"There it is," Gaby said proudly. "Small proper motion, but the plates are only twenty-three hours apart."

Gene called to them. "Orbital elements are coming in," he said. Gaby and Cirocco joined him. Cirocco glanced down and saw his arm go possessively around Gaby's waist, looked quickly away, noting that the Polo sisters had seen it and were just as careful not to notice. They had all learned to stay out of each other's affairs.

Saturn sat in the middle of the tank, fat and brassy. Eight blue circles were drawn around it, each larger than the last, each in the equatorial plane of the rings. There was a sphere on each circle, like a single pearl on a string, and beside the pearls were names and numbers: Mnemosyne, Janus, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Hyperion. Far beyond those orbits was a tenth one, visibly tilted. That was Iapetus. Phoebe, the most distant, could not be shown on the scale they were using.

Now another circle was drawn in. It was an eccentric ellipse, almost tangent to the orbits of Rhea and Hyperion, cutting right across the circle that represented Titan. Cirocco studied it, then straightened. Looking up, she saw deep lines etched on Gaby's forehead as her fingers flew over the keyboard. With each pro- gram she called up, the numbers on her screen changed.

"It had a very close call with Rhea about three million years ago," she noted. "It's safely above Titan's orbit, though perturbations must be a factor. It's far from stabilized."

"Meaning what?" Cirocco asked.

"Captured asteroid?" Gaby suggested, one eyebrow raised doubtfully.

"The proximity to the equatorial plane would make that un- likely," one of the Polo sisters said. April or August? Cirocco wondered. After eighteen months together she still couldn't tell them apart.

"I was afraid you'd see that." Gaby chewed a knuckle. "Yet if it was formed with the others, it ought to be less eccentric."