no lens or tubing, nothing to tell of the existence of this omnipotent AI. And yet she felt it, the presence of a second intelligence up here in the gulf.

"Please, hold up your identification," Valkyrie eventually said.

Leanne felt around for her card. Every astronaut carried one about their person now, whether on an EVA or working around inside a station or even performing a menial task such as preparing the laundry. The potential for all to go wrong lurked everywhere. These ID cards, imprinted with the holder's DNA, were like dog-tags. "Where should I hold it?" she asked. "I can't see any cameras."

"Just hold it up. I will see it."

That sounded ominous. But, with no other choice, Leanne did as she was asked. Seconds ticked over. She could almost sense herself being turned about and dissected.

"Very good," Valkyrie announced. "You may enter."

The door slid open.

The cockpit was unlike any other BASE forward deck Leanne had ever been in. Truth was, she had never pursued a career of being an interplanetary pilot. Many BASE vessels were automated now, a long way from the early days of spaceflight. All she had steered were small shuttle crafts. Cliff had been the pilot, head-hunted from the RAF by the BASE. But even he would have been taken aback by this cockpit.

The only console decks were the ones upon a central plinth. They looked far too simplistic for such a huge ship as this. The damn thing was as big as the Endymion lunar orbiter itself, and yet its only visible means of manoeuvring were the three screens and their accompanying input keyboards and levers in the middle. The rest of the space was open-plan, even more intimidating than if the place had been full of unknown and bewildering machinery. Slots dotted the surrounding walls, which Leanne assumed slid out to reveal further controls.

She gently pulled herself in, thankful for the handholds. The front windows faced the red arc of Mars, an ever-present worry. She forced herself to look away and at the computer deck. To her surprise, they lit up with a soft blue light as she approached. The three screens were interconnected, the software extending across them. She tapped her finger on the middle one but nothing happened.

"The system is under a strict security system," Valkyrie explained. "Only authorised crew personnel can access it, under the approval of Commander Stefania Malinowska."

"Commander Malinowska?" Leanne had never heard the name.

"However," Valkyrie continued, regardless, "I can access relevant systems. I have the highest security clearance onboard."

"I need you to access the guidance and navigation controls."

Another long moment of silence stretched. With every one, Leanne became more aware of the minutes dropping off of her oxygen reserves. If Valkyrie was as omnipotent as it sounded, it would have known about that. It also would have known about Cliff, and Phaethon, and Leanne's entry to the ship.

At last, motion. The screens faded then came back to life with the ship's projected trajectory. It dipped down into the red. The Chimera sat at 98km, still below the line of the exosphere where atmosphere trailed into vacuum. "Alright," Leanne said to herself, strapping into the central chair. "Give me the thruster controls."

Valkyrie obeyed silently. Leanne could feel the computer's judgement, weighing up her every move. It was like she was in training again, her commanders peering over her shoulder. She conjured up what had been drilled into her before leaving Wells station. Safe orbit around Mars needed to be over 200km altitude, at least. Wells station itself was at 500km and employed regular station keeping manoeuvres and aerobraking to keep it secure. Leanne had to get the Chimera above where the atmospheric drag and gravitational pressure currently slowed the ship's orbit.

The control panel which flickered to life on the console before her might as well have been written in alien pictograms. The only things she recognised were the equations of the Chimera's trajectory and the Delta-V required for the manoeuvre. Everything else – all the bars, all the percentages, all the labels – tangled in a blur.

"Valkyrie," she found herself saying. "I have no idea what this means. Where are the thrusters?"

Without a word, Valkyrie highlighted a small section of the panel. It still made no sense to Leanne.

"This is nothing like I've seen in other BASE vessels. What on earth is this ship powered by?"

"I'm sorry, I cannot divulge that information."

There were secrets everywhere on this ship. Leanne had no answers.

"Can you divulge how I'm meant to stop this ship from burning up in the Martian atmosphere?"

"I have the highest security clearance onboard," Valkyrie repeated from earlier. "I can control everything on this ship."

"Help me raise her into a safe orbit. I don't know... I don't know how to control these things."

Leanne felt helpless as she admitted it, but she had no choice. It was either rely on this invisible AI or fail her mission.

"Do you grant me permission to move this ship?"

"Yes, yes. Please."

But no sooner had she submitted the pathetic request, did the ship shudder. Leanne instinctively grabbed for the nearest handhold and Valkyrie chided, "Please keep your hands away from the controls. You might feel a slight discomfort as we raise to a higher altitude but I assure you, there is no danger. Thrusters A, C and E, ignition in three, two, one..."

Leanne's stomach rose as the cockpit lifted around her. She thought for a second she should have unclipped herself so the lack of gravity would counteract the sudden yaw. The Chimera tilted to starboard as she rose, a deep rumbling coming from somewhere in the lower decks. Outside the windows, Leanne looked down at the burnished eye of Mars receding, the surface dropping further away. Nausea swam inside of her, not only due to the motion but the idea of everything familiar retreating. She was alone in this strange vessel with an unknown AI. Cliff was gone. Phaethon was still out there.

And with a terrifying jolt, Leanne realised: the rogue signal had come from inside the Chimera. Only Valkyrie could have prompted it.

She had put the controls of the ship into Valkyrie's hands.

Word count: 2407
Overall word count: 4991

My second ONC shoutout goes to Shivran86 whose ONC entry, Sikander, Don't Die, is an absolutely gorgeous, heart-wrenching historical romance set in Persia. A beautiful piece of writing!