Death By Black Hole Part 11

3.

0.001.

0.05.

Year

6.

0.01.

0.5.

Decade

15.

0.2.

10.

Century

30.

2.

100.

Millennium

100.

50.

2,500.

10,000 years 200.

1,000.

50,000.

1,000,000 years 2000.

1,000,000.

50,000,000.

100,000,000 years 10,000.

100,000,000.

5,000,000,000.

The table is based on a detailed a.n.a.lysis of the history of impact craters on Earth, the erosion-free cratering record on the Moon's surface, and the known numbers of asteroids and comets whose orbits cross that of Earth.

The energetics of some famous impacts can be located on the table. For example, a 1908 explosion near the Tunguska River, Siberia, felled thousands of square kilometers of trees and incinerated the 300 square kilometers that encircled ground zero. The impactor is believed to have been a 60-meter stony meteorite (about the size of a 20-story building) that exploded in midair, thus leaving no crater. The chart predicts collisions of this magnitude to happen, on average, every couple of centuries. The 200-kilometer diameter Chicxulub Crater in the Yucatan, Mexico, is believed to be the calling card of a 10-kilometer asteroid. With an impact energy 5 billion times greater than the atomic bombs exploded in World War II, such a collision is predicted to occur about once in about 100 million years. The crater dates from 65 million years ago, and there hasn't been one of its magnitude since. Coincidentally, at about the same time, Tyrannosaurus rex and friends became extinct, enabling mammals to evolve into something more ambitious than tree shrews.

Those paleontologists and geologists who remain in denial of the role of cosmic impacts in the extinction record of Earth's species must figure out what else to do with the deposit of energy being delivered to Earth from s.p.a.ce. The range of energies varies astronomically. In a review of the impact hazard to Earth written for the fat book Hazards Due to Comets and Asteroids Hazards Due to Comets and Asteroids (Gehrels 1994), David Morrison of NASA Ames Research Center, Clark R. Chapman of the Planetary Science Inst.i.tute, and Paul Slovic of the University of Oregon briefly describe the consequence of unwelcome deposits of energy to Earth's ecosystem. I adapt what follows from their discussion. (Gehrels 1994), David Morrison of NASA Ames Research Center, Clark R. Chapman of the Planetary Science Inst.i.tute, and Paul Slovic of the University of Oregon briefly describe the consequence of unwelcome deposits of energy to Earth's ecosystem. I adapt what follows from their discussion.

Most impactors with less than about 10 megatons of energy will explode in the atmosphere and leave no trace of a crater. The few that survive in one piece are likely to be iron-based.

A 10-to 100-megaton blast from an iron asteroid will make a crater, while its stony equivalent will disintegrate and produce primarily air bursts. A land impact will destroy the area equivalent to that of Was.h.i.+ngton, DC.

Land impacts between 1,000 and 10,000 megatons continue to produce craters; oceanic impacts produce significant tidal waves. A land impact can destroy an area the size of Delaware.

A 100,000-to 1,000,000-megaton blast will result in global destruction of ozone; oceanic impacts will generate tidal waves felt on an entire hemisphere of Earth while land impacts raise enough dust into the stratosphere to change Earth's climate and freeze crops. A land impact will destroy an area the size of France.

A 10,000,000-to 100,000,000-megaton blast results in prolonged climactic effects and global conflagration. A land impact will destroy an area equivalent to the continental United States.

A land or ocean impact of 100,000,000 to 1,000,000,000 megatons will lead to ma.s.s extinction on a scale of the Chicxulub impact 65 million years ago, when nearly 70 percent of Earth's species were suddenly wiped out.

Fortunately, among the population of Earth-crossing asteroids, we have a chance at cataloging everything larger than about a kilometer-the size that begins to wreak global catastrophe. An early-warning and defense system to protect the human species from these impactors is a realistic goal, as was recommended in NASA's s.p.a.ceguard Survey Report s.p.a.ceguard Survey Report, and, believe it or not, continues to be on the radar screen of Congress. Unfortunately, objects smaller than about a kilometer do not reflect enough light to be reliably and thoroughly detected and tracked. These can hit us without notice, or they can hit with notice that is much too short for us to do anything about. The bright side of this news is that while they have enough energy to create local catastrophe by incinerating entire nations, they will not put the human species at risk of extinction.

Of course Earth is not the only rocky planet at risk of impacts. Mercury has a cratered face that, to a casual observer, looks just like the Moon. Recent radio topography of cloud-enshrouded Venus shows plenty of craters too. And Mars, with its historically active geology, reveals large craters that were recently formed.

At over three hundred times the ma.s.s of Earth, and at over ten times its diameter, Jupiter's ability to attract impactors is unmatched among the planets in the solar system. In 1994, during the week of anniversary celebrations for the 25th anniversary of the Apollo 11 Apollo 11 Moon landing, comet Shoemaker-Levy 9, having been broken apart into two dozen chunks during a previous close-encounter with Jupiter, slammed, one piece after another, into the Jovian atmosphere. The gaseous scars were seen easily from Earth with backyard telescopes. Because Jupiter rotates quickly (once every 10 hours), each part of the comet fell in a different location as the atmosphere rotated by. Moon landing, comet Shoemaker-Levy 9, having been broken apart into two dozen chunks during a previous close-encounter with Jupiter, slammed, one piece after another, into the Jovian atmosphere. The gaseous scars were seen easily from Earth with backyard telescopes. Because Jupiter rotates quickly (once every 10 hours), each part of the comet fell in a different location as the atmosphere rotated by.

And, in case you were wondering, each piece hit with the equivalent energy of the Chicxulub impact. So, whatever else is true about Jupiter, it surely has no dinosaurs left!

Earth's fossil record teems with extinct species-life-forms that had thrived far longer than the current Earth-tenure of h.o.m.o sapiens h.o.m.o sapiens. Dinosaurs are in this list. What defense do we have against such formidable impact energies? The battle cry of those with no nuclear war to fight is "nuke them from the sky." True, the most efficient package of destructive energy ever conceived by humans is nuclear power. A direct hit on an incoming asteroid might explode it into enough small pieces to reduce the impact danger to a harmless, though spectacular, meteor shower. Note that in empty s.p.a.ce, where there is no air, there can be no shock waves, so a nuclear warhead must actually make contact with the asteroid to do damage.

Another method is to engage those radiation-intensive neutron bombs (you remember-they were the bombs that killed people but left the buildings standing) in a way that the high-energy neutron bath heats one side of the asteroid to sufficient temperature that material spews forth and the asteroid recoils out of the collision path. A kindler, gentler method is to nudge the asteroid out of harm's way with slow but steady rockets that are somehow attached to one side. If you do this early enough, then all you need is a small push using conventional chemical fuels. If we catalogued every single kilometer-sized (and larger) object whose orbit intersects Earth's, then a detailed computer calculation would enable us to predict a catastrophic collision hundreds, and even thousands, of orbits in the future, granting Earthlings sufficient time to mount an appropriate defense. But our list of potential killer impactors is woefully incomplete, and chaos severely compromises our ability to predict the behavior of objects for millions and billions of orbits into the future.

In this game of gravity, by far the scariest breed of impactor is the long-period comet, which, by convention, are those with periods greater than 200 years. Representing about one-fourth of Earth's total risk of impacts, they fall toward the inner solar system from great distances and achieve speeds in excess of 100,000 miles per hour by the time they reach Earth. Long-period comets thus achieve more awesome impact energy for their size than your runof-the-mill asteroid. More importantly, they are too dim over most of their orbit to be reliably tracked. By the time a long-period comet is discovered to be heading our way, we might have anywhere from several months to two years to fund, design, build, launch, and intercept it. For example, in 1996, comet Hyakutake was discovered only four months before its closest approach to the Sun because its...o...b..t was tipped strongly out of the plane of our solar system, precisely where n.o.body was looking. While en route, it came within 10 million miles of Earth (a narrow miss) and made for spectacular nighttime viewing.

And here's one for your calendar: On Friday the 13th of April, 2029, an asteroid large enough to fill the Rose Bowl as though it were an egg cup, will fly so close to Earth that it will dip below the alt.i.tude of our communication satellites. We did not name this asteroid Bambi. Instead, it's named Apophis, after the Egyptian G.o.d of darkness and death. If the trajectory of Apophis at close approach pa.s.ses within a narrow range of alt.i.tudes called the "keyhole," the precise influence of Earth's gravity on its...o...b..t will guarantee that seven years later in 2036, on its next time around, the asteroid will hit Earth directly, slamming in the Pacific Ocean between California and Hawaii. The tsunami it creates will wipe out the entire west coast of North America, bury Hawaii, and devastate all the land ma.s.ses of the Pacific Rim. If Apophis misses the keyhole in 2029, then, of course, we have nothing to worry about in 2036.

Should we build high-tech missiles that live in silos somewhere awaiting their call to defend the human species? We would first need that detailed inventory of the orbits of all objects that pose a risk to life on Earth. The number of people in the world engaged in this search totals a few dozen. How long into the future are you willing to protect Earth? If humans one day become extinct from a catastrophic collision, there would be no greater tragedy in the history of life in the universe. Not because we lacked the brain power to protect ourselves but because we lacked the foresight. The dominant species that replaces us in postapocalyptic Earth just might wonder, as they gaze upon our mounted skeletons in their natural history musems, why large-headed h.o.m.o sapiens h.o.m.o sapiens fared no better than the proverbially pea-brained dinosaurs. fared no better than the proverbially pea-brained dinosaurs.

THIRTY.

ENDS OF THE WORLD.

Sometimes it seems that everybody is trying to tell you when and how the world will end. Some scenarios are more familiar than others. Those that are widely discussed in the media include rampant infectious disease, nuclear war, collisions with asteroids or comets, and environmental blight. While different from one another, they all can bring about the end of the human species (and perhaps selected other life-forms) on Earth. Indeed, cliched slogans such as "Save the Earth" contain the implicit call to save life on Earth, and not the planet itself. Fact is, humans cannot really kill Earth. Our planet will remain in orbit around the Sun, along with its planetary brethren, long after h.o.m.o sapiens h.o.m.o sapiens has become extinct by whatever cause. has become extinct by whatever cause.

What hardly anybody talks about are end-of-world scenarios that do, in fact, jeopardize our temperate planet in its stable orbit around the Sun. I offer these prognostications not because humans are likely to live long enough to observe them, but because the tools of astrophysics enable me to calculate them. Three that come to mind are the death of the Sun, the impending collision between our Milky Way galaxy and the Andromeda galaxy, and the death of the universe, about which the community of astrophysicists has recently achieved consensus.

Computer models of stellar evolution are akin to actuarial tables. They indicate a healthy 10-billion-year life expectancy for our Sun. At an estimated age of 5 billion years, the Sun will enjoy another 5 billion years of relatively stable energy output. By then, if we have not figured out a way to leave Earth, we will be around when the Sun exhausts its fuel supply. At that time, we will bear witness to a remarkable yet deadly episode in a star's life.

The Sun owes its stability to the controlled fusion of hydrogen into helium in its 15-million-degree core. The gravity that wants to collapse the star is held in balance by the outward gas pressure that the fusion sustains. While more than 90 percent of the Sun's atoms are hydrogen, the ones that matter reside in the Sun's core. When the core exhausts its hydrogen, all that will be left there is a ball of helium atoms that require an even higher temperature than does hydrogen to fuse into heavier elements. With its central engine temporarily shut off, the Sun will go out of balance. Gravity wins, the inner regions of the star collapse, and the central temperature rises through 100 million degrees, triggering the fusion of helium into carbon.

Along the way, the Sun's luminosity grows astronomically, which forces its outer layers to expand to bulbous proportions, engulfing the orbits of Mercury and Venus. Eventually, the Sun will swell to occupy the entire sky as its expansion subsumes the orbit of Earth. Earth's surface temperature will rise until it matches the 3,000-degree rarefied outer layers of the expanded Sun. Our oceans will come to a rolling boil as they evaporate entirely into interplanetary s.p.a.ce. Meanwhile, our heated atmosphere will evaporate as Earth becomes a red-hot, charred ember orbiting deep within the Sun's gaseous outer layers. These layers will impede the orbit, forcing Earth to trace a rapid death spiral down toward the Sun's core. As Earth descends, sinking nearer and nearer to the center, the Sun's rapidly rising temperature simply vaporizes all traces of our planet. Shortly thereafter, the Sun will cease all nuclear fusion; lose its tenuous, gaseous envelope containing Earth's scattered atoms; and expose its dead central core.

But not to worry. We will surely go extinct for some other reason long before this scenario unfolds.

NOT LONG AFTER the Sun terrorizes Earth, the Milky Way will encounter some problems of its own. Of the hundreds of thousands of galaxies whose velocity relative to the Milky Way has been reliably measured, only a few are moving toward us while all the rest are moving away at a speed directly related to their distances from us. Discovered in the 1920s by Edwin Hubble, after whom the the Sun terrorizes Earth, the Milky Way will encounter some problems of its own. Of the hundreds of thousands of galaxies whose velocity relative to the Milky Way has been reliably measured, only a few are moving toward us while all the rest are moving away at a speed directly related to their distances from us. Discovered in the 1920s by Edwin Hubble, after whom the Hubble s.p.a.ce Telescope Hubble s.p.a.ce Telescope was named, the general recession of galaxies is the observational signature of our expanding universe. The Milky Way and the several-hundred-billion-star Andromeda galaxy are close enough to each other that the expanding universe has a negligible effect on their relative motions. Andromeda and the Milky Way happen to be drifting toward each other at about 100 kilometers per second (a quarter-million miles per hour). If our (unknown) sideways motion is small, then at this rate, the 2.4-million light-year distance that separates us will shrink to zero within about 7 billion years. was named, the general recession of galaxies is the observational signature of our expanding universe. The Milky Way and the several-hundred-billion-star Andromeda galaxy are close enough to each other that the expanding universe has a negligible effect on their relative motions. Andromeda and the Milky Way happen to be drifting toward each other at about 100 kilometers per second (a quarter-million miles per hour). If our (unknown) sideways motion is small, then at this rate, the 2.4-million light-year distance that separates us will shrink to zero within about 7 billion years.

Interstellar s.p.a.ce is so vast and empty that there is no need to worry about stars in the Andromeda galaxy accidentally slamming into the Sun. During the galaxy-galaxy encounter, which would be a spectacular sight from a safe distance, stars are likely to pa.s.s each other by. But the event would not be worry-free. Some of Andromeda's stars could swing close enough to our solar system to influence the orbit of the planets and of the hundreds of billions of resident comets in the outer solar system. For example, close stellar flybys can throw one's gravitational allegiance into question. Computer simulations commonly show that the planets are either stolen by the interloper in a "flyby looting" or they become unbound and get flung into interplanetary s.p.a.ce.

Back in Section 4, remember how choosy Goldilocks was with other people's porridge? If Earth gets stolen by the gravity of another star, there's no guarantee that our newfound orbit will be at the right distance to sustain liquid water on Earth's surface-a condition generally agreed to be a prerequisite to sustaining life as we know it. If Earth orbits too close, its water supply evaporates. And if Earth orbits too far, its water supply freezes solid.

If, by some miracle of future technology, Earth's inhabitants manage to prolong the Sun's life, then these efforts will be rendered irrelevant when Earth is flung into the cold depths of s.p.a.ce. The absence of a nearby energy source will allow Earth's surface temperature to drop swiftly to hundreds of degrees below zero Fahrenheit. Our cherished atmosphere of nitrogen and oxygen and other gases would first liquefy and then drop to the surface and freeze solid, encrusting the Earth like icing on a spherical cake. We would freeze to death before we had a chance to starve to death. The last surviving life on Earth would be those privileged organisms that had evolved to rely not on the Sun's energy but on (what will then be) weak geothermal and geochemical sources, deep beneath the surface, in the cracks and fissures of Earth's crust. At the moment, humans are not among them.

One way to escape this fate is to fire up the warp drives and, like a hermit crab and snail sh.e.l.ls, find another planet elsewhere in the galaxy to call home.

WITH OR WITHOUT warp drives, the long-term fate of the cosmos cannot be postponed or avoided. No matter where you hide, you will be part of a universe that inexorably marches toward a peculiar oblivion. The latest and best evidence available on the s.p.a.ce density of matter and energy and the expansion rate of the universe suggest that we are on a one-way trip: the collective gravity of everything in the universe is insufficient to halt and reverse the cosmic expansion. warp drives, the long-term fate of the cosmos cannot be postponed or avoided. No matter where you hide, you will be part of a universe that inexorably marches toward a peculiar oblivion. The latest and best evidence available on the s.p.a.ce density of matter and energy and the expansion rate of the universe suggest that we are on a one-way trip: the collective gravity of everything in the universe is insufficient to halt and reverse the cosmic expansion.

The most successful description of the universe and its origin combines the big bang with our modern understanding of gravity, derived from Einstein's general theory of relativity. As we will see in Section 7, the very early universe was a trillion-degree maelstrom of matter mixed with energy. During the 14-billion-year expansion that followed, the background temperature of the universe has dropped to a mere 2.7 degrees on the absolute (Kelvin) temperature scale. And as the universe continues to expand, this temperature will continue to approach zero.

Such a low background temperature does not directly affect us on Earth because our Sun (normally) grants us a cozy life. But as each generation of stars is born from clouds of interstellar gas, less and less gas remains to comprise the next generation of stars. This precious gas supply will eventually run out, as it already has in nearly half the galaxies in the universe. The small fraction of stars with the highest ma.s.s will collapse completely, never to be seen again. Some stars end their lives by blowing their guts across the galaxy in a supernova explosion. This returned gas can then be tapped for the next generation. But the majority of stars-Sun included-ultimately exhaust the fuel at their cores and, after the bulbous giant phase, collapse to form a compact orb of matter that radiates its feeble leftover heat to the frigid universe.

The short list of corpses may sound familiar: black holes, neutron stars (pulsars), and white dwarfs are each a dead end on the evolutionary tree of stars. But what they each have in common is an eternal lock on the material of cosmic construction. In other words, if stars burn out and no new ones are formed to replace them, then the universe will eventually contain no living stars.

How about Earth? We rely on the Sun for a daily infusion of energy to sustain life. If the Sun and the energy from all other stars were cut off from us then mechanical and chemical processes (life included) on and within Earth would "wind down." Eventually, the energy of all motion gets lost to friction and the system reaches a single uniform temperature. Earth, sitting beneath starless skies, will lie naked in the presence of the frozen background of the expanding universe. The temperature on Earth will drop, the way a freshly baked apple pie cools on a windowsill. Yet Earth is not alone in this fate. Trillions of years into the future, when all stars are gone, and every process in every nook and cranny of the expanding universe has wound down, all parts of the cosmos will cool to the same temperature as the ever-cooling background. At that time, s.p.a.ce travel will no longer provide refuge because even h.e.l.l will have frozen over.