Analog SFF, November 2009 Read online

  There was, Jacques realized, nothing combustible left within many kilometers to cook with.

  The meal was woefully inadequate, but too much better than nothing to complain. They ate it silently, packed up, forded the lake, and headed west. With no point in hurrying any longer, they tried to move as efficiently as possible.

  "Another day,” Helen said, then asked, calmly, “Are we going to last another day?"

  The ground crunched beneath their boots and staffs as clouds of ash flew up to coat their bodies. They passed a scorched Kangasaur. Someone I knew? Jacques asked himself, but he didn't have the energy to examine it closer.

  "Think about lasting another five minutes. Then do that again,” Doc whispered.

  Jacques’ vision focused on the ground ahead of him. Not five minutes, but one more step.

  No! He shook himself. He could not give up; he was supposed to be their leader. He had to think ahead, see ahead. Angry with himself, he straightened up and looked around and up. Then he saw the shadow coming down at them.

  "Megabat,” he croaked. “Overhead."

  "Another one, to our right!” Soob pointed. “Two. Three."

  All over the sky, huge black crescents emerged from the haze, gliding downward.

  "Appears they've just been invited to a barbie,” Soob said.

  There was nowhere to hide or take cover, absolutely nowhere. The megabat Jacques saw got lower and lower, headed directly for them. Too exhausted to run, they huddled together, with Collette, who seemed strongest, in front. The great beak on its thick neck opened wide and Collette waved her staff at it—a disparity in force so incredible that it would have been humorous but for the desperation of their situation.

  Then, with a downbeat of its wings that sent ash flying up all around them, the monster sailed over them for the dead kangasaur upstream.

  "It's about ten times our combined mass and no resistance,” Helen observed.

  Jacques found his voice. “Collette, if we can get on that thing..."

  Doc and Helen looked at him as if he were crazy, but Soob, the hunter, smiled.

  "It's busy with the kangasaur,” he said. “It would hardly notice us if we jump up on it."

  "I can barely lift my arms, let alone jump,” Helen said.

  Doc reached into his pack for a couple of boost patches. “I don't think any of us are in shape to do much jumping. But if two of us take these, we'll be able to help the others on."

  "We can rope ourselves together,” Helen suggested. “Collette, you've done this before. Jacques shouldn't have another dose, Doc has to monitor us, and Soob, you're stronger than I am. It should be Soob and Collette, I think."

  Everyone murmured an assent. Collette was already into her kit. “We should put our suits on, too. Hard to believe now, but it can get cool up there."

  Moving as quickly as they could, they washed off in the river. Lack of towels was no problem—in the desiccated air, they were dry by the time they climbed back up to the trail. Once in the suits, except for the long beards on the men, they looked civilized again. Along with that, having their fill of water to drink and the anticipation of the upcoming adventure raised spirits. Doc administered the drug to Collette and Soob. Then they all approached the feeding monster from behind. It had been diligent; the kangasaur was half gone.

  The megabat shivered slightly as Collette clambered up its stubby tail and onto its hindquarters, but didn't pause from its grisly meal. One by one the rest of the party followed, stepping into Soob's hands to be boosted—essentially to be thrown—onto the megabat back. Then there was nothing to do but wait.

  Helen and Soob were already asleep in the soft fur between the megabat's shoulders, safely secured between Doc and Helen, when it decided it was satiated and took flight. Jacques grabbed the fur and stuck his head up. Collette was on the beast's neck, ready to try to guide it toward the ocean.

  But that proved unnecessary. Riding on a powerful updraft from the fire, the megabat gained a tremendous altitude, then headed for the far shoreline on its own. Amazingly, except for the smoke and haze over the island, the entire sky seemed devoid of clouds. The land below was a perfect square, the ocean a huge circle inside of it, and the island a target in its center. Megabats—Jacques estimated perhaps as many as a hundred—circled over it, not unlike buzzards over a corpse in a desert, though the scale was a hundred times larger. On the shoreline, still green and moist, he saw kangasaurs in the surf, fishing like bears with great swipes of their forearms. Then the shore passed beneath him, and soon they were over a vast ocean.

  Collette released her hold on the monster's ear, worked her way back, settled beside him, and roped herself to him. They kissed briefly and she fell asleep. Too far out to think about swimming back, they were going wherever this megabat was going.

  * * * *

  Two hours out over the ocean, when Collette and Soob revived enough to listen, Helen held forth. She had rigged a circle of lines on the megabat's back by braiding patches of megabat hair into short ropes that could be knotted to their lines. With this, everyone felt more secure, though the megabat had hardly tilted at all during the flight.

  "So I think Doc's cube world idea is correct,” she concluded.

  "On this side, anyway,” Soob said, “the biosphere is a fluid bulge on a square face."

  "Indeed,” Helen responded. “The ocean and the atmosphere respond to gravity and intersect the cube in circles. See how the snow line curves up toward the mountain peaks? If there were only four mountains, the thing would be unbalanced, I think. It works better as a sphere with eight huge mountains the shape of triangular pyramids, arranged symmetrically."

  "Why don't the mountains collapse?” Soob asked. “My memory is a little hazy, but I thought planets this size would inevitably assume a more or less spherical form."

  Helen shook her head. “They should. I can only guess that the mountains are made of something very strong and lightweight. Made is the operative word. I think this is a manufactured world."

  "But why would anyone do this?” Collette asked.

  "The mountains and the ridges connecting their peaks extend beyond the atmosphere,” Soob observed.

  Helen nodded. “All but the last traces. Each face of the cube would be isolated from the other faces, for things not able to travel through vacuum. It could harbor six separate biomes, but each with the same insolation and resources."

  "A zoo?” Jacques speculated. “With life forms from different worlds?"

  "You wouldn't need a perfect cube for that,” Soob said. “I suspect some esthetic motivation as well—form and function. It's architecture on a scale we've only begun to think about. I'd like to meet whoever came up with this."

  Everyone fell silent at that point, gazing at the incredible sight below them. Jacques felt an unjustified, giddy relief. Saboteurs, murderers, and fire lay behind them. In the alien forests and cliffs ahead of them, somewhere, was a link to the civilization they had lost. If they could find it. If they could feed themselves. If they could avoid being eaten.

  Copyright © 2009 G. David Nordley

  TO BE CONCLUDED.

  [Back to Table of Contents]

  Science Fact: ROCK! BYE-BYE, BABY by Edward M. Lerner

  What can we do about the remote but potentially catastrophic danger of Earth-asteroid collisions? Some people are beginning to take the question seriously....

  In the early morning of December 14, 1807, something streaked across the sky above Weston, Connecticut and exploded. At least six rocky fragments reached Earth, the largest (now on display at the Yale Peabody Museum) weighing twenty-eight pounds. Yale University professors Benjamin Silliman and James L. Kingsley collected fragments and published a detailed report in the Connecticut Herald.

  Upon first learning of the incident, President Jefferson said, “I would more easily believe that two Yankee professors would lie than that stones would fall from heaven."[1]

  Times change.

  That objects fall
to Earth from the sky is now common knowledge.

  This is (as I write) the hundredth anniversary year of the Tunguska Event, in which an aerial explosion (the site did not show an impact crater) from an asteroid or comet flattened and burned forest across a broad expanse of Siberia. Atmospheric disturbances extended to Western Europe, with Londoners able to read newspapers by the night sky's sudden eerie luminescence.[2] And an asteroid or comet strike is a leading explanation for the sudden disappearance of the dinosaurs—the so-called Cretaceous-Tertiary extinction event.

  Atlantic Magazine recently ran an article, “The Sky Is Falling."[3] That article was headlined with the provocative teaser: “The odds that a potentially devastating space rock will hit Earth this century may be as high as one in 10. So why isn't NASA trying harder to prevent catastrophe?"

  Is Earth wearing a bull's eye? We'll look at that, relying on more primary sources, and at realistic approaches toward handling space-rock hazards. We'll also consider obstacles to preparing—and motivations beyond fear for developing—a planetary-defense capability.

  First, let's get some definitions out of the way.

  * * * *

  Meteoroids and Asteroids and Comets (Oh, My)

  What might smack into Earth?

  Many more things than planets (however defined—I won't go there) whiz about the solar system. We call these sub-planetary objects:

  * meteoroids, if boulder-sized or smaller (and meteorites if they survive reentry and reach the Earth's surface), or

  * asteroids, or,

  * comets, if they exhibit a coma (gaseous envelope).

  Meteoroids, by definition, are too small to endanger many people or any significant area. Only size distinguishes asteroids from meteoroids.

  Asteroids orbit mostly between Mars and Jupiter, in the too inclusively named Asteroid Belt, at a distance from the Sun of between about two and four astronomical units[4]. Of course, if all asteroids orbited in that belt, this would be a short article. And maybe dinosaurs would still rule the Earth.

  The threat to Earth comes from Near Earth Objects (NEO). How near? The current standard is approaching within 1.3 AU of the Sun. Stated another way, a NEO is anything that comes within 28 million miles of Earth's orbit.

  Astronomers distinguish several populations of near-Earth asteroids. Generally speaking:

  * The Aten asteroids orbit within Earth's orbit.

  * The Amor asteroids approach from the outside, but do not cross, Earth's orbit.

  * The Apollo asteroids have orbits that cross Earth's own orbit.

  NEO can be comets rather than asteroids. Short-period comets originate in the Kuiper Belt, a disk-shaped region that begins beyond Neptune, about thirty AU from the Sun, and extends (depending on whose guess you favor) to fifty or fifty-five AU from the Sun. Long-period comets, it is hypothesized, originate in a spherical region even larger and more remote (and whose existence remains speculative): the Oort Cloud.

  Asteroids and comets generally differ in composition. The ice line marks the distance from a star at which water can condense as ice. For our sun, the ice line is at about 2.7 AU—within the Asteroid Belt. Water evaporates from objects that orbit within the ice line, and is driven outward by the solar wind, possibly to condense in the outer solar system. Other volatile materials (like ammonia and methane) behave in similar ways and have their own ice lines.

  Objects originating in the inner solar system generally consist of rock or metal. Objects originating in the outer solar system add ices of various kinds. We call the latter comets. If a comet has an eccentric[5] orbit that crosses the ice line(s), sunlight starts turning the volatile materials to gas. Gases escaping the object's usually minuscule gravitational attraction, and any dust carried away by the gases, are pushed and shaped by the solar wind to form the comet's tail.

  Most space-rock discussions focus on asteroids. Comets certainly can be dangerous, but their long, slow orbits are much harder to characterize. Known and suspected asteroids with Earth-approaching orbits far outnumber known and suspected comets with Earth-approaching orbits.

  Not all objects represent the same threat. Size is a factor, obviously, but it's not the only factor. Speed of encounter (the net velocity difference between Earth and space rock) matters, naturally. Composition also matters. Metallic objects will better survive an atmospheric passage than stone. Solid objects survive better than loosely-packed rubble piles. And the angle of impact matters. Objects entering the atmosphere at a shallow angle undergo more friction from entry than rocks entering at a steep angle.

  Solar-system objects do not have fixed orbits. True, many objects have stable orbits. They must, because the solar system has an age on the order of 4.5 billion years. The familiar planets have avoided a plunge into the Sun or being hurled into the interstellar darkness. Still, forces are constantly at work on orbiting objects, including the gravitational perturbations from other orbiting bodies, tidal drag[6], the propulsive effects of gas eruptions, and light pressure.

  When orbits change, things can collide....

  * * * *

  Expert Opinions

  The Asteroid Deflection Research Symposium (ADRS), held October 23-24, 2008, in Arlington, VA, drew invited attendees from concerned U.S. government agencies:

  * the National Aeronautics and Space Administration,

  * the National Research Council,

  * the National Science Foundation,

  * the Air Force,

  * the Defense Threat Reduction Agency (part of the Department of Defense)

  * the Department of Energy, and

  * the Department of Homeland Security.

  A small number of attendees came from academia (especially from the aerospace department of Iowa State University, organizer of the event), the aerospace industry, and science-oriented media outlets. A few were science-fiction authors like me.[7]

  ADRS was hardly the first symposium on the topic of planetary defense, but it was my first. It appears to have been a good place to start. The organizers kept the conference small to facilitate candid discussion—and succeeded. Most attendees stressed that their presentations and participation reflected personal opinions and not the official viewpoints of their organizations. For that reason, I have not attributed information to individual participants.

  ADRS provided the original impetus and much of the source material for this article.

  * * * *

  How Serious Is the Threat?

  The conference opened with an overview of the problem. Congress has tasked NASA to survey the threat from NEOs. NASA's Near Earth Objects Program maintains a running count of the findings at neo.jpl.nasa.gov/stats/. New NEOs are found more or less daily; discovery trends suggest the cumulative number of such objects will reach 6000 in 2009. The count of large objects, a kilometer or more across, will likely reach 800 during 2009.[8]

  A kilometer—besides being a nice, round number—is about the size at which a rock's impact would have global repercussions. (One kilometer isn't yet dinosaur-killer sized: that impactor, whether an asteroid or a comet head—a question still in dispute—is estimated at about ten kilometers.) Initial effects would include fireball, air and ground shock waves, and (with a water impact) tsunami. Dust and debris would add acid rain, diminished sunlight, and secondary impacts.

  How precise are the statistics? That's open to debate.

  * The search has used telescopes neither designed for asteroid hunting nor dedicated to the task.[9]

  * An object must be observed several times to derive its orbit. A “new” discovery may prove to be a repeat sighting of an object whose orbit had not yet been characterized or has shifted. The less massive an object, all others things being equal, the more likely it is that the object's orbit will change with time.

  * Objects this small aren't round, being too small for gravity to have collapsed them into a generally spherical shape. Glimpses of a rock from different sides differ, complicating the recognition, sizing, and orbital-determ
ination problems.

  * Size estimates often derive from a pinpoint of reflected light. A dark rock is harder to spot than a light rock. Size estimates derive from estimates of both average asteroid albedo—we don't have up-close data about very many asteroids—and distance. The sizing model will mistakenly conclude that a dark rock is smaller than it truly is.

  * * * *

  Here's the good news: a decreasing rate of discovery for large NEO. This suggests that we've found most of them.

  The not-so-good news: It doesn't take a kilometer-sized rock to do a lot of damage. Impact of a rock as “small” as 140 meters would cause a regional disaster. In 2005, Congress tasked NASA to find 90% of such medium-sized objects by 2020. Congress did not provide funding for this mandate; NASA's current forecast shows it missing the deadline by a few years.

  The most worrisome NEO are dubbed Potentially Hazardous Objects (PHO). These are large objects—about 500 feet or greater—whose orbits approach too close to Earth. “Too close” is defined as a 0.05 AU: about 4.65 million miles. In comparison, the mean Earth-moon distance is about 240 thousand miles.

  NASA's NEO Program has identified about 1,000 PHO.[10]

  Sky surveys probably won't give much warning of potentially dangerous comets (mostly Kuiper Belt Objects with highly eccentric orbits). Until a comet begins to show a significant coma or tail—i.e., as it gets closer to the Sun—it is very difficult to spot and track. Even short-period comets have orbits of decades (the famous Halley's Comet has an orbital period of seventy-six years), so modern surveys have likely characterized very few of them.

  The bigger the rock, the rarer, of course. Table 1, adapted from NASA's 2006 Near-Earth Object Survey and Deflection Study[11] and several ADRS presentations, gives one view (based on a variety of estimates, uncertainties, and assumptions) of the probabilities. The bigger the rock (all others things equal), the more dangerous. Estimates vary, but impact by an object as small as 50 meters might deliver five to nine megatons of energy.

  And that alarming forecast in Atlantic? Sky surveys do not back it up. Nor does the historical record. Nor, in fact, does the article itself. Its only substantiation is an expert's statement citing—without context, definition, or explanation—"a one-in-10 chance per century of a dangerous space-object strike.” For some level of danger, that is certainly true.