[extropy-chat] SURVIVAL: When asteroids or comets strike

scerir scerir at libero.it
Sat Apr 10 17:57:04 UTC 2004

> Question: If an asteroid is spinning, would its angular momentum be
> translated into explosive energy upon impact? If so, how much would a 3
> km body tumbling at 1 rotation/10 minutes add to the impact from its
> rotation?

It seems too slow that rotation to cause extra damages, imo.
Interesting the impact of an asteroid on the ocean.

WASHINGTON, D.C., Jan. 7, 1998 -- Los Alamos National Laboratory
researchers are demonstrating the enormous damage of an asteroid
strike -- not from an impact on land but from tsunamis caused by an
asteroid hitting Earth's oceans. Computer models show how impacts of
various sizes will generate waves that could devastate entire coastlines
on several continents. A surveillance and defense system could prevent
such a disaster.

Astrophysicist Jack Hills of the U.S. Department of Energy's Los Alamos
National Laboratory presented his findings today at a news conference
and a scientific session at the Washington, D.C., meeting of the American
Astronomical Society.

A tsunami is a fast-moving ocean wave, usually caused by underwater
earthquakes or volcanic eruptions, that runs up on a coastline, causing
widespread damage. A tsunami retains its destructive energy while it
travels enormous distances. When the wave strikes a continental shelf,
its speed decreases and its height increases. An asteroid impact would
induce a series of waves that could scour thousands of miles of coastline
with walls of water and roiling debris.

Hills and his colleague Charles Mader use a detailed numerical
simulation with a one kilometer spatial resolution and comparative data
from historical tsunami events.

The Los Alamos model estimates that an asteroid three miles across
hitting the mid-Atlantic would produce a tsunami that would swamp the
entire upper East Coast of the United States to the Appalachian
Mountains. Delaware, Maryland and Virginia would be inundated,
including Long Island and all the coastal cities in this region. It would
drown the coasts of France and Portugal.

Alternately, Hills' model shows how much of Los Angeles and Waikiki
would be lost if the same rock cratered the ocean between Hawaii and
the West Coast.

Fortunately, Earth is likely to take a hit from an object that large only
once every 10 million years. However, the chance of a strike by a
relatively small asteroid is two or three thousand times more likely, or
once every few thousand years.

Objects larger than about 600 feet across are virtually unaffected by the
atmosphere and will reach Earth's surface at nearly full velocity to cause
a crater on land or sea. Most of the damage from such an impact would
come from a tsunami.

For example, the Los Alamos model shows that an asteroid about 1,300
feet in diameter would devastate the coasts on both sides of the ocean
with a tsunami more than 300 feet high.

Asteroids smaller than the threshold 600 feet across lose most of their
energy in the atmosphere but can still cause unprecedented damage. A
"small" impactor hit near the Tunguska River in central Siberia in 1908.
Though it never hit the ground, the shock wave flattened 800 square
miles of forest.

An impact like Tunguska, which hit with a force a thousand times greater
than the Hiroshima bomb, occurs over land every 300 years on average.
Hills and Mader have received Laboratory funding for an additional three
years of model development. They expect increasingly sophisticated
models to predict more extensive coastal damage than previously
calculated. And Hills would like to see the research yield a practical plan
of defense. "An impact from the smaller asteroids is one disaster that is
preventable," Hills said.

But to deflect an asteroid on a collision course, first it must be seen
ahead of time. Then a nuclear-armed rocket must be ready to intercept it.
A nuclear blast in space could either shatter or re-direct the incoming
asteroid, Hills said. Currently, there is no such surveillance or defense
capability in place.

"It's a problem that could be solved for much less than the cost of one
hurricane. We could just set it up and be done with it," said Hills.

Los Alamos National Laboratory is operated by the University of California
for the U.S. Department of Energy.


http://www.sthjournal.org/212/gisler.pdf  (about 16 pages)
Galen Gisler, Robert Weaver, Charles Mader
Los Alamos National Laboratory, Los Alamos,
Michael Gittings
Science Applications International Los Alamos

We have performed a series of two-dimensional and three-dimensional
simulations ofasteroid impacts into an ocean using the SAGE code from
Los Alamos NationalLaboratory and Science Applications International
Corporation. The SAGE code is a compressible Eulerian hydrodynamics
code using continuous adaptive meshrefinement for following discontinuities
with a fine grid while treating the bulk of the simulation more coarsely.
We have used realistic equations of state for the atmosphere,sea water,
the oceanic crust, and the mantle. In two dimensions, we simulated asteroid
impactors moving at 20 km/s vertically
through an exponential atmosphere into a 5 km deep ocean. The impactors were
composed of mantle material (3.32 g/cc) or iron (7.8g/cc) with diameters
from 250m to 10 km. In our three-dimensional runs we simulatedasteroids of 1
km diameter composed of iron moving at 20 km/s at angles of 45 and 60
from the vertical. All impacts, including the oblique ones, produce a
large underwater cavities with nearly vertical walls followed by a collapse
starting from the bottom and subsequent vertical jetting. Substantial
of water are vaporized and lofted high into the atmosphere. In the larger
impacts, significant amounts of crustaland even mantle material are lofted
as well. Tsunamis up to a kilometer in initial heightare generated by the
collapse of the vertical jet. These waves are initially complex inform, and
interact strongly with shocks propagating through the water and the
crust.The tsunami waves are followed out to 100 km from the point of impact.
Their periods and wavelengths show them to be intermediate type waves, and
not (in general) shallow-water waves. At great distances, the waves decay as
the inverse of the distance from the impact point, ignoring sea-floor
topography. For all impactors smaller thanabout 2 km diameter, the impacting
body is highly fragmented and its remains lofted into the stratosphere with
the water vapor and crustal material, hence very little trace of the
impacting body should be found for most oceanic impacts. In the oblique
impacts,the initial asymmetry of the transient crater and crown does not
persist beyond a tsunami propagation length of 50 km.
-Science of Tsunami Hazards, Volume 21,(2003)

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