[meteorite-list] Small Stony Asteroids Will Explode and Not Hit Earth, Study Shows

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:05 2004
Message-ID: <200307161919.MAA19338_at_zagami.jpl.nasa.gov>

http://space.com/scienceastronomy/asteroid_breakup_030716.html

Small Stony Asteroids Will Explode and Not Hit Earth, Study Shows
By Robert Roy Britt
space.com
16 July 2003

When asteroids fall through Earth's atmosphere, a variety of things can
happen. Large iron-heavy space rocks are almost sure to slam into the
planet. Their stony cousins, however, can't take the pressure and are more
likely to explode above the surface.

Either outcome can be dismal. But the consequences vary.

So scientists who study the potential threat of asteroids would like to know
more about which types and sizes of asteroids break apart and which hold
together. A new computer model helps to quantify whether an asteroid
composed mostly of stone will survive to create a crater or not.

A stony space rock must be about the size of two football fields, or 720
feet (220 meters) in diameter, to endure the thickening atmosphere and slam
into the planet, according to the study, led by Philip Bland of the
Department of Earth Science and Engineering at Imperial College London.

"Stones of that size are just at the border where they're going to reach the
surface -- a bit lower density and strength and it'll be a low-level air
burst, a bit higher and it'll hit as a load of fragments and you'll get a
crater," said Bland, who is also a Royal Society Research Fellow.

The distinction would mean little to a person on the ground.

Two ways to destroy a city

"An airburst would be a blast somewhere in the region of 500-600 megatons,"
Bland said in an e-mail interview. "As a comparison, the biggest-ever
nuclear test was about 50 megatons."

A presumed airburst in 1908, over a remote region of Siberia called
Tunguska, flattened some 800 square miles (2,000 square kilometers) of
forest. The object is estimated to have been just 260 feet wide (80 meters).
Bland said the event was probably equal to about 10 megatons.

"If most of it made it to the ground you might actually be a bit better off,
because the damage would be a little more localized," he said. "A lot of
energy would still get dumped in the atmosphere, but you'd probably also
have a ragged crater, or crater field, extending over several kilometers,
with the surrounding region flattened by the blast."

Smaller stony asteroids, say those the size of the car, enter the atmosphere
more frequently but typically disintegrate higher up and cause no damage. In
fact, as many as two or three dozen objects ranging from the size of a
television to a studio apartment explode in the atmosphere every year,
according to data from U.S. military satellites.

Separate research in recent years has shown that stony asteroids are often
mere rubble piles, somewhat loose agglomerations of material that may have
been shattered in previous collisions but remain gravitationally bound.

Pieces and parts

The new computer model is detailed in the July 17 issue of the journal
Nature. It was created with the help of Natalia Artemieva at the Russian
Academy of Sciences.

Previous models treated the cascade of fragments from a disintegrating
asteroid as a continuous liquid "pancake." The new model tracks individual
forces acting on each fragment as the bunch descends.

The researchers can plug in asteroid size, density, strength, speed and
entry angle at the top of the atmosphere. With "reasonable confidence" a
computer program then details how that rock should behave in the air and
what will happen at the surface.

The model has implication not just for land-based impacts, but also
splashdowns in the ocean that can trigger devastating tsunamis. An airburst
is not likely to generate much of a tsunami, possibly lowering that risk
compared to what scientists had figured.

The results suggest rocks about 720 feet across (220 meters) are likely to
actually hit the surface every 170,000 years or so. Some previous research
has suggested a frequency of every 4,000 years or less.

Looking back

The model can also "hindcast" what sort of rock might have generated a
certain known crater.

"You see a crater field on Mars, we can tell you what sort of object caused
it," Bland said.

In fact, he and Artemieva have done just that. In their most recent tests,
which are not discussed in the Nature paper, they plugged in the atmospheric
details of Mars, as well as Venus, and hurled some hypothetical space rocks
at those planets.

"The simulated crater fields that the model produces look almost exactly
like the real thing," Bland said.

For now, the model does not handle very large asteroids, those that could
cause widespread regional or even global damage, though Bland said the flaw
may be fixable. He is careful to point out that computer models do not
provide solid proof for what might happen.

"There are still a lot of unknowns in this," he said.
Received on Wed 16 Jul 2003 03:19:41 PM PDT