It happens once in a great while: a mountain of rock crashes out of the sky at a rate 20 times faster than a speeding bullet. With an indescribable explosion, it turns Earth into a cataclysm of fire and dust storms, earthquakes, tidal waves. When the last such disaster occurred on Earth 65 million years ago, it wiped out three quarters of the planet's life forms-including the dinosaurs. What hit?
An asteroid, one of many billions of space rocks orbiting the sun. Scientists think asteroids, which range in size from a meter in diameter to as wide as 932 kilometers (580 miles)-may be leftover "bits" that didn't clump together to form a planet in the early days of the solar system 4.6 billion years ago.
Today, 90 percent of all known asteroids reside in a donut-shape region called the asteroid belt, between Jupiter and Mars. Others, called "Trojan asteroids," cloud beside Jupiter, held captive by the planet's and the sun's gravity (attracting force). Then there are Near-Earth asteroids with orbits that regularly swing near Earth. Depending on how gravity yanks at their orbits, these rocks could slowly spin toward a collision course with Earth.
As scientists learn more about the massive space rock that doomed the dinos, read on to see how they're ratcheting up the hunt for potentially lethal asteroids, and how they plan to prevent a disastrous sequel.
It's hard to imagine that a giant rock could devastate an entire planet, but 65 million years ago, a 10 kilometer (6 mile)-wide asteroid speeding between 22,000 and 67,000 miles per hour did just that, says Dan Durda, an asteroid scientist at the Southwest Research Institute in Colorado: "It was every ecological disaster you can think of happening at the same time."
At that speed, the asteroid packed enough energy to pound out a nearly 200-km-wide hole called the Chicxulub (CHEEK-zhu-lube) crater; the crater now lies submerged under eons of accumulated seafloor mud and water north of Mexico's Yucatan Peninsula.
Seconds after impact, a shockwave (powerful blast of air) buckled Earth's crust (surface), killing every living thing and triggering earthquakes as far as thousands of kilometers away. Some Chicxulub quakes would shatter today's Richter scale-the measurement of an earthquake's energy from 1 to 10-and score a 12. (Compare them with a 1999 quake that measured 7.4 in Izmet, Turkey, leveling the city and claiming more than 30,000 human lives.) Then underwater earthquakes unleashed tsunamis, monster waves that buried entire coastlines and caused mountains to topple into oceans.
But the grand slam: On impact, the Chicxulub asteroid spewed 25 trillion tons of earth across the planet and into space. "Some was thrown halfway to the Moon," Durda says. Within an hour, most debris rained back toward Earth. With high velocity (speed), it tore through the atmosphere. And friction (rubbing forces) caused much of the debris to incinerate in the air. Plant life erupted into flames and ignited wildfires worldwide.
In the fiery aftermath, soot obliterated sunlight for years and the planet suffered a long cold spell. Chemical reactions from the fires increased the amounts of toxic chemicals like nitric acid in the atmosphere, letting loose life-devastating acid rain.
In the end, one quarter of all life on Earth survived. Chicxulub's lesson: Humanity couldn't endure a monster asteroid hit. Fortunately, says Durda, "Large impacts of this scale happen once in hundreds of millions of years."
Today, astronomers around the world routinely survey space to scope out and chart the course of any asteroid. In the U.S. alone, at least seven surveys are underway. Their goal by 2008: to pinpoint 90 percent of Near-Earth Asteroids (NEAs) with a width larger than 1 kilometer. Thousands of asteroids stray near our neighborhood, and space rocks 1 km or larger in diameter could spell global catastrophe; approximately 1,000 NEAs larger than 1 km roam near space, according to the National Optical Astronomy Observatory.
How do astronomers search for asteroids? "Basically, you take a picture of the sky and minutes later take another, to see which objects have moved," says David Tholen of the University of Hawaii's Institute for Astronomy. Scientists use computer-controlled telescopes and CCD (Charge-Coupled Device) cameras-with the same light sensing devices found in digital cameras that are 20 times more sensitive than film to faint starlight. "The astronomer divides the sky into a chessboard grid," says Tholen, "and the telescope repeatedly searches the grid."
The light from the sun reflected off asteroids is captured by telescopes and directed on to a focal plane loaded with CCDs; the CCDs transmit the light into electrical data sifted by a computer. "The computer chugs
through all space objects and creates a list of what's moved," Tholen says. Asteroids that orbit near Earth are likely to move a lot in the images, while stars hundreds of trillion kilometers away don't seem to move at all. When astronomers spy an asteroid, they use its changing position in each image to calculate the rock's orbit. If the estimated orbit takes it suspiciously close to Earth and the asteroid is wider than a half kilometer-the length of 5 football fields astronomers keep an eye glued on it.
Until recently, whenever astronomers discovered an NEA, news headlines blared doomsday scenarios. "But scientists now realize thousands of small asteroids [under 1 km wide] have the potential to hit Earth," says planetary astronomer Richard Binzel of the Massachusetts Institute of Technology. Some asteroids consist of crumbly granite-like rocks. And small asteroids of this composition easily break apart when entering the atmosphere. Other asteroids are metallic-made mostly of nickel and iron-and are less likely to splinter. Even so, the probability of a small metallic asteroid striking human-populated areas is low. That's because open spaces or oceans cover most of Earth.
To quell public panic, however, in 1998 Binzel and colleagues devised an asteroid danger scale. Called the Torino Impact Hazard Scale, it rates NEAs from 0 to 10. Zero means no hazard. "Ten means we're certain to be hit, and it'll be as bad as the dinosaurs," says Binzel. Currently, no asteroid has rated above 2, meaning it virtually has no chance to hammer Earth in the foreseeable future.
That's not to say there hasn't been some nail-biting. Last August, an 800 meter (2,500 foot)-wide asteroid strayed 530,000 km (330,000 mi) from Earth. "2002 NY40" veered so close that many sky watchers caught a speck of gliding light through binoculars. 2002 NY40 had been spotted only a month before. "We can't find every single one of them," says Tholen. "We're going to miss some."
The most easily missed asteroids are those that take hundreds of years to orbit the sun. (Asteroids' orbits vary, but most take between 3 to 6 years to circle the sun once.) They're so far away they can't be seen "until, surprise, they suddenly show up next door," Tholen says. But 2002 NY40 isn't typical or scary-it rates a zero on the scale. Astronomers detect most potentially threatening asteroids years or decades before their orbits swing them too close.
How to deal with killer asteroids? Scientists are concocting some far-out ideas. One strategy: Send a spacecraft equipped with powerful engines to nudge it into a different orbit. "It wouldn't take much," says Binzel. As a tiny tap causes a billiard ball to veer and miss a pool-table pocket, a small nudge on an asteroid creates a huge difference in its orbit, he says (see diagram, p. 14-15). Another option: set off a bomb on the asteroid. But the consequences of turning a potentially planet killing rock into a mere 20 city-killer have squared off scientists in debate.
A less violent method to knock an asteroid off course is to give it a paint job (see diagram, p 14)! While asteroids vary in shape and color, most are dark rock. And like wearing black pants in summer, the rock heats up because dark colors absorb sunlight. Heat radiating off sunlit sides of large asteroids actually acts as a weak "rocket engine" that propels the asteroid.
Scientists think this principle, called the Yarkovsky effect, could be altered by painting an asteroid a less heat-absorbing color like white to alter its natural orbit. With 20 or 30 years warning of a potential killer, "it's a case where time is on our side," says Binzel.