[meteorite-list] Undetectable Asteroids Could Destroy Cities, Experts Say

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:14 2004
Message-ID: <200404151727.KAA05275_at_zagami.jpl.nasa.gov>

http://news.nationalgeographic.com/news/2004/04/0414_040414_earthkillers.html

Undetectable Asteroids Could Destroy Cities, Experts Say
Stefan Lovgren
National Geographic News
April 14, 2004

When a massive asteroid, measuring ten kilometers (six miles) across,
smashed into Earth off Mexico's Yucatán Peninsula some 65 million years ago,
it most likely changed the shape of life on Earth.

The dust from the impact, perhaps exacerbated by other asteroid blasts,
blocked the sun, darkening and cooling the Earth. When the dust settled,
increasing greenhouse gases sent temperatures soaring. The violent climate
change, most scientists believe, is what finished the dinosaurs, along with
70 percent of all plants and animals living at the time.

So, could such an asteroid strike again?

Absolutely. But while the dinosaurs didn't know what was about to hit them,
humans probably would. Scientists have already identified more than 700 of
the estimated 1,100 "Earth killers"-asteroids bigger than one kilometer
(about a thousand yards) across-out there. They concluded that none are on a
collision course with the Earth during the next century.

The bad news, however, is there are also about ten million "smaller"
asteroids out there. These could not destroy humankind, if they were to hit
Earth, but could cause widespread damage, possibly even wiping out an entire
city. Because they have not been identified, the smaller asteroids could
potentially strike without warning.

"Finding and cataloguing the big [asteroids] is relatively easy and
inexpensive," said Brian Marsden, director of the Minor Planet Center in
Cambridge, Massachusetts, a clearinghouse for asteroid observations. "But
when it comes to ... the likelihood that there really would be an impact in
the foreseeable future, it is the smaller objects that are of more concern,
and to make a serious search for them would cost a fair bit of money."

Spaceguard

Formed during the creation of the universe, most asteroids are made of rock,
but about 3 percent are made of metals like iron. They range in size from
small boulders to objects that are hundreds of miles in diameter. In our
solar system most asteroids orbit the vast region of space between Mars and
Jupiter.

Debris from asteroids and comets called meteoroids collide with Earth all
the time. But these simply burn up on their descent through Earth's
atmosphere, producing "shooting stars."

On the other hand, the probability of a large asteroid hitting Earth is
extremely slim. However, such an impact could be devastating, which is why
NASA in the 1990s started a program known as Spaceguard. Its goal: to
identify 90 percent of the large near-Earth asteroids-those bigger than a
kilometer in diameter-by the year 2008.

"To date, more than 700 objects of an estimated population of about 1,100
have been discovered," Lindley Johnson, the manager of NASA's Near-Earth
Object Observation Program in Washington, D.C., told the U.S. Senate's
Subcommittee on Science, Technology, and Space last week.

The estimate of the large asteroids vary. Some speculate there may be as
many as 1,500. About 100 objects have been found per year in the last four
years, though experts agree it's unlikely that every single large asteroid
will be found.

"As we discover more, and hopefully conclude that each one cannot hit within
the next century or so, the remaining threat will shift to the smaller
ones," said Clark Chapman of the Southwest Research Institute in Boulder,
Colorado.

Shock Waves

Tracking smaller asteroids is almost impossible, mainly because there are so
many of them-ten million in Earth's neighborhood, according to David
Morrison, a NASA scientist.

The Minor Planet Center receives observations of up to 15,000 new objects
every day from two telescopes in New Mexico alone. An informal network of
amateur astronomers around the world does much of the follow-up observation
work.

While they are not potential Earth killers, smaller asteroids can cause
considerable harm. "Any asteroid larger than 50 meters [164 feet] is a
threat to the place it hits," Morrison said.

In 1908 an asteroid believed to be about 60 meters (197 feet) in diameter
exploded in the atmosphere over Siberia. The resulting shock wave knocked
down trees for hundreds of square miles.

An asteroid made of iron, on the other hand, would crash through the
atmosphere intact and plunge into Earth. If it fell in the ocean, it could
create a giant tsumani that could threaten coastal cities.

According to one expert scenario, there is a 10 percent chance that a
70-meter (230-foot) asteroid will impact Earth in our lifetime, striking
with an energy of 10 megatons, equivalent to 700 Hiroshima-size atomic
bombs.

"There is a better than even chance that an asteroid big enough to do some
damage will impact in a person's lifetime," Johnson said. "However, it will
probably be relatively small and the area of damage fairly isolated, so the
probability of any specific person being affected is quite small, maybe one
in 300,000 in any one year period."

No Warning

A potential hit by a large asteroid is likely to be discovered decades in
advance, allowing scientists to find ways of deflecting the object by, say,
setting off a nuclear bomb on the object to change its orbit.

A smaller asteroid, on the other hand, is likely to slip under the radar.
"Most likely, we'll have no warning at all," Chapman said.

On March 18 of this year, an asteroid measuring perhaps 50 meters across
passed Earth at a distance of 30,000 miles (48,000 kilometers). It was
announced with 23 hours notice.

"Generally, we get a couple of scares each year," Marsden said. "Most of
them are pretty silly, because someone says something unwise."

Chances of getting killed by any asteroid are slim.

"If you are a smoker, or drive without seat belts, forget it," Chapman said.
"If you are worried about shark attacks, or terrorist attacks, or the
chances of another Three Mile Island, then pay attention. The impact hazard
is more likely to kill you than any of those."
Received on Thu 15 Apr 2004 01:27:47 PM PDT