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Killer asteroids. (Earth/space science: asteroids).

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--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 a disastrous sequel.

DINO KILLER

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 mined 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."

SPACE WATCH

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 pin-point 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.

ARMAGEDDON ALERT?

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.

[GRAPHIC OMITTED]
THREAT OF NEAR-EARTH ASTEROIDS

 AN ASTEROID ENTERS EARTH'S POTENTIALLY
 THE SIZE OF ... ATMOSPHERE ... CAUSING ...

Dust and small rocks Continually Shooting stars

A Car Twice a month Explosions high in the
 atmosphere with the
 force of a small
 atomic bomb

A blue whale Every few centuries Powerful shockwave
 traveling 100 miles

Titanic Every few hundred Tsunami, if it hit an
 centuries ocean

Half mile A few per million Regional calamity
 years

One mile Every 1 million years Worldwide calamity

Three mils Every 10 million years Human extinction; an
 impact this size is
 believe to have killed
 off the dinosaurs

SOURCE: NASA, ERIC ASPHAUG, AND WALL STREET JOURNAL

ASTEROID ATTACK ODDS

How likely is death by an asteroid hit? Before you
panic, check out the chart below, which compares
the odds and causes of dying in the U.S.

 CAUSE OF DEATH CHANCES

MOTOR VEHICLE ACCIDENT 1 IN 100
HOMICIDE 1 IN 300
FIRE 1 IN 800
FIREARMS ACCIDENT 1 IN 2,500
ELECTROCUTION 1 IN 5,000
ASTEROID/COMET IMPACT 1 IN 20,000
PASSENGER AIRCRAFT CRASH 1 IN 20,000
FLOOD 1 IN 30,000
TORNADO 1 IN 60,000
VENOMOUS BITE OR STING 1 IN 100,000
FIREWORKS ACCIDENT 1 IN 1 MILLION
FOOD POISONING BY BOTULISM 1 IN 3 MILLION
DRINKING WATER WITH EPA LIMIT 1 IN 10 MILLION
OF TRICHOLOETHYLENE

SOURCE: C.R. CHAPMAN & D. MORRISON, 1994, NATURE 367, 33-40


WHERE ARE THE ASTEROIDS?

Asteroids range from 1 meter up to 932 km wide. Ninety percent of all known asteroids reside in the asteroid belt between Mars and Jupiter--254 to 598 million km from the sun.
THE TWO BIGGEST ASTEROIDS

Name
CERES

Diameter
932 Km

Name
PALLAS

Diameter
608 Km


[ILLUSTRATION OMITTED]

INSTEAD OF BRUTE FORCE A GENTLE NUDGE

Scientists who study asteroids are rethinking what might be done if they discovered one on a collision course with Earth. Rather than trying to destroy the asteroid with a nuclear weapon, it might be safer to use one of several gentler approaches that would slowly push it out of the way.

[ILLUSTRATION OMITTED]

Paint It Black?

The Yarkovsky effect, the very slight momentum imparted by the radiation of heat from an asteroid's surface, might be used to slowly deflect an object that threatened the earth.

Incoming light PHOTONS hit the asteroid and heat its surface.

The heat is radiated as THERMAL PHOTONS, which have a very slight mass.

The slight force of the thermal photons leaving the surface creates an equal and opposite force.

There is a delay between light photons' arrival and thermal photons' departure. Because the asteroid rotates, the delay means that thermal photons leave in a direction more in line with the asteroid's orbital path, either increasing or decreasing its momentum.

ALTERING THE SURFACE

Changing the reflectivity of an asteroid would slightly alter how much it heats up. The effect would build up and, given enough time, could eventually alter the orbit. Several methods have been suggested for changing reflectivity.

The asteroid could be covered with dirt, powder, or even paint.

The object could be peppered with small explosive charges, removing the top surface and exposing material beneath that had different thermal properties.

A huge, thin solar sail could be used to power a spacecraft sent to the asteroid, then detached from the craft and wrapped around the object.

OTHER WAYS TO MOVE AN ASTEROID

A NUCLEAR BLAST, some distance from the asteroid, would create a plume of dust and debris, forcing the asteroid in the opposite direction.

A MASS DRIVER, an electromagnetic slingshot on the asteroid, would hurl dirt and rocks from the asteroid into space, creating an opposing force to move the asteroid.

A SOLAR CONCENTRATOR, a mirror that reflects and concentrates sunlight, would vaporize part of the surface, creating a trajectory-changing plume.

FINDING ASTEROIDS

Asteroids are detected by taking multiple images of the same part of the sky over time. A fast-traveling, relatively close object like an asteroid will appear to have moved from image to image. In the past, astronomers had to pore over conventional photographs looking for differences between them; now digital images are compared by computers. Once an asteroid is found and tracked over time its orbit can be calculated to see whether there is a risk it will hit Earth.

SPACE ROCKS: WHAT'S THE DIFFERENCE?

ASTEROIDS Millions of these large rocks orbit the inner solar system. They come in three main types: 1) rock, 2) metal--iron and nickel, and 3) both rock and metal.

COMETS "Dirty snowballs" are made of snow and dust grains. They orbit nearly one light year (9.5 trillion km) from the sun. Some orbits swing comets near the sun--then heat vaporizes them.

METEOROIDS These bits of dust and rock are comet or asteroid debris. Thousands enter the atmosphere daily. Friction burns them up--you see a streak of light, or meteor.

Earth/Space Science: Asteroids

[ILLUSTRATION OMITTED]

Did You Know?

* The Chicxulub asteroid impact was such a significant event in Earth's history, paleontologists use it to mark the end of the Cretaceous period (from 140 million to 65 million years ago) and the beginning of the Tertiary period (65 million to 4 million years ago).

* Near-Earth Asteroids are classified into three types: Amors, which orbit between the orbits of Earth and Mars; Apollos, which cross Earth's orbit; and Atens, asteroids that are always closer to the sun than Earth is.

* If all asteroids in the solar system were pieced together, the combined mass would only total approximately 15 percent of Earth's moon.

Cross-Curricular Connection

History: Have students research and report on the contributions of scientist Giuseppe Piazzi.

Critical Thinking: Screen a Hollywood asteroid-disaster flick. Then have students research to distinguish what may be fiction from what may be factual.

Resources

For an informational and easy-to-understand site on asteroid and comet impact hazards, visit impact.arc.nasa.gov/

NASA's Near-Earth Object Program is located at neo.jpl.nasa.gov/

For an extensive list of Web sites on asteroids and comets, visit:

nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html

Directions: Answer the following questions in complete sentences.

1. What are Near-Earth Asteroids? And why do scientists watch them carefully?

2. Name at least three natural disasters triggered by the mountain-size asteroid that impacted Earth 65 million years ago.

3. How do scientists using computerized telescopes and CCD cameras tell the difference between a star and an asteroid?

4. Scientists sometimes miss detecting asteroids like 2002 NY40. Why?

5. Scientists are concocting some far-out ideas to keep killer asteroids from colliding with Earth. Name one of the three ways described in the article.

ANSWERS

1. Near-Earth Asteroids are asteroids with orbits that regularly swing near Earth. Scientists keep a close watch on these asteroids because depending on how gravity yanks at their orbits, these rocks could slowly move toward a collision course with Earth.

2. When the giant asteroid impacted Earth 65 million years ago, it triggered a massive shockwave, earthquakes--some would score a 12 on today's Richter scale--and tsunamis. The impact also spewed 25 trillion tons of earth across the planet and as far as halfway to the moon. As the debris rained back down to Earth, friction with atmospheric gases caused the debris to incinerate. This ignited wildfires worldwide. Soot from the fires blocked out the sun, causing a cold spell. And chemical reactions from the fires increased toxic chemicals like nitric acid in the atmosphere, which caused acid rain.

3. In images taken by computerized telescopes and CCD cameras, asteroids would move. But stars, hundred of trillions kilometer away, don't seem to move at all.

4. The most easily missed asteroids are the ones that take hundreds of years to orbit the sun. They are usually very far away and then they suddenly seem to show up close to Earth.

5. Send a spacecraft with powerful engines to knock the asteroid off course. Paint the asteroid a different color to alter its path. Use a bomb to blow up the asteroid.
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Title Annotation:colliding into planet Earth
Author:O'Hanlon, Larry
Publication:Science World
Date:Mar 28, 2003
Words:2724
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