Coming Attractions

Natural History Magazine

by Neil deGrasse Tyson

From Natural History Magazine, September 1997

One needn't look far to find scary predictions of a global holocaust by killer asteroids. That's good, because most of what you might have seen read or heard is true.

The chances that your or my tombstone will read killed by asteroid are about the same for killed in an airplane crash. About two-dozen people have been killed by falling asteroids in the past 400 years, but thousands have died in crashes during the relatively brief history of passenger air travel. So how can this comparative statistic be true? Simple. The impact-record shows that by the end of 10 million years, when the sum of all airplane crashes has killed a billion people (assuming a death-by-airplane rate of 100 per year), an asteroid is likely to have hit Earth with enough energy to kill a billion people. What confuses the interpretation is that while airplanes kill people a few at a time, our asteroid might not kill anybody for millions of years. But when it hits, it will take out hundreds of millions of people instantaneously and many more hundreds of millions in the wake of global climatic upheaval.

The combined asteroid & comet impact rate in the early solar system was frighteningly high. Theories of planet formation show that chemically rich gas condenses to form molecules, then particles of dust, then rocks and ice. Thereafter, it's a shooting gallery. Collisions serve as a means for chemical and gravitational forces to bind smaller objects into larger ones. Those objects that, by chance, accreted slightly more mass than average will have slightly higher gravity and attract other objects even more. As accretion continues, gravity eventually shapes blobs into spheres and planets are born. The most massive planets had sufficient gravity to retain gaseous envelopes. All planets continues to accrete for the rest of their days, although at a significantly lower rate than when formed. Still, there remain billions (possibly trillions) of comets in the extreme outer solar system (up to a thousand times the size of Pluto's orbit) that are susceptible to gravitational nudges from passing stars and interstellar clouds that set them on their long journey inward toward the Sun. Solar system leftovers also include short-period comets, of which two dozen are known to cross Earth's orbit, and thousands of catalogued asteroids, of which at least a hundred do the same.

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. We cannot simultaneously be happy we live on a planet; happy that our planet is chemically rich; and happy we are not dinosaurs; yet resent the risk of planet-wide catastrophe. Some of the energy of an asteroid collision with Earth gets deposited into our atmosphere through friction and an airburst of shock waves. Sonic booms are shock waves too, but they are typically made by airplanes with speeds anywhere between one and three times the speed of sound. The worst damage they might do is jiggle the dishes in your cabinet. But with a speeds upwards of 45,000 miles per hour—nearly 70 times the speed of sound—the shock waves from your 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 is deposited on Earth's in an explosive event that heats the ground and blows a crater that can measure twenty times the diameter of the original object. If many impactors strike with little time between each event then Earth's surface would not have enough time to cool between impacts. We infer from the pristine cratering record on the surface of the Moon (our nearest neighbor in space) that Earth experienced an era of heavy bombardment 4.6 billion – 4 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 unrelentingly sterilized. The formation of complex molecules, and thus life, was inhibited, although all the basic ingredients were being delivered nonetheless. How long did life take to emerge? An often-quoted figure is 800 million years (4.6 billion – 3.8 billion = 800 million). But to be fair to organic chemistry, you must first subtract all the time Earth's surface was forbiddingly hot. That leaves a mere 200-million years over which life emerged from a rich chemical soup, which, as does all good soups, includes water.

Yes, the water you drink each day was delivered to Earth by comets more than 4 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. Ejection occurs when impactors carry so much energy that smaller rocks near the impact zone are thrust upwards with sufficient speed to escape the gravitational grip of the planet. Afterward, the 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, though circumstantial, evidence that simple life on the Red Planet thrived a billion years ago. Mars has boundless geological evidence for a history of running water that includes dried river beds, river deltas, and flood plains. 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 when you speculate whether life arose on Mars first, was blasted off its surface as the solar system's first bacterial astronaut, and then arrived to jump-start Earth's own evolution of life. There's even a word for the 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 over two and a half times the energy than that required to leave Mars. Furthermore, Earth's atmosphere is about 100 times denser. Air resistance on Earth (relative to Mars) is formidable. Bacteria would have to be hardy indeed to survive the several million years of interplanetary wanderings before landing on Earth. Fortunately, there is no shortage of liquid water and rich chemistry on Earth, so we do not require theories of panspermia to explain the origin of life as we know it, even if we still cannot explain it.

Ironically, we can (and do) blame impacts for major episodes of extinction in the fossil record. But what are the risks to life and society? Below is a table that relates average collision rates on Earth with the size of impactor and the equivalent energy in millions of tons of TNT. For reference, I include a chart below that compares the impact energy in units of the atomic bomb that the United States dropped on the city of Hiroshima in 1945. The data are adapted from a graph by NASA's David Morrison, as reported in the congressionally mandated study, The Spaceguard Survey: Report of the NASA International Near-Earth Object Detection Workshop (Pasadena: JPL).

Once per... Asteroid Diameter Impact Energy
[Meters] [Megatons of TNT] [A-Bombs]
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 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 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.

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 mid-air, 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 have been left by 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 100 million years. The crater is dated 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 space. The range of energies varies astronomically. In a review of the impact hazard to Earth written in 1994 for the fat book Hazards Due to Comets and Asteroids (ed. Tom Gehrels; Tucson: University of Arizona Press), David Morrison (NASA Ames), Clark R. Chapman (Planetary Science Institute), and Paul Slovic (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 ten 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 Washington, 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 to 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 mass extinction on a scale the Chicxulub impact 65 million years ago, when nearly seventy 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 Spaceguard Survey Report. Unfortunately, objects smaller than about a kilometer do not reflect enough light to be reliably detected and tracked. These can hit us without notice, or they can hit with notice that is much too short for us do anything about it. 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. Have a nice day.

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 no shortage of craters. And Mars, with its historically active geology, reveals large craters that were recently formed.

At over 300 times the mass of Earth, and at over ten times its diameter, Jupiter's ability to attracts 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 Moon landing, comet Shoemaker-Levy 9, having been broken apart into two dozen pieces during a previous close encounter with Jupiter, slammed, one chunk 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 piece of the comet fell in a different location as the atmosphere rotated by. Each piece hit with the equivalent energy of the Chicxulub impact.

Earth's fossil record teems with extinct species—life forms that had thrived far longer than the current Earth-tenure of Homo sapiens. Dinosaurs are in this list. What defense do we have against such formidable impact energies? The battle cry of those with no war to fight is blow them out of the sky with nuclear weapons. 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. (In empty space, 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 harms way with slow but steady rockets that are somehow attached to one side. If you do this early enough then only a small nudge will be required 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 our ability to predict the behavior of objects much farther into the future (for millions and billions of orbits) is severely compromised by the onset of chaos (e.g. see Chaos in the Solar System, Natural History, July 1995).

In this game of gravity, by far the most dangerous breed of impactor is the long-period comet, which, by convention, are those with periods greater than two hundred 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 a much higher impact energy for their size than your run-of-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 orbit was tipped strongly out of the plane of our solar system and nobody was looking. While en route, it came within 10 million miles of Earth (a narrow miss) and made for spectacular nighttime viewing.

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 for all objects that pose a risk to life on Earth. The number of people in the world engaged in this search totals one or two 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 post-apocalyptic Earth just might wonder why we fared no better than the proverbially peabrained dinosaurs.

Neil deGrasse Tyson, an astrophysicist, is the Frederick P. Rose Director of New York City’s Hayden Planetarium and a Visiting Research Scientist at Princeton University.