crew compartment.
For the past few days, Mercury had been growing in the long-range camera displays, from a brilliant crescent rising out of the Sun’s glare, waxing to a perfect half-full as the ship chased the planet along its orbit. Now the planet lay directly ahead of them, filling the sky, bisected at the line between night and day.
The tug was turning round, aligning itself so that the engine pointed forward, ready to fire against their direction of motion and brake them into orbit round Mercury. Far away on the body of the tug, the thrusters burned again briefly, and the tug slowed and stopped in its new attitude, accurate to a second of arc.
The six large solar panels and the giant sunshade, that had shielded them from the relentless glare of the Sun over the past three months, started to fold up now. The panels and sunshade were too fragile to withstand the forces of the orbit insertion burn, and the tug could manage without the sunshade for a short time, before the Sun’s heat made it essential to spread the protective shadow around the vessel again.
Monitors on the command deck showed the crew the forward view from one of the inspection cameras. The four passengers watched the scene in fascination. Lit at an extreme angle by the Sun, the stark shadows of craters and scarps gave the planet’s edge a strange, half-eaten appearance. From this distance, it was impossible to believe that they were going to miss the planet; it filled the field of view, and the tug seemed to be plummeting headlong towards its centre. The navigation displays, however, told the true story; their flight path would take them over the North Pole, missing Mercury by a scant 200 kilometres.
The minutes to ignition counted down.
In the tense quiet of the command deck, Clare spoke.
‘Okay, electrical power to batteries. Shut down power generators.’
Wilson punched switches on the engine controller panel, watching as the power levels fell. On another console, he brought up the fuel system, readying the giant tanks of ammonia and the turbopumps to deliver hundreds of tonnes of fuel to the engine.
‘Arm engine for firing.’ Clare spoke quietly, watching the approach display and the minutes and seconds to the orbit insertion.
‘Control drums armed. Fuel system is pressurised and armed. Safeties off.’ He looked across at Clare. ‘Ready to bring up power.’
‘Reactor to fast idle.’
Wilson flipped a safety cover off a switch and pressed it, holding it down while the reactor power increased, ready for the insertion burn.
‘Reactor power rising.’ Wilson released the starting switch as the temperature readings rose inside the reactor’s deadly heart. He watched as the core stabilised at the lowest power setting, verifying items from the checklist on the screen in front of him. Finally, he was satisfied, and flipped off the safeties for full power.
‘Reactor stable at fast idle. Coolant flow normal. Neutron flux is normal.’ He glanced round the instruments one last time. ‘Ready for ignition.’
Clare gave her full attention to the approach display, which showed the relative orbital paths of the ship and Mercury, and the long deceleration vector that they had to achieve for successful orbital insertion. In all the Solar System, few manoeuvres were more dangerous if anything went wrong, and they were on their own, far from any help.
She took a deep breath. It was now or never.
‘Okay, let’s do it. Set engine to autopilot.’
‘Engine to autopilot. Ignition sequence armed.’ Wilson’s eyes flickered round the engine and fuel situation displays, watching for any signs of trouble.
Clare checked the plan loaded in the autopilot for the twentieth time, looking for any error. Her hands moved in an unconscious cycle, hovering over the reactor scram button, then the autopilot mode panel, then the abort button, in a practised sequence.
She wished she wasn’t so nervous; she was convinced the others were picking it up and were watching her for signs of not being able to cope.
Nearby, Matt gripped the arms of his seat. He wished he were more than a passenger, and that he had something to do to take his mind off the anxiety. He watched Clare as she sat in the commander’s seat, monitoring the countdown, obviously in control. He hated orbital insertion burns; they were the tensest part of any flight, and he couldn’t relax until the burn was over.
‘Deep Space Control, Mercury Two Zero Seven commencing Mercury orbit insertion, firing in sixty seconds.’ Wilson’s voice would not reach Earth for another nine minutes, and by then, the crew’s fate would be determined, one way or another.
‘Firing in forty seconds.’
Ahead of the ship, Mercury had grown so large that fine detail in the surface features could be made out on the displays. Wandering lines of rilles and lobate scarps, smooth plains, and crater upon crater filled the field of view, ready to smash them to fragments if they miscalculated their approach.
The planet’s surface continued to expand, until it filled their field of view in every direction. The sensation of falling straight towards the surface was overpowering, and Matt nervously checked the navigation displays, to reassure himself that they weren’t going to smash into the planet.
At last, however, just as he thought that the displays were wrong and that impact was unavoidable, he realised that the landscape was slowing in its headlong rush towards them, and had begun to move past in front of the ship. As he watched, the sense of lateral motion increased, the surface moving faster and faster until it seemed to flow past them like a river, a mere hundred kilometres below.
The Sun slid towards the horizon on their left, and the shadowed terrain below them, great bowls of craters made billions of years ago, darkened into night.
‘Fifteen seconds to firing. Ignition sequence started.’ Wilson reached up and tightened his seat straps.
The bloated disc of the Sun shrank to a point, and disappeared below the horizon, plunging the space tug into the darkness of Mercury’s shadow.
‘Ignition sequence commit. Firing in five, four, three—’
Far behind them, at the end of the long propellant tanks of chilled liquid ammonia, the waiting vanes of the turbopumps spun up to their full rated power, a dizzying 26,000 revolutions per minute, forcing the cold ammonia fuel at tremendous pressure into the waiting engine. Huge volumes of ammonia surged into the reactor, right into the heart of the blizzard of neutrons, as the reactor came to full power.
The incandescent core of the reactor blazed a searing ultra-violet, a captive sun pouring gigawatts of pure energy into its surroundings. The ammonia fuel flashed into superheated gas, and erupted through the engine nozzles in an enormous, spreading flame.
‘—zero.’
A giant hand gripped the space tug and shoved it backwards in space, pressing the crew into their seats. The command deck quivered, and a steady rumble came through the ship’s structure from the distant nuclear furnace.
‘Full thrust. Reactor stable.’ Wilson’s eyes never left the engine status displays, flicking over the readouts as he watched the fuel inlet pressures and flow, the reactor power output, and the exhaust gas temperature.
After so long in low-gravity and weightless conditions, the deceleration was hard on their bodies; Matt found it an effort to breathe as his chest was squeezed in by the tremendous thrust.
By turning his head, he could see the planetary approach display on Clare’s navigation display. The white line of the tug’s track was changing, curving closer to the globe of Mercury with each passing minute, the ship’s headlong rush braked by the mighty thrust of the nuclear engine. The magenta line of the target orbit seemed to curve impossibly close to the planet – surely there was no way the ship could slow down enough?
The tug’s structure quivered with the tremendous thrust of the engine. Behind the passenger seats, something escaped from its fastenings and fell onto the floor with a clang.
On the forward view monitors, a wavering, glowing nimbus hovered ahead of the ship, all that could be seen of the engine flame and the enormous energies being expended to slow them down.
Wilson watched the fuel displays sink as the nuclear engine drank its way through their fuel reserves at nearly fifty tonnes a minute, slowly bringing the tug’s colossal speed down so that Mercury’s gravity could take hold.
The ship slid down through the darkness, its engine flame a bright star in the night sky.
