Russian expert says nuclear devices can defend against asteroids

From: http://www.globalsecurity.org/space/library/news/2008/space-080701-rianovosti01.htm

Russian expert says nuclear devices can defend against asteroids
RIA Novosti
01/07/2008 18:45
MOSCOW, July 1 (RIA Novosti) - Nuclear explosive devices are the most effective means of protecting Earth from possible collisions with space bodies, including comets and asteroids, a Russian nuclear physicist told RIA Novosti on Tuesday.
Scientists around the world have long been seeking means of protecting the Earth from the threat of dangerous Near Earth Objects (NEOs). Scientists say such collisions pose a threat on average once every 200-1,000 years.
Vadim Simonenko, deputy head of the Russian Federal Nuclear Center, believes that nuclear explosive devices are more energy efficient, compact and less heavy than lasers or the so-called "gravitational tractors" in terms of their practical application as "weapons against NEOs."
"We in Russia have a wealth of experience in the controlled use of nuclear explosions for peaceful purposes," Simonenko said. "A nuclear device in skillful hands is like a scalpel in the hands of a surgeon."
The scientist said special nuclear devices must be created for effective use against NEOs. In order to disperse an asteroid with diameter of up to 100 meters, these devices must have a yield of about one megaton of TNT equivalent, and weigh several hundred kilograms.
"It is a difficult technical task but it can be accomplished. Besides, there is no need for actual testing [of these devices] because it may be done through computer modeling," he said.
Existing nuclear devices, both military and civilian, were created for specific purposes and do not meet the requirements for "precision strikes" against NEOs, especially comets, which are less predictable than asteroids, Simonenko said.
An earlier report at a Moscow scientific conference said 99942 Apophis, or Asteroid 2004 MN4, with a diameter of 350 meters, currently poses biggest space threat to Earth.
In 2029, this NEO will be at a distance of only 36,000 km (22,400 miles) - closer than satellites in geostationary orbit. Earth's gravity could alter the path of Apophis in such a way that it would collide with Earth on its next approach in 2036.
The explosion could surpass the famous Tunguska explosion of June 30, 1908, which affected a 2,150 square kilometer (830 sq miles) area of Russia felling over 80 million trees in the Krasnoyarsk Territory in Siberia.
Some researchers believe, however, that blowing up NEOs in space poses could result in large fragments surviving the fiery passage through the atmosphere and still hitting the planet.
They propose a more cautionary approach toward dealing with NEOs, by deflecting them from their collision path toward the Earth.
"Deflection is the most favorable strategy, but it requires a considerable early warning period - up to a few years," Simonenko said. "We may not have such a luxury because small asteroids [100-150 m in diameter] are hard to detect."

From: http://impact.arc.nasa.gov/news_detail.cfm?ID=3

Following are reports on the status of the Russian efforts toward planetary defense. The information has been communicated by Vadim Simonenko and Anatoly Zaitsev of the Russian Federal Nuclear Center at Snezhinsk (Chelyabinsk-70). It is primarily the results of three meetings of the Space Protection of the Earth conference.

More at: http://abob.libs.uga.edu/bobk/ccc/cc020701.html

Discovery Channel to Show Space Solar Power!

From: http://dsc.discovery.com/tv/project-earth/highlights/highlights.html

Orbital Power Plant
Premieres Friday, Sept. 12 at 10 p.m. ET/PT.We could have a source of never-ending power and, at the same time,reduce our carbon emissions to virtually zero. This is theastonishing vision of former NASA physicist John Mankins. He has aplan to send thousands of satellites into space, which will gatherenergy from the sun and then beam the solar energy down to Earth asmicrowave energy. The microwave energy will be collected by antennason the ground. These then convert the energy to electricity. CanMankins make it all work?

Space Sunshield
Premieres Friday, Sept. 12 at 9 p.m. ET/PT.Astronomer and professor Roger Angel thinks he can diffract the powerof the sun by placing trillions of lenses in space and creating a 100,000-square-mile sunshade. He intends to use electromagneticpropulsion to get the lenses into space. Professor Angel hasproduced a diffraction pattern that will be etched onto each lens.The pattern will cause the sun's rays to change direction. The taskforce tests this pattern by etching it onto a lens. A scale model isbuilt in a giant hanger with a model of Earth, a single lens representing the sunshade and a laser representing the sun. When the laser is turned on, it should hit the lens as a single beam and thensplit into a number of smaller beams that are diffracted away from the model.

"It's always science fiction until someone goes out and does it"

From: http://www.wired.com/science/space/news/2008/08/space_limits

Rocket Scientists Say We'll Never Reach the Stars

Many believe that humanity's destiny lies with the stars. Sadly for us, rocket propulsion experts now say we may never even get out of the Solar System.
At a recent conference, rocket scientists from NASA, the U.S. Air Force and academia doused humanity's interstellar dreams in cold reality. The scientists, presenting at the Joint Propulsion Conference in Hartford, Connecticut, analyzed many of the designs for advanced propulsion that others have proposed for interstellar travel. The calculations show that, even using the most theoretical of technologies, reaching the nearest star in a human lifetime is nearly impossible.
"In those cases, you are talking about a scale of engineering that you can't even imagine," Paulo Lozano, an assistant professor of aeronautics and astronautics at the Massachusetts Institute of Technology and a conference attendee, said in a recent interview.
The major problem is that propulsion -- shooting mass backwards to go forwards -- requires large amounts of both time and fuel. For instance, using the best rocket engines Earth currently has to offer, it would take 50,000 years to travel the 4.3 light years to Alpha Centauri, our solar system's nearest neighbor. Even the most theoretically efficient type of propulsion, an imaginary engine powered by antimatter, would still require decades to reach Alpha Centauri, according to Robert Frisbee, group leader in the Advanced Propulsion Technology Group within NASA's Jet Propulsion Laboratory.
And then there's the issue of fuel. It would take at least the current energy output of the entire world to send a probe to the nearest star, according to Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute. That's a generous figure: More likely, Cassenti says, it would be as much as 100 times that.
"We just can't extract the resources from the Earth," Cassenti said during his presentation. "They just don't exist. We would need to mine the outer planets."
A 160-Million-Ton Needle
Interstellar propulsion systems are not a new idea. Rocket scientists, aeronautical engineers and science-fiction enthusiasts have proposed such designs for several decades.
In 1958, U.S. scientists explored the possibility of a spaceship propelled by dropping nuclear bombs out the back, a so-called nuclear-pulsed rocket. The research, called Project Orion, was killed by the signing of the Nuclear Test Ban Treaty and the budgetary requirements of the Apollo Project.
In 1978, the British Interplanetary Society designed a mission to Barnard's Star, almost 6 light years away, using a pulsed fusion rocket fueled by deuterium. Building such a spaceship would require mining the outer planets for fuel for at least two decades, scientists said at the Joint Propulsion Conference this year.
But the thought experiments continue. At the conference, Frisbee presented a theoretical design for a ship using antimatter to propel its way to nearby stars.
Frisbee's design calls for a long, needle-like spaceship with each component stacked in line to keep radiation from the engines from harming sensitive equipment or people.
At the rocket end, a large superconducting magnet would direct the stream of particles created by annihilating hydrogen and antihydrogen. A regular nozzle could not be used, even if made of exotic materials, because it could not withstand exposure to the high-energy particles, Frisbee said. A heavy shield would protect the rest of the ship from the radiation produced by the reaction.
A large radiator would be placed next in line to dissipate all the heat produced by the engine, followed by the storage compartments for the hydrogen and antihydrogen. Because antihydrogen would be annihilated if it touched the walls of any vessel, Frisbee's design stores the two components as ice at one degree above absolute zero.
The systems needed to run the spacecraft come after the propellant tanks, followed by the payload. In its entirety, the spaceship would resemble a large needle massing 80 million metric tons with another 40 million metric tons each of hydrogen and antihydrogen. In contrast, the Space Shuttle weighs in at a mere 2,000 metric tons.
"Interstellar missions are big," Frisbee said, in part because of the massive amounts of energy (and hence fuel) required to get moving fast enough to make the trip in anything like a reasonable amount of time. "Any time you try to get something up to the speed of light, Newton is still God."
With that fuel, it would still take nearly 40 years to travel the 4.3 light years to Earth's nearest neighbor, Alpha Centuri, he said.
Down and Out On Earth
Even improving humans' access to near space is not easy.
Scientists have all but discarded ideas for rockets that can reach orbit using a single stage. Instead, private space ventures have focused on lightening the payload and rocket and on increasing reliability. If space tourism comes into vogue, then launch providers could benefit from economies of scale.
But alternative-propulsion systems? They are not in short supply in people's imaginations, but most fail the test of reality, Marcus Young, a researcher at the U.S. Air Force Research Lab's Advanced Project Group, told conference attendees. Young and his team surveyed ideas for launch vehicles that could be accomplished in the next 15 to 50 years and found most to be unworkable.
Space elevator? Even if the engineering made sense, the design requires a breakthrough in materials science to create cables long and strong enough. Rail guns? A vehicle would have to shoot down a 100-kilometer track at 50 times the force of gravity to achieve orbit. Nuclear power? Radioactivity would limit its use to outside Earth's atmosphere, and the politics are positively toxic.
"There are a lot of ideas that initially you say, 'Hey, that might work,'" Young said. "But after a little research, you quickly find that it won't."
Yet, just because science fiction is not yet a reality is not a reason to make science suffer, said MIT's Lozano.
"There is a lot of interesting stuff that you cannot do even in the solar system," he said. "We have the technical means to do it. But some of the most sophisticated technologies ... we have not developed. Not because we can't, but because we have not made it a priority."
As for interstellar travel, even the realists are far from giving up. All it takes is one breakthrough to make the calculations work, Frisbee said.
"It's always science fiction until someone goes out and does it," he said.

SSP on Video

Message: http://www.youtube.com/watch?v=YiU9MibyBJ0
Mankins: http://www.nss.org/settlement/ssp/sspnasavideo.htm
ESA: http://www.youtube.com/watch?v=iIXI_pl-Akk&feature=related
Animation: http://www.youtube.com/watch?v=BoxXlF9mepU&feature=related
Goldstone Demo: http://www.youtube.com/watch?v=7O44WM1Q9H8&feature=related
Hoffert @ Google: http://www.youtube.com/watch?v=V9YD9-_WTjk&feature=related
Guy Pinolet: http://www.liftconference.com/space-solar-power
Houle: http://www.youtube.com/watch?v=ub5nlv2vmI0&feature=related

Beautiful Pillars of Fire

Terrific supercomputer simulations of Tunguska-like airbursts:
http://www.sandia.gov/news/resources/releases/2007/asteroid.html

What about the Damacloids?

The global inaction on the known and solvable problem of asteroids is bad enough.


It is worse that the results of SpaceGuard seem to be making people complacent.
See http://www.nature.com/news/2008/080625/full/4531164a.html Fewer people are searching for near-Earth asteroids, astronomer David Morrison said in the 1990s, than work a shift in a small McDonalds. But that group — a little larger now — has over the past two decades discovered a host of happily harmless rocks, and in doing so reduced the risk of an unknown asteroid blighting civilization. [Note, SpaceGuard has not taken any rocks out of the sky, and has found less than 1% of the estimated population of Tunguska size impactors, and is still missing 21% of the expected civilization ending Asteroid population]

But what about the Damacloids?


http://en.wikipedia.org/wiki/Damocloid_asteroid
http://www.ifa.hawaii.edu/~jewitt/papers/DAMO/Jewitt.damo.pdf


The Dark Comets?


In this paper http://astrobiology.cf.ac.uk/MNPAPER.pdf authors W. M. Napier, J. T. Wickramasinghe and N. C. Wickramasinghe argue:



Dynamical balance arguments that involve the capture of long-period comets from the Oort
cloud imply that there should be ∼1000 times more Halley-type objects than are actually
observed...Here we demonstrate that the surfaces of inactive comets, if composed of loose, fluffy organic material like cometary meteoroids, develop reflectivities that are vanishingly small in visible light. The near-Earth objects may therefore be dominated by a population of fast, multi-kilometre bodies too dark to be seen with current near-Earth object surveys. Deflection strategies that assume decades or centuries of warning before impact are inapplicable to this hazard.

We propose, as a solution to the paradox, that the surfaces of inert comets become extremely dark. An immediate consequence of the above solution of the ‘missing comet’ problem is that there exists a large population of extremely dark comets in Earth-crossing orbits, which are undetectable with current near-Earth object (NEO) search programmes but are nevertheless impact hazards (cf. Bailey & Emel’yanenko 1998)...the mean interval between impacts of such bodies is ∼0.67 Myr, with impact energies 1.5 × 106 Mt, for a mean impact speed ∼60 km s−1 (Jeffers et al. 2001). Impacts of at least 1.5 × 107 Mt energy are expected at mean intervals ∼2.3 Myr...Rickman et al. (2001), on the other hand, find that comets yield a large, perhaps dominant, contribution to km-sized impactors, estimating for example a terrestrial impact rate of about one Jupiter family comet (active or dormant) per Myr. While the uncertainties are large, it seems unavoidable that, if the present hypothesis is correct, a dormant Halley population represents a major if not dominant global impact hazard at the present time...The Oort cloud is demonstrably sensitive to Galactic perturbers of various sorts – stars, nebulae and tides (Napier & Staniucha 1982; Byl 1986, etc.). Nurmi, Valtonen & Zheng (2001) find that the flux of comets from the Oort cloud, and hence the impact rate, may fluctuate by an order of magnitude arising from the motion of the Sun with respect to the Galactic mid-plane. As we are at present passing through the plane of the Galaxy, it is expected that the current impact rate is several times higher than that deduced from the lunar cratering record, which is time-averaged over one or two Gyr. The dark Halleys, on this hypothesis, are a link in the chain between Galactic disturbances and huge impacts. That passages of the Sun through the spiral arms of the Galaxy might induce terrestrial disturbances has been discussed by a number of authors (e.g. McCrea 1975). Napier & Clube (1979) specifically proposed that bombardment episodes might occur during such passages, leading to mass extinctions and other trauma. Leitch & Vasisht (1998) have shown that, indeed, the Sun was passing through a spiral arm during the Cretaceous-Tertiary and Permo-Triassic extinctions of 65 and 250 million years ago, which also coincided with the Deccan and Siberian trap flood basalts. On this model, each of these was caused, not by a stray asteroid, but by an episode of cometary bombardment. For at least some of the great mass extinctions, multiple impacts coupled with climatic trauma appear to have been involved (e.g. McGhee 1996; Keller 2002). Current detection and deflection strategies involve the assumption that decades or centuries of warning will be available following the discovery of a threat asteroid. However if the major impact hazard indeed comes from this essentially undetectable population, the warning time of an impact is likely to be at most a few days. A typical Halley-type dormant comet spends 99 per cent of its time beyond the orbit of Mars and so a full mapping of this population is beyond current technology.


[Lesson: We need to get "out there" with IR telescopes an deflection capabilities, not sit on our complacent hands trusting we are several rolls of the dice away from a double six.]

100 Years & Nat Geo

A conference 100 YEARS SINCE TUNGUSKA PHENOMENON: PAST, PRESENT AND FUTURE was held June 26 - 28, 2008 inMoscow, Presidium of Russian Academy of Sciences, Leninskiy Prospekt 32а

http://tunguska.sai.msu.ru/index.php?q=program_e

The Conference is devoted to the 100-year anniversary of the Tunguska phenomenon. The purpose of the conference is to integrate the efforts of inter-disciplinary experts in understanding the Tunguska event and similar impact phenomena. Including: Hazards due to comets and asteroids
1. The role of the Tunguska event in the problem of asteroidal and cometary hazards

2. Investigation of impact craters on the Earth and other bodies of the Solar System

3. Means of mitigation of asteroidal and cometary hazards

An exceptional article can be found in this month (Aug 2008) National Geographic Magazine:

SMALL expert From: http://ngm.nationalgeographic.com/print/2008/08/earth-scars/stone-text

An estimated ten million rocky asteroids and ice-and-dirt comets pirouette in outer space, and once in a while their paths fatefully intersect our planet’s. One such encounter took place a hundred miles from present-day Washington, D.C., where a 53-mile-wide crater lies buried beneath Chesapeake Bay—the scar left when a two-mile-wide rock smashed into the seafloor 35 million years ago. More notorious is the titan, six miles in diameter, that barreled into the Gulf of Mexico around 65 million years ago, releasing thousands of times more energy than all the nuclear weapons on the planet combined. “The whole Earth burned that day,” says Ed Lu, a physicist and former astronaut. Three-quarters of all life-forms, including the dinosaurs, went extinct.
Astronomers have identified several hundred asteroids big enough to cause a planet­wide disaster. None is on course to do so in our lifetimes. But the heavens teem with smaller, far more numerous asteroids that could strike in the near future, with devastating effects. On June 30, 1908, an object the size of a 15-story building fell in a remote part of Siberia called Tunguska. The object—an asteroid or a small comet—exploded a few miles before impact, scorching and blowing down trees across 800 square miles. The night sky was so bright with dust from the explosion, or icy clouds from the water vapor it blasted into the upper atmosphere, that for days people in Europe could read newspapers outdoors at night. On Tunguska’s hundredth anniversary, it’s unsettling to note that objects this size crash into Earth every few centuries or so...Every day, dozens of tons of detritus from outer space—dust from comets, tiny shards of asteroids—burn up in the Earth’s upper atmosphere, leaving bright meteor trails at night. Most days a chunk or two of rock or metal, fist size or bigger, survives the fiery plunge. ..Since then, there have been some spectacular near misses. On August 10, 1972, an object around 15 feet across and weighing 150 tons skipped off the upper atmosphere. Hundreds of eyewitnesses saw the glowing streak, dazzling on a sunny afternoon, as it traversed the sky from Utah to Alberta before whizzing back out into space. On March 22, 1989, a rock as much as a thousand feet across came within a few hundred thousand miles of Earth—an uncomfortably close shave. ..Erosion and vegetation have erased most of the scars left by impacts in the geologic past. Perhaps the best preserved lies about half an hour east of Flagstaff, Arizona. .. Fifty thousand years ago this was a forested plain inhabited by mammoths, giant ground sloths, and other Ice Age animals. Shoemaker, an asteroid expert with the Lowell Observatory in Flagstaff, imagines the day the sky fell. “Suddenly, there’s a terrific, brilliant light,” she says. In a flash, a searing-hot iron-nickel mass, 150 feet wide and weighing 300,000 tons, tears into the Coconino sandstone, flinging boulders and molten iron for miles. A blast of wind more powerful than any earthly tornado scours the landscape. ..If Edward Teller is the father of the hydrogen bomb, Simonenko is the father of the asteroid bomb. In the mid-1960s the superpowers dreamed of using their nuclear arsenals for peaceful purposes, such as leveling mountains and digging canals. Simonenko, a new recruit to the lab, was asked to study the effects of a torpedo-shaped charge that would explode laterally, ideal for earthmoving. It occurred to him that such a device could also be used to deflect an object in space. He told his boss, who laughed and ordered the eager young physicist to get back to work.
Though nuclear excavation never became a reality, Simonenko went on studying nuclear asteroid deflection. He and Voloshin concluded that the best way to deflect an asteroid up to a mile or so wide would be to detonate a nuclear charge nearby. The intense radiation would fry the surface, driving off a "sacrificial layer" of rock. The expanding vapor would act as a rocket motor, nudging the asteroid onto a new trajectory. For a smaller, Tunguska-size rock, Simonenko says, "it would be simpler: We vaporize it."
Simonenko has a brother-in-arms in nuclear physicist David Dearborn of Lawrence Livermore National Laboratory in northern California. Dearborn's day job is determining whether the aging weapons in the U.S. nuclear stockpile are reliable. In his spare time, he ponders asteroid defense. He, too, favors a standoff nuclear blast. "Not too close—then the blast is too intense, and things shatter too much. And not too far, or you don't get enough energy."
Although it may be technically straightforward to dust off a few warheads and sling them at an asteroid, deciding whether to press the red button—and which nation gets to press it—could be excruciating. First, the nation with its finger on the trigger would have to withdraw from the Outer Space Treaty, which bans the use of nuclear weapons in space. But if catastrophe looms, says Dearborn, "people would really have to say, 'Can we be brighter than the dinosaurs?'"

[Note, this last part is probably incorrect. The OST forbits the stationing of weapons of mass destruction is space, but a nuclear device for the purpose of asteroid deflection would be nothing of the kind, but rather a form of Nuclear Propulsion, which is specifically allowed. However, in order to TEST such a device, a country would have to withdraw (or amend) the Limited Nuclear Test-Ban Treaty, which forbids the testing of any sort of nuclear explosion in space]

New Clovis-Age Comet Impact Theory

From: http://www.spacedaily.com/reports/New_Clovis_Age_Comet_Impact_Theory_999.html

Two University of Oregon researchers are on a multi-institutional 26-member team proposing a startling new theory: that an extraterrestrial impact, possibly a comet, set off a 1,000-year-long cold spell and wiped out or fragmented the prehistoric Clovis culture and a variety of animal genera across North America almost 13,000 years ago...Kennett said that 35 animal genera went extinct at the end of the Pleistocene, with at least 15 clearly being wiped out close to 12,900 years ago. There would have been major ecological shifts, driving Clovis survivors into isolated groups in search of food and warmth. There is evidence, he said, that pockets of Clovis people survived in refugia, especially in the western United States.

From: http://archaeology.about.com/b/2007/05/30/did-an-exploding-comet-end-clovis.htm

Researchers argue that the theory explains why the Murray Springs site in Arizona has a thick carbon mat, a deposit similar in age and context to ones seen at other Paleoindian sites such as Lindenmeier (Colorado), Hell Gap (Wyoming), and Lange-Ferguson (South Dakota). The radiocarbon dates on the mat are uniformly about 10,800 +/- 200.

More at: http://allendale-expedition.net/publications/comet.pdf

http://media.www.dailyemerald.com/media/storage/paper859/news/2007/11/28/News/The-End.Of.An.ice.Age-3119506.shtml
Researchers say the comet - a ball of ice, rock and dust from outer space - hit the Laurentide ice sheet in Canada."It wouldn't have left a crater because it would have exploded and sent a shower of debris and chunks of ice," said Erlandson, adding the impact would have been ice on ice.Heat from impact may have sparked fires across the continent, and the melting of the ice sheet may have caused flooding. Researchers say the comet may have contributed to human population reduction and localized animal extinction.Researchers say the comet may have helped cause a cold climate period that lasted for about 1,300 years, called the Younger Dryas, and may have wiped out or fragmented the prehistoric Clovis tribe. A carbon-rich black layer found at more than 50 sites around North America is evidence for the impact, according to the article published in the Proceedings of the National Academy of Sciences. The layer contains iridium, carbon spherules and fullerenes with helium 3. Evidence of mammoths and other animals, along with early human hunters, are found beneath the black mat, but are missing within or above the strip.More evidence is found in the Carolina Bays, which are depressions from New Jersey to Florida. Erlandson says the depressions point toward the ice sheet, possibly a result of the comet.The theory, which is known as the "YDB Comet Theory," was first proposed by University of California at Santa Barbara paleoceanographer James Kennett, Douglas Kennett's father, Richard Firestone of the Lawrence Berkeley National Laboratory and Arizona researcher Allen West. Douglas Kennett often collaborates with his father.

Big Bang In Antarctica: Killer Crater Found Under Ice

From: http://www.sciencedaily.com/releases/2006/06/060601174729.htm or

http://www.physorg.com/printnews.php?newsid=68455520

Big Bang In Antarctica: Killer Crater Found Under Ice
ScienceDaily (June 1, 2006) — Planetary scientists have found evidence of a meteor impact much larger and earlier than the one that killed the dinosaurs -- an impact that they believe caused the biggest mass extinction in Earth's history.
The 300-mile-wide crater lies hidden more than a mile beneath the East Antarctic Ice Sheet. And the gravity measurements that reveal its existence suggest that it could date back about 250 million years -- the time of the Permian-Triassic extinction, when almost all animal life on Earth died out.
Its size and location -- in the Wilkes Land region of East Antarctica, south of Australia -- also suggest that it could have begun the breakup of the Gondwana supercontinent by creating the tectonic rift that pushed Australia northward.
Scientists believe that the Permian-Triassic extinction paved the way for the dinosaurs to rise to prominence. The Wilkes Land crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub meteor is thought to have been 6 miles wide, while the Wilkes Land meteor could have been up to 30 miles wide -- four or five times wider.
"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs, and probably would have caused catastrophic damage at the time," said Ralph von Frese, a professor of geological sciences at Ohio State University...The scientists used gravity fluctuations measured by NASA's GRACE satellites to peer beneath Antarctica's icy surface, and found a 200-mile-wide plug of mantle material -- a mass concentration, or "mascon" in geological parlance -- that had risen up into the Earth's crust.
Mascons are the planetary equivalent of a bump on the head. They form where large objects slam into a planet's surface. Upon impact, the denser mantle layer bounces up into the overlying crust, which holds it in place beneath the crater.
..."There are at least 20 impact craters this size or larger on the moon, so it is not surprising to find one here," he continued. "The active geology of the Earth likely scrubbed its surface clean of many more." ..."On the moon, you can look at craters, and the mascons are still there," von Frese said. "But on Earth, it's unusual to find mascons, because the planet is geologically active. The interior eventually recovers and the mascon goes away." He cited the very large and much older Vredefort crater in South Africa that must have once had a mascon, but no evidence of it can be seen now. ..Approximately 100 million years ago, Australia split from the ancient Gondwana supercontinent and began drifting north, pushed away by the expansion of a rift valley into the eastern Indian Ocean. The rift cuts directly through the crater, so the impact may have helped the rift to form, von Frese said.
But the more immediate effects of the impact would have devastated life on Earth.
"All the environmental changes that would have resulted from the impact would have created a highly caustic environment that was really hard to endure. So it makes sense that a lot of life went extinct at that time," he said.

Witness Account of Ancient Asteroid Destruction?

From: http://www.timesonline.co.uk/tol/news/uk/science/article3649054.ece
A clay tablet that has baffled scientists for 150 years has been identified as a witness’s account of the asteroid suspected of being behind the destruction of Sodom and Gomorrah. Researchers who cracked the cuneiform symbols on the Planisphere tablet believe that it recorded an asteroid thought to have been more than half a mile across...He said the size and route of the asteroid meant that it was likely to have crashed into the Austrian Alps at Köfels. As it travelled close to the ground it would have left a trail of destruction from supersonic shock waves and then slammed into the Earth with a cataclysmic impact. Debris consisting of up to two thirds of the asteroid would have been hurled back along its route and a flash reaching temperatures of 400C (752F) would have been created, killing anyone in its path. About one million sq km (386,000 sq miles) would have been devastated and the impact would have been equivalent to more than 1,000 tonnes of TNT exploding. Dr Hempsall said that at least 20 ancient myths record devastation of the type and on the scale of the asteroid’s impact, including the Old Testament tale of the destruction of Sodom and Gomorrah and the Ancient Greek myth of how Phaeton, son of Helios, fell into the River Eridanus after losing control of his father’s sun chariot.

http://blog.beliefnet.com/feilerfaster/2008/05/did-an-asteroid-cause-sodom-an.html had some more details, and brought up something I had not previously heard:

Wikipedia says:
An alternative theory, first suggested in 1882, is that the Great Chicago Fire was caused by a meteor shower. At a 2004 conference of the Aerospace Corporation and the American Institute of Aeronautics and Astronautics, engineer and physicist Robert Wood suggested that the fire began when Biela's Comet broke up over the Midwest and rained down below. That four large fires took place, all on the same day, all on the shores of Lake Michigan (see Related Events), suggests a common root cause. Eyewitnesses reported sighting spontaneous ignitions, lack of smoke, "balls of fire" falling from the sky, and blue flames. According to Wood, these accounts suggest that the fires were caused by the methane that is commonly found in comets.
http://en.wikipedia.org/wiki/1871_Great_Chicago_Fire

And then there is the Hindu Mahabharata narrative, which records:
Gurkha flying in his swift and powerful Vimana hurled against the three cities of the Vrishis and Andhakas a single projectile charged with all the power of the Universe. An incandescent column of smoke and flame as bright as the thousand suns rose in all its splendor. An iron thunderbolt, a gigantic messenger of death, which reduced to ashes the entire race of the Vrishnis and the Andhakas. The corpses were so burned as to be unrecognizable. The hair and nails fell out; pottery broke without apparent cause, and the birds turned white.... after a few hours all foodstuffs were infected.... to escape from this fire, the soldiers threw themselves in streams to wash themselves and their equipment..."

A discussion with other references and an alternate explanation can be found here:

http://www.americanchronicle.com/articles/68842