Space Chronicles: Facing the Ultimate Frontier (8 page)

• • •
CHAPTER FIVE

KILLER ASTEROIDS
^*

T
he chances that your tombstone will read “K
ILLED BY
A
STEROID
” are about the same as they’d be for “K
ILLED IN
A
IRPLANE
C
RASH
.” Only about two dozen people have been killed by falling asteroids in the past four hundred years, while 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 ten million years, when the sum of all airplane crashes has killed a billion people (assuming a death-by-airplane rate of a hundred per year), an asteroid large enough to kill the same number of people will have hit Earth. The difference is that while airplanes are continually killing people a few at a time, that asteroid might not kill anybody for millions of years. But when it does hit, it will take out a billion people: some instantaneously, and the rest in the wake of global climatic upheaval.

The combined impact rate for asteroids and comets in the early solar system was frighteningly high. Theories of planet formation show that chemically rich gas cooled and condensed to form molecules, then particles of dust, then rocks and ice. Thereafter, it was a shooting gallery. Collisions served as a means for chemical and gravitational forces to bind smaller objects into larger ones. Those objects that, by chance, had accreted slightly more mass than average had slightly higher gravity, attracting other objects even more. As accretion continued, gravity eventually shaped blobs into spheres, and planets were born. The most massive planets had sufficient gravity to retain the gaseous envelope we call an atmosphere.

Every planet continues to accrete, every day of its life, although at a significantly lower rate than when it first formed. Even today, interplanetary dust rains down on Earth in vast quantities—typically a hundred tons of it a day—though only a small fraction reaches Earth’s surface. The rest harmlessly vaporizes in Earth’s atmosphere as shooting stars. More hazardous are the billions, likely trillions, of leftover rocks—comets and asteroids—that have been orbiting the Sun since the early years of our solar system but haven’t yet managed to join up with a larger object.

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.

I
n 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.

M
uch 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.

O
f 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.

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-Earth 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.