A gas torus is the result of a light mass in orbit around a heavy mass, as Titan orbits Saturn. It may be that the light mass is too weak to hold its atmosphere. The faster molecules of air escape-but they go into orbit about the heavy mass. Thus, Titan circles Saturn within a ring of escaped Titanian atmosphere, as lo orbits Jupiter within a ring of sulfur ionized by Jupiter's ferocious magnetic field.

A gas torus is thin. The gas must be so rarefied that each molecule can be considered to be in a separate orbit: it must reasonably expect to circle halfway round the primary mass without bumping another molecule. Under such circumstances, a gas torus is stable. The occasional stray photon will bump a molecule into interstellar space; but the molecules are continually reencountering the satellite body.

Titan-smaller than Mars, no larger than Ganymede-carries an atmosphere of refined smog at one and a half times Earth's sea level pressure. The atmosphere is continually being lost, of course, but some of it continually returns from the gas torus.

Levoy's Star was an extreme case, and a slightly different proposition too.

The Smoke Ring was the thickest part of the gas torus around Levoy's Star. At its median it was as dense as Earth's atmosphere a mile above sea level: too dense for stability. It must be continually leaking into the gas torus. But the gas torus was stable: dense, but held within a steep gravitational gradient. Molecules continually returned from the gas torus to the Smoke Ring, and from the Smoke Ring to the storm of atmosphere surrounding Goldblatt's World.

"Goldblatt's World must have started life as a gas giant planet like, say, Saturn. Probably it didn't fall into range until the pulsar had lost a good deal of its heat and spin." Sharon Levoy's crisp voice spoke within Kendy's memory. "Then it was captured by strong Roche tides. It may have dropped close enough to lose water and soil as well as gas. For something like a billion years Goldblatt's World has been leaking gas into the Smoke Ring, and the Smoke Ring has been leaking to interstel

lar space. It's not stable, exactly, but hell, planets aren't stable over the long run."

"It won't be stable that much longer," Dennis Quinn interrupted. "Most of Goldblatt's World is already gone. Ten million years, or a hundred million, and the Smoke Ring will be getting rarefied."

Kendy remembered these things. The records had been made while Discipline's instruments probed the Smoke Ring from close range. Already some of the crew were exploring the Smoke Ring via CARMs. Their reports were enthusiastic. There was life, DNA-based, the air was not only breathable, but tasted fine

Kendy didn't remember bringing Discipline into orbit around Levoy's Star. He must have expended his onboard fuel, postponing by several years his arrival at the target stars along his course. Why?