From a ballistic standpoint, Themis was a nightmare.

No one had ever tried to orbit a toroidal body. Themis was 1300 kilometers across and only 250 kilometers wide. The torus was flat along the outside, and 175 kilometers from top to bottom. The density of the torus varied radically, supporting the view that it was composed of a thick floor along the outside, an atmosphere about that, and a thin canopy arching overhead holding the air inside.

Then there were the six spokes, 420 kiloineters tall. They were elliptical in cross-section, with major and minor axes of .100 kilometers and 50 kilemeters, respectively, except near the base where they flared out to join the torus. in the center was the hub, more massive than the spokes, 160 kilometers in diameter, with a 100-kilometer hole in the center.

Trying to cope with a body like that was tantamount to a nervous breakdown for the ship's computer, and for Bill, who had to make a model the computer would believe in.

The easiest orbit would have been in the equatorial plane of Saturn, enabling them to use the velocity they already had. But that was not possible. Themis was oriented with its axis of rotation parallel to the equatorial plane. Since the axis passed

through the hole at the center of Themis, any Saturn-equatorial orbit Cirocco might assume would have Ringmaster passing through areas of wildly fluctuating gravitational attraction.

The only viable possibility was an orbit in the equatorial plane of Themis. Such an orbit would be expensive in terms of angular momentum. it had the single advantage of being stable, once achieved.

The maneuvering began before they reached Saturn. During the last day of approach their course was re-calculated. Cirocco and Bill relied on Earth-based computers and navigational aids as far away as Mars and Jupiter. They lived in CONMOD and watched Saturn grow larger in the aft television screens.

Then the long burn was initiated.

During a lull in her work, Cirocco turned on the camera in SCIMOD. Gaby looked up with a harried expression.

"Rocky, can't you do something about that vibration?" "Gaby, the engine function is, as they say, nominal. They're just going to shake, that's all."

"Best observing time of the whole fucking trip," Gaby muttered. In the seat next to Cirocco, Bill laughed.

"Five minutes, Gaby," he said. "And I really think we ought to let them burn as long as we planned. It would work out so much nicer that way."

The engines shut down on the tick and they watched for final confirmation that they were where they wanted to be.

"This is Ringmaster; C. bones commanding. We have arrived in Saturn orbit at 1341.453 hours, Universal Time. I'll send up the prelims for a correcting burn when we come out from behind. Meanwhile, I'm going off this channel."

She slapped the appropriate switch.

"Anybody who wants to take a look outside, this is going to be your only chance."

It was tight, but August and April and Gene and Calvin man- aged to squeeze into the cramped room. After checking with Gaby, Cirocco turned the ship ninety degrees.

Saturn was a dark gray hole, seventeen degrees wide, covering 1000 times the area of the moon as seen from Earth. The rings were an incredible forty degrees from side to side.

They looked like solid, brilliant metal. Ringmaster had come in north of the equator, so the upper face was presented to them. Each particle was being lit from the opposite side, presenting a thin crescent, like Saturn. The sun was a brilliant point of light in the ten o'clock position, approaching Saturn.

No one spoke as the sun drew nearer to eclipse. They saw Saturn's shadow fall across the part of the ring nearest them, cutting it like a razor.

Sunset lasted fifteen seconds. The colors were deep and changed rapidly, pure reds and yellows and blue-blacks like those seen from an airliner in the stratosphere.

There was a soft chorus of sighs in the cabin. The glass depolarized and everyone gasped again as the rings grew brighter, bracketing the deep blue glow that outlined the northern hemi- sphere. Gray striations became visible on the planetary surface, illuminated by ringlight. Down there were storms as big as the Earth.

When she looked away at last, Cirocco saw the screen to her left. Gaby was still in SCIMOD. There was an image of Saturn on the screen above her head, but she didn't look up at it.

"Gaby, don't you want to come up and see this?" Cirocco saw her shake her head. She scanned the numbers marching across a tiny screen.

"And lose the best observing time of the whole trip? You've got to he out of your mind."

They first assumed a long, elliptical orbit with a low point 200 kilometers above the theoretical radius of Themis. It was a mathematical abstraction because the orbit was tilted thirty degrees from Themis, equator, which put them above the dark side. They passed the spinning toroid to emerge on the sun side. Themis lay spread out before them as a naked-eye object.

Not that there was a lot to see. Themis was nearly as black as space, even with the sun shining on it. She studied the huge mass of the wheel with the triangular solar absorption sails rimming it like sharp gear teeth, presumably soaking up sunlight and turning it into heat.

The ship moved over the interior of the great wheel. The spokes became visible, and the solar reflectors. They seemed nearly as dark as the rest of Themis, except where they mirrored some of the brighter stars.

The problem that still worried Cirocco was the lack of an entrance. There was a lot of pressure from Earth to get into the thing, and Cirocco, despite her cautious instincts, wanted to as badly as anyone else.

There had to be a way. No one doubted Themis was an artifact. The debate concerned whether it was an interstellar space vehicle or an artificial world, like O'Nell One. The differences were movement and origin. A spaceship would have an engine, and it would be at the hub. A colony would have been built by somebody close at hand. Cirocco had heard theories that included inhabitants of Saturn or Titan, Martians--though no one had found so much as a flint arrowhead on Mars-and ancient space- faring races from the Earth. She didn't believe any of them, but it hardly mattered. Ship or colony, Themis had been built by someone, and there would be a door.

The place to look was the hub, but the constraints of ballistics forced her to orbit as far from the hub as she could get.

Ringmaster settled into a circular orbit 400 kilometers above the equator. They traveled in the direction of spin, but Themis turned faster than their orbital speed. It was a black plane outside Cirocco's window. At regular intervals one of the solar panels would sweep by like the wing of a monstrous bat.

Some details could now be seen on the outer surface. There were long, puckered ridges that converged on the solar panels, presumably covering huge pipes to carry a fluid or gas to be warmed by the sun. Scattered widely in the darkness were a few craters, some of them 400 meters deep. There was no rubble scattered around them. Nothing could stay on the outer surface of Themis that wasn't fastened down.

Cirocco locked her control board. At her elbow, Bill nodded in his couch, asleep. The two of them had not left CONMOD in two days.

She moved through SCIMOD like a sleepwalker. Somewhere down there was a bed with soft sheets and a pillow, and a comfortable quarter gee now that the carousel was turning again.

"Rocky, we've got something strange here."

She stopped with one foot on the ladder of D Spoke, stood very still for a moment.

"What did you say?" The edge in her voice made Gaby look up.

"I'm tired, too," she said, irritably. She palmed a switch, and an image appeared on the overhead screen.

it was a view of the approaching edge of Themis. There was a swelling on it that seemed to grow larger as it caught up with them.

"That wasn't there before." Cirocco's brow furrowed as she tried to shake off the exhaustion.

A buzzer sounded faintly and for a moment she could not place it. Then things became sharp and clear as adrenalin ate the cobwebs. It was the radar alarm in CONMOD.

"Captain," Bil,1 said over the speaker, "I've got a strange reading here. We're not getting closer to Themis, but something's getting closer to us."

"I'll be there." Her hands felt like ice as she grabbed a stanch- ion to swing herself up. She glanced at the screen. The object exploded. It looked like a starburst, and it was growing.

"I can see it now," Gaby said. "It's still attached to Themis. It's like a long arm or a boom, and it's opening out. I think-"

"The docking facilities!" Cirocco yelled. "They're gonna grab us! Bill, start the engine sequence, stop the carousel, get ready to move."

"But it'll take us thirty minutes-"

"I know. Move!"

She caromed off the viewport and into her seat, reached for her microphone.

"All hands. Emergency status. Depressurization alert. Evacuate the carousel. Acceleration stations. Strap in." She slammed the alarm button with her left hand and heard the eerie hooting begin in the room behind her. She glanced to her left.

"You too, Bill. Get suited."

"But-"

"Now!"

He was out of his seat and diving through the access hatch. She turned and called over her shoulder.

"Bring my suit back with you!"

The object was visible out the window now, approaching fast. She had never felt so helpless. By overriding the attitude control system's programing she was able to fire all the thrusters on the side of the ship facing Themis, but it was not nearly enough. The great mass of Ringmaster barely moved. Other than that, she could only sit and monitor the automatic engine sequencing and count the seconds as they dragged by. In a short time she knew they could not escape. That thing was big, and moving faster.

Bill appeared, suited, and she scrambled into SCIMOD to don her own suit. Five anonymous figures sat belted to acceleration couches, not moving, staring at the screen. She clamped her helmet, and heard chaos.

"Quiet down." The chatter died away. "I want silence on the suit channel unless 1 ask you to speak."

"But what's happening, Commander?" It was Calvin's voice.