The conversation, and her view of Julia’s face, was cut off by the Flivver’s hatch swinging shut.

ANY CURVE YOU COULD MAKE BY SLICING A CONE WITH A PLANE—A circle, an ellipse, a parabola, or a hyperbola—could be the shape of an orbit. For practical purposes, though, all orbits were ellipses. And most of the naturally occurring orbits in the solar system—those of the planets around the sun, or of moons around planets—were ellipses so round as to be indistinguishable, by the naked eye, from circles. This was not because nature especially favored circles. It was because highly elongated elliptical orbits tended not to last for very long. As a body in a highly eccentric orbit went rocketing in toward the central body and executed a hairpin turn at the periapsis—the point of closest approach—it was subject to tidal forces that could break it up. It might strike the central body’s atmosphere or, in the case of heliocentric orbits, come too close to the sun’s heat and suffer thermal damage. If it survived the plunge through periapsis, it would fly out on a long trajectory that would take it across the orbits of other bodies. After rounding the turn at apoapsis—the point of maximum distance—it would cycle back across the same set of orbits on its way back in toward the center. The solar system was sparse, and so the odds that it would strike, or come close to, any given planet or asteroid on any given circuit were small. But over astronomical spans of time, the likelihood of a close encounter or a collision was high. Collision would, of course, result in a meteorite strike on the planet and the destruction of the formerly orbiting body. A mere close encounter would perturb the body’s orbit into a new and different ellipse, or possibly into a hyperbola, which would eject it from the solar system altogether. The sun still maintained a stable of comets and asteroids in highly eccentric orbits, but their number dwindled over time, and they were rare events to astronomers. In its early aeons the solar system had been a much more chaotic place, with a wider range of orbits, but the processes mentioned had gradually swept it clean and, by a kind of natural selection, produced a system in which nearly everything was moving in an almost circular orbit.

What was true of the solar system as a whole had also been true of the Earth-moon system. The moon had circled the Earth in a nearly circular orbit. From time to time, a wandering stone from deep space would blunder in through a libration point and get captured into a geocentric orbit, but sooner or later it would hit the moon, hit the Earth, or be ejected by a close encounter with one of those bodies. Thus had the moon swept Earth’s skies for billions of years and protected it from most big meteor strikes, making it a suitable place for the development of complex ecosystems and civilizations.

All the rocks that made up the White Sky had once shared the moon’s orbit, and most of them, for the time being, remained at a safe distance of about four hundred thousand kilometers. Their orbits, for now, were of low eccentricity, meaning that they were nearly circular. However, the vast number of chaotic interactions within the White Sky had spawned a diversity of orbits. Some of those orbits were highly eccentric, meaning that their apogees might be far away, but their perigees were close to the Earth: close enough to get caught up in its atmosphere or to strike it directly. Any rock whose orbit was eccentric enough to come near the Earth could also come near Izzy. In general, rocks in such orbits were moving at about eleven thousand meters per second when they were that close to Earth. A bolide the size of a peppercorn, moving at that velocity, would have the same kinetic energy as a high-powered rifle bullet.

Of course, high-powered bullets were designed to strike things with great force and do damage in a predictable way, while moon rocks weren’t designed at all. So the results of collisions could be unpredictable.

What had probably happened in this case was that a rock closer in size to a chickpea, and packing the energy of several rifle bullets, had punched through the wall of the capsule but, in so doing, fractured into several pieces that had sprayed outward across the capsule’s volume in a narrow cone, striking Pete Starling’s body something like a shotgun blast but with much more total kinetic energy. Most of that energy had gone into his flesh and caused him to basically explode. The largest single piece of the original rock had kept going through his body, or perhaps missed him entirely, and punched its way out through the opposite side of the capsule.

If the rock had passed a couple of meters to either side, it would have missed them entirely and they wouldn’t even have known it was there. In the Earth’s atmosphere, of course, it would have been a different story. The rock would have dissolved in a bright streak, turning most of its kinetic energy into heat. The air in its immediate vicinity would have gotten warmer for a bit. Had it happened at night, keen observers might have seen a streak of light. When the same thing happened on a large scale, all over the Earth, the air became so hot that it glowed, as it was doing now.

In any case Dinah now found herself locked into a capsule, lit only by a few strips of white LEDs that were darkened by blood spatter, as the air leaked out of it. She had, of course, been drilling for events such as this one for a significant part of her life. One of the first things they taught you was that the air wasn’t really leaking out as quickly as you thought. Only so much air could get through a small hole. Nevertheless, plugging those holes was life-or-death. So Dinah’s first move, once she had recovered from surprise, was to shove Pete Starling’s remains up toward the larger of the two holes: the one through which the bolide had entered. With a wet sucking sound his bloody flesh sealed that hole. Her ears now enabled her to find the smaller exit hole, which was about the size of her pinkie. She slapped her bloody hand over it. The hiss stopped and she immediately felt a space hickey beginning to form where the Big Hoover was trying to pull her flesh out into the void. It hurt, but not that badly. She listened for a few moments until she was satisfied that there were no other hissing noises—no other leaks.

A bloody bandage floated past. She snatched it out of the air, peeled her hand away from the hole, and stuffed it in there. Some of it got sucked out into space, but then it formed into a wad that moved no further. The hole was still hissing, though, so she grabbed an empty plastic bag and shaped that over the irregular mound of wet gauze. The vacuum sucked it inward and created a nearly airtight seal.

A softer hiss, more of a whooshing noise, emanated from the “back” of the capsule. Dinah’s ears felt a change in pressure, but they didn’t pop—suggesting that the pressure had just increased. She knew nothing about this capsule, but she did know how simple life support systems worked, and she knew that they would likely contain a store of compressed oxygen that would be bled in to compensate for what was being turned into CO₂ in the occupants’ bodies and absorbed by the scrubbers. The mechanism was probably trying to compensate for the air that had just been voided into space, bringing the pressure back up to normal.

If that were the case, then it should now be possible to open the hatch on the Flivver. Dinah floated toward it, reached through the capsule’s open hatch, and rapped on the metal, leaving bloody knuckle prints.

Nothing happened for a moment, and so she rapped out SOS: three dots, three dashes, three dots.

The hatch opened to reveal Ivy’s face. “My. Goodness. Gracious,” she said.

“Thanks, sister,” Dinah said, and vaulted through as Ivy got out of the way—partly just to be accommodating but largely, Dinah assumed, to avoid getting smeared with the bodily fluids of Julia’s late science advisor. Julia herself was strapped into one of the jump seats, buckled over into a fetal position suffering from the dry heaves, and keeping an eye on Dinah out of the corner of her eye.