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[Lazarus Long]

This is another reason we might experience "periodic" effects of meteor showers and potentiallty greater hazardous impacts at various times in the past. I thought this article deserves inclusion, not so much as a "present threat" but to understand better how something that might have happened over 400 million yeasrs ago could still be effecting us, and more importantly how it may have effected us over that whole time period.

Two Asteroids Collided, Showered Earth with Debris
1 hour, 6 minutes ago Science - Space.com
By Robert Roy Britt
Senior Science Writer, SPACE.com

Scientists have found evidence on Earth for an ancient asteroid collision in space that appears to have generated a modest rain of fire on the planet for a few million years.

The explosive collision might have been one of the largest in the solar system's recent history, the researchers say, involving two space rocks each up to 620 miles wide (1,000 kilometers).

Asteroid collisions were once common in the solar system. They are less frequent now, but still occur in a region between Mars and Jupiter known as the asteroid belt, where most of these leftovers from planet formation orbit the Sun.

In ancient marine sediments across a large swath of southern Sweden, researchers found sand-sized grains of the mineral chromite that are low in iron, a sign of extraterrestrial origin. The stuff appears to have fell from the sky about 480 million years ago.

Based on the number and size of the grains, the scientists determined that bright fireballs would have graced Earth's skies about 100 times more often than occurs today, said lead researcher Birger Schmitz at Göteborg University in Sweden.

A small fraction of the fragments fell to the ground over a period of 5 million to 10 million years, Schmitz told SPACE.com via e-mail.

When large asteroids strike Earth directly, mass extinctions can result, experts believe. But that's probably not what happened with this slow shower of debris.

"We see prominent changes in the biota in the sections that we study," Schmitz said, "but there is no mass extinction." The biological shifts evident in the soil are due to environmental change, he said, but it's not clear if the asteroid collision was behind the environmental change or not.

More Asteroid News

Another Recent Asteroid Collision

[Lazarus Long]

http://us.rd.yahoo.c...ion_020612.html
Recent Crash Created Youngest Known Asteroid Family
By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
12 June 2002

A few million years ago, two asteroids collided in interplanetary space. The smaller, aggressor rock was pulverized to dust as it shattered the larger target rock into millions of small and large fragments which were violently dispersed in all sorts of new directions.

Today astronomers said they have traced the paths of a handful of these fragments back to their origin, piecing together what is now the most well documented and recent example of asteroid destruction and creation. The work will provide a wealth of new information about rocks from space and the overall development of the solar system, including Earth.

It could also help scientists model what would happen if they ever try to blow up an asteroid that is heading toward our planet.

Fresh faces

Asteroids were originally formed more than 4 billion years ago, during a chaotic time when the planets developed around a new Sun.

Since then most of them -- including the handful that have been visited by spacecraft -- have undergone multiple impacts and are mere vestiges of their parent bodies. Some are piles of rubble, the result of many impacts. Most are scarred and pitted, their courses altered many times over, their origins difficult to trace.

About 20 asteroid families, however, were created recently enough to be identified as having common origins.

Now David Nesvorny and his colleagues at the Southwest Research Institute (SwRI) have identified 39 known asteroids as debris from a collision that took place practically yesterday in the history of the solar system. These new creations are expected to be largely unaltered since their violent generation just 5.8 million years ago.

The largest remnant is an asteroid named Karin, roughly 12.5 miles wide (20 kilometers). The cluster of boulders, which all exhibit similar composition, has now been given the same name.

The Karin cluster was born when an asteroid estimated to be 1.9 miles wide (3 kilometers) slammed into a 16-mile-wide (25 kilometers) rock at about 11,180 mph (5 km/s), Nesvorny explained. The target rock was 600 times more massive than the smaller one.

At least hundreds and perhaps thousands of fragments larger than 0.62 miles (1 kilometer) were produced, Nesvorny said. An asteroid this large could cause a global catastrophe if it met up with Earth. The collision also generated up to 100 million fragments as big as a football field, he said. Such rocks could destroy a city. Preliminary observations also found space dust that appears to be associated with the crash.

The results will be published in the June 13 issue of the journal Nature.

Glimpsing our past and future

University of Maryland researcher Derek Richardson, who was not involved in the study, said it offers "unprecedented insight into the dynamics of asteroid collisions -- and hence into how the planets of the solar system formed." Here's why:

Earth and the other rocky planets had humble beginnings as rocks, essentially asteroids that grew by gentle collisions to become planets shortly after the Sun was born.

Back in those days, before Jupiter was fully formed, asteroid collisions were more frequent. They also tended to be gentler, however, because most of the material was orbiting the nascent Sun in the same direction. Rocks could join forces and grow into larger objects, eventually able to absorb almost any punch and continue on as a planet.

When Jupiter evolved into the massive object it is now, it began to fling asteroids on wilder courses, thereby generating more catastrophic collisions. What had been a freeway with well-designed onramps that led to mild fender benders gained intersections with no stop lights that forced some serious crackups.

The more violent collisions put a lid on further planet formation among all but the most stout objects -- the four that became Mercury, Venus, Earth and Mars.

[Most astronomers believe a Mars-sized object once hit Earth. The result? Our Moon was forged during 24 hours of chaos. And yet Earth had enough bulk to hang in there.]

Richardson, who wrote a review that is also published in Nature, said the Karin cluster "will no doubt be the focus of attention for the asteroid community for some time" and is a compelling target for a space mission. Asteroids as small as Karin cannot be photographed or studied in detail any other way.

Because the family-building crash occurred relatively recently, Richardson said, "many erosional and weathering processes thought to occur on asteroid surfaces may not have had time to erase the tell-tale signatures of the break-up event."

The Bruce Willis factor

The cluster could also serve as a laboratory for scientists bent on blowing up space rocks that might threaten Earth.

Most asteroids orbit the Sun in a belt between Mars and Jupiter. Astronomers already knew the objects sometimes collide and send fragments on new trajectories around the Sun. A few fragments, large and small, can be gravitationally booted (by Jupiter) or lured (by the Sun) into the inner solar system where they cross the path of Earth's orbit.

That's when they become dangerous, of course.

Some researchers have suggested that if an asteroid is ever found to be on a collision course with our planet, a bomb or missile might be used to destroy or deflect it. But since the idea hasn't been tested, no one knows how an asteroid might come apart. It's possible that the fragments would end up doing more harm than a single object, experts say.

"This event may teach us about how asteroid material breaks up when an energetic impact and explosion occurs," Nesvorny said.

The study team also included William F. Bottke Jr, Luke Dones & Harold F. Levison, all of the SwRI, which is in Boulder, Colorado.

[Lazarus Long]

http://www.space.com...g_010815-1.html

24 Hours of Chaos: The Day The Moon Was Made
By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
15 August 2001

For 25 years, scientists have pondered a theory that the Moon was created when an object the size of Mars crashed into Earth less than 100 million years after the Sun was born, some 4.6 billion years ago. The general idea has been run through the paces and massaged into shape and is now the favored explanation.

But attempts to model cousins of that theory on computers generate inexplicable side effects.

In one of two leading computer models, Earth was creamed while it was still gathering mass, during a brief time when it was only half its current size. All the rocky inner planets are thought to have formed this way, a method rapid accumulation of matter called runaway growth.

But if the Moon was carved out during Earth's growth phase, then it would have been around when Earth continued bulking up by swallowing tremendous numbers of large asteroids. Some of these iron-rich rocks would have hit the Moon, too. Yet the iron is not there.

In the other model, the aggressor was three times as massive as Mars and created an excess of rotation in the Earth-Moon system that simply doesn't exist today.

Now researchers have harnessed the latest in computing power to provide the most detailed model ever made of the cosmic scene that supposedly created the Moon. The result, a 3-D animation of the blast and subsequent chaos, is comforting. It shows that the Moon could have formed when a Mars-sized object hit a fully formed Earth.

The collision would have given Earth its spin, defined what we now call an equator, and put enough material into orbit at the right distance from Earth to allow the formation of a satellite that generations would later swoon over.

24 hours of chaos

Robin Canup of the Southwest Research Institute has been modeling the Moon's formation for eight years. On previous studies, she has worked with William Ward and Alastair Cameron, who represent one of two separate research groups that developed the original impact theory back in the mid-70s. (William K. Hartmann and Donald R. Davis were the other team.)

As Canup knows, all ideas about how the Moon formed must contend with one important fact: The Moon contains very little iron. Earth, on the other hand, is loaded with iron, the bulk of it tied up in the planet's core.

So the Moon is thought to have been pieced together by the bits that got blown off the upper layers of Earth, as well as the outer portions of the object that hit Earth.

Canup's latest effort, produced with the help of Erik Asphaug of the University of California, Santa Cruz, is like a small scene in a blockbuster disaster movie -- the first 24 hours of time in the epic calamity that made the Moon. It is detailed in the Aug. 16 issue of the journal Nature.

The model treats the debris created by the collision as more than 20,000 computational lumps, or particles, all of which are given their own gravity to play with as the cataclysm unfolds.

In a telephone interview, Canup described the day the Moon was made:

A dark, lifeless object less than half as massive as Earth careens around a newborn Sun. It is one of many planet-sized bodies hoping for a long career. But its orbit is shaky. It's future grim. It is a character actor on the grand stage of the solar system, a player of great ultimate consequence but one destined to never see its name in lights.

This doomed "protoplanet" travels a path that crosses the orbits of similar objects and, ultimately, cannot last. Eventually, the nameless protoplanet meets up with a fledgling Earth.

It is not a head-on collision, but rather a glancing blow. The impact imparts what astronomers call angular momentum into the system. It sets Earth to spinning on its axis and creates a Moon that would go round and round the host planet for billions of years.

The shock of the impact strips material from the outer layers of Earth and the impacting object. The mostly iron cores of both bodies meld into Earth's core. It is like a compact car merging onto the highway and colliding with an S.U.V. -- glass, trim and hubcaps fly, but the two chassis remain hopelessly tangled.

All told, about 2 percent of the combined mass of the objects -- mostly rocky stuff that's largely bereft of iron -- begins to orbit the Earth. About half of this eventually becomes the Moon.

Some of the stripped material is heated so fantastically that it vaporizes and expands into the surrounding vacuum of space.

"The material that was vaporized expands into a cloud that envelops the whole planet," Canup explained.

Meanwhile, a long arm of solid matter is winging its way around Earth. Some of it develops into a clump that slams back into the planet. The rest is flung into orbit, all pretty much along a plane that mimics the path of the incoming object. This plane slices through what is now Earth's equator, and it is roughly the same plane along which the Moon orbits.

"The object came in and hit, and that's what set the Earth's rotation and what its equator would be," Canup said.

The model assumes Earth was not spinning before the impact, though it might have been. If it were already spinning, Canup said the model could be tweaked to account for that fact and would still work.

"For the first time, we demonstrated with simulations that a single impact can give you an iron-depleted Moon of the right mass, and the current mass of the Earth, and the current angular momentum of the Earth-Moon system," Canup said.

Though the model covers only a day's time, Canup said shortly thereafter the material in outer regions began to cool. Gradually, small clumps would have formed, collided with one another, and grown. Based on other models, she said it would have taken between 1 and 100 years to make a Moon after the impact.

Case is not airtight

The new model is a significant improvement over previous efforts, which treated gravity as an overall issue or worked with no more than 3,000 computational lumps. But it is just one step toward a fuller understanding of what really happened.

Jay Melosh, a University of Arizona researcher who is known for his work in modeling asteroid impacts, told SPACE.com the new model is an incremental step rather than a trailblazing one. And there are outstanding questions about some assumptions made.

"Their case is not airtight," Melosh said.

In a review of the work that also appears in Nature, Melosh argues that the real promise is in how computers are becoming powerful enough to handle the complicated scenario of a such a colossal impact.

"Not only does such a collision involve all the details of shock physics, melting and vaporization, but the mutual interactions of all those hot fluids squirting around in space have to be taken into account," Melosh writes.

He says that as with any attempt to model the Moon formation, the results hinge on an incomplete understanding of how the energy, density and pressure would affect the material of which the Earth and Moon are composed.

And Canup acknowledges that there is not, and never will be, direct physical remains of the Moon-forming impacter. The ensuing drama was so hot, and the characters so well-mixed, that there are no ancient layered deposits to provide clues, as are found by people like Melosh who study smaller and more recent asteroid impacts.

But Melosh said the prospects for better models are promising. In fact, he is working with Canup and Asphaug on ways to refine the new model to better account for the shock and fluid dynamics. And he figures others will soon use the improved computing power and more capable software packages to produce their own scenarios.

"More studies of this kind will be published in the not-too-distant future," Melosh said.

Other ways the Moon might form

The new study strengthens just one theory of how the Moon might have formed. Other scientists have suggested that the Moon developed elsewhere in the solar system and was captured by Earth. An even more remote possibility is that the Earth and Moon condensed together out of the material that formed the solar system.

Another idea is that gravitational interactions between the Earth, the Sun, and other developing planets simply tore Earth apart and the Moon formed from this debris.

But the majority of researchers prefer the impact theory. And though a similar impact would be extremely unlikely today, it was a fairly common occurrence back when the solar system was forming.

"The last stages of planetary accumulation were very violent," Melosh said. "An event of this type within 100 million years of the birth of the solar system is not rare at all."

The Moon is not the only result of this chaos. In fact, the present spacing between planets "evolved by a sort of natural selection involving the demise of intervening objects whose orbits were not so stable," Melosh said.