But here was Malenfant’s voice crackling in her headset, barking orders in his practical way. “Make sure you’re attached to at least two tethers at all times. Do you all understand? Cornelius, Emma, Michael?”

One by one they answered — even Michael, in his eerie translated voice. Yes. I won ‘tfall off.

“Let’s get on with it,” Cornelius murmured.

Malenfant led them to a pair of guide cables. They were made of yellow nylon and had been pinned to the dirt by the fireflies.

Looking ahead, Emma saw how the tethers snaked away over the asteroid’s tight, broken horizon. Malenfant said, “Clip yourself to the guide cables. We’ve practiced with the jaw clips; you know how to handle them. Remember, always keep ahold of at least two cables…”

Emma lifted herself with her toes, tilted, and let herself fall gently forward. It was like falling through syrup. The complex, textured surface of the asteroid approached her faceplate; reflections skimmed across her gold visor.

She let her gloved hands sink into the regolith. She heard a soft squeaking, like crushed snow, as her gloves pushed into the dust.

This was the closest she had come to Cruithne.

On impulse, she undipped her outer glove, exposing her skin-suited hand. She could actually see her skin, little circles of it amid the orange spandex, exposed to vacuum, forty million miles from Earth. Her hand seemed to prickle, probably more from the effects of raw sunlight than from the vacuum itself.

She pushed her half-bare hand into the asteroid ground. The surface was sun-hot, but the regolith beneath was cold and dry. She felt grains — sharp, shattered, very small, like powder. But the dust was very loose, easily compacted; it seemed to collapse under her gentle pressure, and soft clouds of it gushed away from her fingers.

When she had pushed her hand in maybe six inches, the dust started to resist her motion, as if compacting. But her probing fingers found something small and hard. A pebble. She closed her hands around it and pulled it out. It was complex, irregularly shaped, the size of her thumb joint. It was made of a number of different rock types, she could see, smashed and jammed together. It was a breccia, regolith compacted so the grains stuck together, analogous to sandstone on Earth.

She rolled the pebble in her fingers, letting dust flake off on her skin, relishing the raw, physical contact, a window to reality.

She tucked the pebble back in its hole. She rubbed her fingers over each other to scrape off a little of the dust that clung to her skinsuit glove, and put back her outer glove. Snug in its layers of cooling and meteorite-protection gear, her hand tingled after its adventure.

When they were done, clipped to the cables in a line, Malenfant stood briefly to inspect them, then let himself fall back to the surface. “Here we go.” And he crawled away, toward the horizon.

Emma dug her gloved hands into the regolith and pulled herself along the ground. She could see the feet of Michael ahead, was aware of Cornelius bringing up the rear behind her. It was like skimming along the floor of a swimming pool; she just paddled at the regolith with one hand, occasionally pushing at the ground to keep up.

They covered the ground rapidly. Fireflies ghosted alongside them, scrabbling over the surface in a blur of pi tons and tethers, making this an expedition of scrambling humans and spiderlike robots.

Her perspective seemed to swivel around so that she no longer felt as if she were sailing over a sea-bottom floor but climbing, scrambling up the face of some dusty cliff. But this cliff bulged outward at her, and there was nothing beneath her to catch her.

And now the world seemed to swivel again, and here she was clinging to a ceiling like a fly. She found herself digging her gloves deep into the regolith. But she couldn’t support her weight here, let alone keep herself pinned flat against the roof. Her heart thumped, so loud in her ears it was painful.

A hand grabbed her shoulder.

It was dark, she realized. Without noticing she’d sailed into the shadow of the asteroid. She flipped up her gold visor, and now Malenfant loomed, a fat, ghostly snowman. There were stars all around his head. “You okay?”

She took stock. Her stomach seemed to have calmed, the thumping of her heart slowing. “Maybe moving around this damn rock is harder than I expected.”

She looked back. Cornelius came clambering along the guide ropes after her, paddling at the regolith like a clumsy fish. Despite the darkness of the asteroid’s short “night,” Cornelius wouldn’t lift his sun visor.

Malenfant grinned at Emma and made a starfish sign in front of his face, a private joke from their marriage. The poor sap has barfed in his suit.

Somehow that made Emma feel a whole lot better.

“Anyhow it’s over.”

“It is?”

Malenfant helped her to her feet. “We’re here.”

And she found herself facing the artifact.

It was just a hoop of sky blue protruding from the asteroid ground, rimmed by stars. It sat in a neat craterlike depression maybe fifty yards across.

She could see the marks of firefly pitons and tethers, the regular grooves made by scoops as the robots had dug out this anomaly from the eroded hulk of Cruithne. The fireflies had fixed a network of tethers and guide ropes around the artifact. They looked, bizarrely, like queuing ropes around some historic relic.

Malenfant, tethered to the dirt, stood before the artifact, facing it boldly. Cornelius and Michael were clambering along more tethers toward him, ghosts in the pale starlight, just outlines against a background of black dirt and wheeling stars and alien blue.

Emma approached the artifact. It was perfectly circular, as far as she could see, like a sculpture. A small arc at the base was buried in the dirt of Cruithne. There were stars all around the ring, in the night sky — but not within its hoop, she noticed now. The disc of space cut out by the hoop was black, blacker than the sky itself.

It was obviously artificial. A made thing, in a place no human had been before.

And it was glowing, here in the asteroid night. She glanced down at herself. There was blue artifact light on her, too, highlighting from the folds of her meteorite-protection oversuit.

Malenfant said, “Let’s not freak out. It’s not going to bite us. We’re not going to slacken up on our tether drill, and we’re going to watch our consumables every second of the stay here. Is that understood? Okay, then.”

Clipping themselves to the guide ropes, Emma firmly gripping Michael’s hand, the four humans moved in on the artifact.

Reid Malenfant:

Malenfant got to maybe six feet from the base of the hoop, where it slid into the regolith. The hoop towered over him. That interior looked jet black, not reflecting a single photon cast by his helmet lamp.

He glared into the disc of darkness. What are you for? Why are you here?

There was, of course, no reply.

First things first. Let’s do a little science here, Malenfant.

Sliding his tether clips along the guide ropes, he paced out the diameter of the hoop. Thirty feet, give or take. He approached the hoop itself. It was electric blue, glowing as if from within, a wafer-thin band the width of his palm. He could see no seams, no granularity.

He reached out a gloved hand, fabric encasing monkey fingers, and tried to touch the hoop.

Something invisible made his hand slide away, sideways.

No matter how hard he pushed, how he braced himself against the regolith, he could get his glove no closer than an eighth of an inch or so to the material. And always that insidious, soapy feeling of being pushed sideways.

He reported this to Cornelius, who grunted. “Run your hand up and down, along the hoop.”