Credit: Justin Randall

The hissing vibration of the sled's thruster hummed through the frame of the sled, through the thinly padded seats and pressure suits of the two men, into their bodies, their bones, and became something like sound, reverberating inside their helmets.

And it didn't sound good. For minutes at a time it would drone on smoothly, then catch and sputter, sometimes producing an almost human-sounding cough, then settling back into normality. Until the next time.

Occasionally Saunders twisted in his seat to look back at the engine behind the open cab, or turned his head to look at Robeson. The technician was driving the sled, and he stared straight ahead. Neither man spoke.

Suddenly there was a ragged metallic shriek, and the sled veered to the right. Robeson swore, fighting the control stick with one hand as he shut down the thruster with the other.

"What the hell?" Saunders yelled.

Robeson didn't answer. When the sled bumped to a stop, he unfastened his seatbelt and hopped down to the icy ground. A moment later Saunders heard a soft sound, like a grunt, in his helmet radio. He looked down at Robeson. "What is it?" he said. "What's wrong?"

"You may as well come down, sir. We'll be walking from here."

"What's the matter with the damn thing?" Saunders lowered himself from the sled and joined the driver.

Robeson pointed at the sled's engine. "Burn-through in the main reaction chamber."

Saunders looked. "That little hole? Can't you patch it?"

"What you're seeing is the outer housing of the engine. Underneath, everything's burned to hell. This engine is dead." Robeson glanced toward the horizon, a smooth curve of white against the black sky. Then he looked upward. Saturn was high, to the left. Nearly full, it filled a great swath of the sky. It was waxing gibbous now, so in about an hour it would be full.

"But you haven't even opened it up," Saunders said in disbelief. "How do you know it's that bad?"

"Because that's my job," said Robeson. "And we don't have time for me to start taking it apart. My suit shows we're 71 klicks from Jansha Base. What's yours say?"

"Seventy-eight point seven. And 92 minutes of oxygen. God damn it, that's too far, Robeson." Saunders cocked his head back slightly. "Mayday! Mayday! Mayday! This is Enceladus Transport Sled zero-five, Stanley Saunders and Joe Robeson calling. Our position is approximately 75 km from Jansha Base, bearing approximately 287 degrees. Our sled is disabled and we are heading for Jansha on foot. We may not have enough oxygen to make it. This is a Mayday call. Anyone receiving please respond."

They stood in silence, listening to the radio's background hiss in their helmets.

"No one can hear you," said Robeson. "We're too far from Jansha."

"It's not that we're too far, damn it! They can't receive us because we're over the damn horizon! But something in orbit might pick us up, or the signal might bounce off another moon."

"Okay," Robeson answered. "We can hope. But in the meantime we better start walking."

It wasn't really walking, more a shuffling skip. Some liked to call it the Enceladus two-step. With .01 g, walking doesn't really work. And while a person could make supermanstyle leaps of 50 m or more, doing so was both slow and dangerous. An energetic jump would leave a person trapped in a slowly drifting parabolic trajectory for over a minute. With no atmosphere there was no way to control where you landed, and there were places you wouldn't want to land. The surface of Enceladus is ice, sometimes skating rink smooth, sometimes fine ice powder, sometimes with kilometres-deep fissures, sometimes a field of jagged shards and deadly sharp spires, as hard as Earthly glass.

The trick with the two-step is to stay low. Bend your knees, keep one foot well ahead of the other, and nudge yourself forward, not up, with each hop. Watch the ground, watch where you'll touch down after this hop, and the next one and the one after that. On rough ground you have to keep your hops short and slow. When it's smooth you can glide a dozen metres or more at a time and build up some speed - if you don't mind risking your life. Even with needle-sharp adaptive crampons, boots provide almost no traction on Enceladus, so slowing down in a hurry isn't an option. If something catches your toe, you'll fall. If a patch of ice crumbles underneath you, you'll fall. If your crampons slip, you'll fall. If you're moving fast when you fall, you'll tumble and spin and bounce for a long, long time. Depending on how all that tumbling and bouncing ends, it will either be a time-wasting and undignified nuisance or the end of your life. Being in a hurry on Enceladus is not a good idea.

They skipped. After a few minutes Saunders made another Mayday call. To their left, near the horizon, a geyser made a ghostly white funnel-shaped silhouette against the black sky.

"How much oh-two does your suit show, Robeson?"

"Same as you, 85 minutes now."