Long-period comets—icy vagabonds from the extreme reaches of the solar system (as much as a thousand times the radius of Neptune’s orbit)—are susceptible to gravitational nudges from passing stars and interstellar clouds, which can direct them on a long journey inward toward the Sun, and therefore to our neighborhood. Several dozen short-period comets from the nearer reaches of the solar system are known to cross Earth’s orbit.
As for the asteroids, most are made of rock. The rest are metal, mostly iron. Some are rubble piles—gravitationally bound collections of bits and pieces. Most asteroids live between the orbits of Mars and Jupiter and will never ever come near Earth.
But some do. Some will. About ten thousand near-Earth asteroids are known, with more surely to be discovered. The most threatening of them number more than a thousand, and that number steadily grows as spacewatchers continually survey the skies in search of them. These are the “potentially hazardous asteroids,” all larger than about five hundred feet across, with orbits that bring them within about twenty times the distance between Earth and the Moon. Nobody’s saying they’re all going to hit tomorrow. But all of them are worth watching, because a little cosmic nudge here or there might just send them a little closer to us.
In this game of gravity, by far the scariest impactors are the long-period comets—those whose orbits around the Sun take longer than two hundred years. Representing about one-fourth of Earth’s total risk of impacts, such comets fall toward the inner solar system from gargantuan distances and achieve speeds in excess of a hundred thousand miles an hour by the time they reach Earth. Long-period comets thus achieve more awesome impact energy for their size than your run-of-the-mill asteroid. More important, they are too distant, and too dim, throughout 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 just a couple of years—or a couple of months—to fund, design, build, and launch a craft to intercept it. In 1996, for instance, comet Hyakutake was discovered only four months before its closest approach to the Sun because its orbit was tipped strongly out of the plane of our solar system, precisely where nobody was looking. While en route, it came within ten million miles of Earth: a narrow miss.
The term “accretion” is duller than “species-killing, ecosystem-destroying impact,” but from the point of view of solar-system history, the terms are the same. Impacts made us what we are today. So, we cannot simultaneously be happy that we live on a planet, happy that our planet is chemically rich, and happy that dinosaurs don’t rule the Earth, and yet resent the risk of a planet-wide catastrophe.
In a collision with Earth, some of an impactor’s energy gets deposited into our atmosphere through friction and an airburst of shock waves. Sonic booms are shock waves too, but they’re typically made by airplanes with speeds between one and three times the speed of sound. The worst damage they might do is jiggle the dishes in your china cabinet. But at speeds in excess of 45,000 miles per hour—nearly seventy times the speed of sound—the shock waves from the average collision between an asteroid and Earth can be devastating.
If the asteroid or comet is large enough to survive its own shock waves, the rest of its energy gets deposited on Earth. The impact blows a crater up to twenty times the diameter of the original object and melts the ground below. If many impactors hit one after another, with little time between each strike, then Earth’s surface will not have enough time to cool between impacts. We infer from the pristine cratering record on the surface of our nearest neighbor, the Moon, that Earth experienced such an era of heavy bombardment between 4.6 billion and 4.0 billion years ago.
The oldest fossil evidence for life on Earth dates from about 3.8 billion years ago. Before that, Earth’s surface was being relentlessly sterilized. The formation of complex molecules, and thus life, was inhibited, although all the basic ingredients were being delivered. That would mean it took 800 million years for life to emerge here (4.6 billion – 3.8 billion = 800 million). But to be fair to organic chemistry, you must first subtract all the time that Earth’s surface was forbiddingly hot. That leaves a mere 200 million years for life’s emergence from a rich chemical soup—which, like all good soups, included liquid water.
Much of that water was delivered to Earth by comets more than four billion years ago. But not all space debris is left over from the beginning of the solar system. Earth has been hit at least a dozen times by rocks ejected from Mars, and we’ve been hit countless more times by rocks ejected from the Moon.
Ejections occur when impactors carry so much energy that, when they hit, smaller rocks near the impact zone are thrust upward with sufficient speed to escape a planet’s gravitational grip. Afterward, those rocks mind their own ballistic business in orbit around the Sun until they slam into something. The most famous of the Mars rocks is the first meteorite found near the Alan Hills section of Antarctica in 1984—officially known by its coded (though sensible) abbreviation, ALH-84001. This meteorite contains tantalizing, yet circumstantial, evidence that simple life on the Red Planet thrived a billion years ago.
Mars has abundant “geo”-logical evidence—dried river beds, river deltas, floodplains, eroded craters, gullies on steep slopes—for a history of running water. There’s also water there today in frozen form (polar ice caps and plenty of subsurface ice) as well as minerals (silica, clay, hematite “blueberries”) that form in standing water. Since liquid water is crucial to the survival of life as we know it, the possibility of life on Mars does not stretch scientific credulity. The fun part comes with the speculation that life-forms first arose on Mars and were blasted off the planet’s surface, thus becoming the solar system’s first microbial astronauts, arriving on Earth to jump-start evolution. There’s even a word for that process: panspermia. Maybe we are all Martians.
Matter is far more likely to travel from Mars to Earth than vice versa. Escaping Earth’s gravity requires more than two and a half times the energy required to leave Mars. And since Earth’s atmosphere is about a hundred times denser, air resistance on Earth (relative to Mars) is formidable. Bacteria on a voyaging asteroid would have to be hardy indeed to survive several million years of interplanetary wanderings before plunging to Earth. Fortunately, there is no shortage of liquid water and rich chemistry here at home, so even though we still cannot definitively explain the origin of life, we do not require theories of panspermia to do so.
Of course, it’s easy to think impacts are bad for life. We can and do blame them for major episodes of extinction in the fossil record. That record displays no end of extinct life-forms that thrived far longer than the current Earth tenure of Homo sapiens. Dinosaurs are among them. But what are the ongoing risks to life and society?
House-size impactors collide with Earth, on average, every few decades. Typically they explode in the atmosphere, leaving no trace of a crater. But even baby impacts could become political time bombs. If such an atmospheric explosion occurred over India or Pakistan during one of the many episodes of escalated tension between those two nations, the risk is high that someone would misinterpret the event as a first nuclear strike, and respond accordingly. At the other end of the impactor scale, once in about a hundred million years we’re visited by an impactor capable of annihilating all life-forms bigger than a carry-on suitcase. In cases such as those, no political response would be necessary.
Space Tweet #8
For some people, space is irrelevant. But when the asteroid comes, I bet they’ll think differently
Apr 13, 2011 8:40 PM
What follows is a table that relates average collision rates on Earth to the size of the impactor and the equivalent energy in millions of tons of TNT. It’s based on a detailed analysis 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. These data are adapted from a congressionally mandated study titled The Spaceguard Survey: Report of the NASA International Near-Eart
h Object Detection Workshop. For comparison, the table includes the impact energy in units of the atomic bomb dropped by the US Air Force on Hiroshima in 1945.
• • • RISK OF IMPACTS ON EARTH • • •
Once per
Asteroid Diameter
(meters)
Impact Energy
(megatons of TNT)
Impact Energy
(atomic bomb equivalent)
Month
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
2,000
1,000,000
50,000,000
100,000,000 years
10,000
100,000,000
5,000,000,000
The energetics of some famous impacts can be located on the table. For example, a 1908 explosion near the Tunguska River in Siberia felled thousands of square kilometers of trees and incinerated the three hundred square kilometers that encircled ground zero. The culprit is believed to have been a sixty-meter stony meteorite (about the size of a twenty-story building) that exploded in midair, thus leaving no crater. The chart indicates that collisions of this magnitude happen, on average, every couple of centuries. A much rarer sort of event created the nearly two-hundred-kilometer-wide Chicxulub crater on Mexico’s Yucatán Peninsula, which is believed to have been left by an asteroid perhaps ten kilometers wide, with an impact energy five billion times greater than the atomic bombs exploded in World War II. This is one of those collisions that take place once in a hundred million years. The crater dates from about sixty-five million years ago, and there hasn’t been one of similar 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.
It’s useful to consider how strikes by comets and asteroids impact Earth’s ecosystem. In a fat book titled Hazards Due to Comets and Asteroids, several planetary scientists do just that regarding these unwelcome deposits of energy. Here’s a bit of what they sketched out:
• Most impactors with less than about ten megatons of energy will explode in the atmosphere, leaving no trace of a crater. The few that survive in one piece are likely to be iron based.
• A blast of 10 to 100 megatons from an iron asteroid will make a crater, whereas its stony equivalent will disintegrate, producing primarily airbursts. On land, the iron impactor will destroy an area equivalent to Washington, DC.
• A land impact of 1,000 to 10,000 megatons will produce a crater and destroy an area the size of Delaware. An oceanic impact of that magnitude will produce significant tidal waves.
• A blast of 100,000 to 1,000,000 megatons will result in global destruction of ozone. An oceanic impact will generate tidal waves on an entire hemisphere, while a land impact will raise enough dust into the stratosphere to alter Earth’s weather and freeze crops. A land impact will destroy an area the size of France.
• A blast of 10,000,000 to 100,000,000 megatons will result in prolonged climatic change and global conflagration. A land impact will destroy an area equivalent to the continental United States.
• A blast of 100,000,000 to 1,000,000,000 megatons, whether on land or sea, will lead to mass extinction on the scale of the Chicxulub impact, when three-quarters of Earth’s species were wiped out.
Earth, of course, 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. Radio topography of cloud-enshrouded Venus shows no shortage of craters. And Mars, with its historically active geology, reveals large, recently formed craters.
At more than three hundred times the mass of Earth, and more than ten times its diameter, Jupiter’s ability to attract impactors is unmatched among the planets of our solar system. In 1994, during the week of anniversary celebrations for the twenty-fifth anniversary of the Apollo 11 Moon landing, comet Shoemaker-Levy 9, having broken into a couple dozen chunks during a previous close encounter with Jupiter, slammed—one chunk after another, at a speed of more than 200,000 kilometers an hour—into the Jovian atmosphere. Backyard telescopes down here on Earth easily detected the gaseous scars. Because Jupiter rotates swiftly (once every ten hours), each piece of the comet plunged into a different location as the atmosphere slid by.
In case you were wondering, each piece of Shoemaker-Levy 9 hit with the equivalent energy of the Chicxulub impact. So, whatever else is true about Jupiter, it surely has no dinosaurs left.
You’ll be happy to learn that in recent years, more and more planetary scientists around the world have gone in search of vagabonds from space that might be heading our way. True, our list of potential killer impactors is incomplete, and our ability to predict the behavior of objects millions of orbits into the future is severely compromised by the onset of chaos. But we can focus on what will happen in the next few decades or centuries.
Among the population of Earth-crossing asteroids, we have a chance at cataloguing everything larger than about one kilometer wide—the size that begins to wreak global catastrophe. An early-warning and defense system to protect the human species from these impactors is a reachable goal. Unfortunately, objects much smaller than a kilometer, of which there are many, reflect much less light and are therefore much harder to detect and track. Because of their dimness, they can hit us without notice—or with notice far too short for us to do anything about them. In January 2002, for instance, a stadium-size asteroid passed by at about twice the distance from here to the Moon—and it was discovered just twelve days before its closest approach. Given another decade or so of data collecting and detector improvements, however, it may be possible to catalogue nearly all asteroids down to about 140 meters across. While the small stuff carries enough energy to incinerate entire nations, it will not put the human species at risk of extinction.
Any of these we should worry about? At least one. On Friday the 13th, 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 altitude of our communication satellites. We did not name this asteroid Bambi. Instead, we named it Apophis, after the Egyptian god of darkness and death. If the trajectory of Apophis at close approach passes within a narrow range of altitudes called the “keyhole,” then the influence of Earth’s gravity on its orbit will guarantee that seven years later, in 2036, on its next trip around, the asteroid will hit Earth directly, likely slamming into the Pacific Ocean between California and Hawaii. The five-story tsunami it creates will wipe out the entire west coast of North America, dunk Hawaiian cities, and devastate all the landmasses of the Pacific Rim. If Apophis misses the keyhole in 2029, we will have nothing to worry about in 2036.
Once we mark our calendars for 2029, we can either pass the time sipping cocktails at the beach and planning to hide from the impact, or we can be proactive.
The battle cry of those anxious to wage nuclear war is “Blow it out of 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 space, where there is no air, there can be no shock waves, and so a nuclear warhead must actually make contact with the asteroid to do damage.
Another method would be to engage a radiation-intensive neutron bomb (that’s the Cold War–era bomb that kills people but leaves buildings intact). The bomb’s high-energy neutron bath would heat up one side of the asteroid, causing materi
al to spew forth and thus induce the asteroid to recoil. That recoil would alter the asteroid’s orbit and remove it from the collision path.
A kindler, gentler method would be to nudge the asteroid out of harm’s way with slow but steady rockets that have somehow been attached to one side. Apart from the uncertainty of how to attach rockets to an unfamiliar material, if you do this early enough, then all you need is a small push using conventional chemical fuels. Or maybe you attach a solar sail, which harnesses the pressure of sunlight for its propulsion, in which case you’ll need no fuel at all.
The odds-on favorite solution, however, is the gravitational tractor. This involves parking a probe in space near the killer asteroid. As their mutual gravity draws the probe to the asteroid, an array of retro rockets fires, instead causing the asteroid to draw toward the probe and off its collision course with Earth.
The business of saving the planet requires commitment. We must first catalogue every object whose orbit intersects Earth’s. We must then perform precise computer calculations that enable us to predict a catastrophic collision hundreds or thousands of orbits into the future. Meanwhile, we must also carry out space missions to determine in great detail the structure and chemical composition of killer comets and asteroids. Military strategists understand the need to know your enemy. But now, for the first time, we would be engaged in a space mission conceived not to beat a spacefaring competitor but to protect the life of our entire species on our collective planetary home.
Whichever option we choose, we will first need that detailed inventory of orbits for all objects that pose a risk to life on Earth. The number of people in the world engaged in that search totals a few dozen. I’d feel more comfortable if there were a few more. The decision comes down to how long into the future we’re willing to protect the life of our own species on Earth. If humans one day become extinct from a catastrophic collision, it won’t be because we lacked the brainpower to protect ourselves, but because we lacked the foresight and determination. The dominant species that replaces us on postapocalyptic Earth might just wonder why we fared no better than the proverbially pea-brained dinosaurs.
Space Chronicles: Facing the Ultimate Frontier Page 6