Lenny and Sharon had seen in the meltings the signature of a fascinatingly violent event: the passage of a Sunlike star within only one or two light-days of the solar system. Both stars would have torn at their outermost planets and, given the extreme age of the galaxy, and six hundred billion stars with an average separation of only five light-years—each one of them moving on a different course—at least one close brush in Sunspace’s past seemed statistically inevitable.

Even at a distance of light-hours, an intruding sun would be too distant to be seen as anything more than a bright star in Earthly or Martian skies, too distant to affect planetary climate and etch recognizable traces in the fossil record. But to the outer planets—those most weakly bound to the Sun— even a light-day was close enough to be felt.

All over Triton there were hints of exploded volcanoes whose lava had been water; and drillings beneath the nitrogen “seas” revealed them to have condensed on top of frozen ice-lava lakes. Vinny suspected that Triton was still reeling from the gravitational tug-of-war. In a region he named Yellowstone Park, liquid nitrogen geysers jetted up from hot spots below the surface. Given that his own Sun was merely a bright star on the horizon, he saw no heat source except the gravitational pulse, and a subsequent fraction of a degree-per- megayear cooling process that was not yet finished, to explain where the energy to drive geysers would come from on a frigid, geologically moribund moon. And if Triton was, to one degree or another, still active after all these years, Vinny could do little more than wonder what greater surprises awaited him on Neptune, whose volume, through which frictional heat would have been distributed, was far greater than Triton’s, and whose relative surface area, through which heat could be radiated, was even smaller.

For all he and his companions knew, they were about to become the very last page of human history. But there was nothing left to do except turn the page, as the spiders capped a vent named Old Faithful and jetted streams of hot nitrogen through Lenny’s maze of bristle-lined pipes. Each bristle, as it bent and twisted and strained to snap back to its original position, produced a faint electrical current, and the collective whole became a surprisingly efficient charger for Gaius’s batteries. Once below the Neptunian cloud tops, those same pipes would derive their power from changing pressures and temperatures.

“Neutrino free,” Lenny had said. “Everything has to be neutrino free.”

“Surely the power requirements of four people won’t outshine the galactic neutrino background,” Vinny had protested. “We ought to go nuclear. We’ll find all the deuterium and helium-three we’d ever need in Neptune’s atmosphere.”

But Lenny would not hear of it. He was cautious to the point of paranoia. To him, any neutrino glow at all was an unacceptable risk that might bring with it endless uncertainty as they waited to be proven right or wrong. Why make it a test at all?