NEO News (03/27/09) To Catch a Falling Star

From Dave Morrison.

NEO News (03/27/09) To Catch a Falling Star

On October 6, 2008, the small NEA 2008TC3 was discovered approaching the Earth; 36 hours later it exploded in the atmosphere over the northern Sudan desert with energy of about 2KT. This represented the first detection and prediction of an impact before it happened. Now Nature reports on a remarkable continuation of this story with the identification and analysis of about 4 kg of fragments from 2008TC3. Below are the abstract from the Nature paper and several news reports on this unique sequence of events.

David Morrison

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RECOVERED PIECES OF ASTEROID HOLD CLUES TO EARLY HISTORY
By Kenneth Chang
New York Times: March 25, 2009

Scientists who for the first time tracked an asteroid on a collision course with Earth, and watched as it exploded in the atmosphere, have now picked up some of the remnants on the ground. The discovery and analysis of the meteorites, reported in Thursday's issue of Nature, give scientists solid data on the composition of meteorites that originate from at least one type of asteroid, known as F-class.

Millions of asteroids, mostly small, whirl around the solar system, and over the years people have picked up tens of thousands of meteorites, the surviving rock fragments of asteroids that collide with Earth. "But we don't know where a single one of them comes from," said Michael E. Zolensky, a cosmic mineralogist at the Johnson Space Center in Houston, during a NASA-sponsored news conference on Wednesday.

That changed when Petrus M. Jenniskens, a scientist at the SETI Institute in Mountain View, Calif., organized a search team to comb through a Sudan desert to look for pieces of an asteroid that had been spotted less than a day before it hit Earth last year. "For the first time, we can dot the line between the meteorite in our hands and the asteroid astronomers saw in space," said Dr. Jenniskens, the lead author of the Nature paper.

The 280 pieces, about 10 pounds in total, are of a rare type of meteorite known as ureilites. The hodgepodge of minerals in ureilites indicates they were heated up but not fully melted, suggesting that they were once part of a much larger asteroid that possessed planet like geological processes.

Because ureilites are now linked to F-class asteroids, also rare, the hope is that scientists can now determine the history of asteroids, which contain some of the most primitive materials left over from the early solar system. "It's like the first step towards a Rosetta stone of understanding asteroids," Dr. Zolensky said.

The cascade of discovery started when Richard Kowalski, working with the of the University of Arizona, spotted a moving white dot on his computer screen late Oct. 5 at an observatory on Mount Lemmon outside Tucson. He sent the coordinates to the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics. A computer program at the center automatically calculates the orbits of reported projects, but it failed for the object Mr. Kowalski reported, because Earth's gravity appeared to be greatly distorting its orbit. The next morning, when Timothy B. Spahr, the center's director, took a closer look, the asteroid, designated 2008 TC3, looked as if it was being pulled directly into Earth.

Dr. Spahr notified Steven R. Chesley, a scientist in NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. "For the first time ever, I saw an impact probability of 100 percent pop up on the computer screen," Dr. Chesley said. "And this was, needless to say, the kind of thing that makes you sit up straight in the chair."

Because the asteroid was dim, the astronomers knew that it was small, about the size of a car and 80 tons, and would not cause any significant damage. Notice quickly spread, and asteroid watchers, professional and amateur, pointed their telescopes toward it.

With hundreds of observations coming in during the day, the computers at the Jet Propulsion Laboratory refined the trajectory. "Our last pre-impact prediction was accurate to about a kilometer and a couple tenths of a second in the impact time," Dr. Chesley said.

The asteroid disintegrated about 23 miles over the Nubian desert of northern Sudan about an hour before sunrise, 20 hours after Mr. Kowalski discovered it. It released the energy of one to two kilotons of TNT.

"We figured that probably was the end of the story," Dr. Chesley said. The expectation was that none of 2008 TC3 survived the passage through the atmosphere. But still, Dr. Jenniskens, an expert on meteor showers, wondered. "If we could find something, it would be tremendous," he said. "So you have to try. It was really a long shot."

In December, he flew to Sudan and organized a team of 45 students and staff members from the University of Khartoum to search through the desert for fragments of 2008 TC3. And they found the shiny black fragments that had come from space.

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Nature 458, 485-488 (26 March 2009)

THE IMPACT AND RECOVERY OF ASTEROID 2008 TC3

P. Jenniskens1, M. H. Shaddad2, D. Numan2, S. Elsir3, A. M. Kudoda2, M. E. Zolensky4, L. Le4,5, G. A. Robinson4,5, J. M. Friedrich6,7, D. Rumble8, A. Steele8, S. R. Chesley9, A. Fitzsimmons10, S. Duddy10, H. H. Hsieh10, G. Ramsay11, P. G. Brown12, W. N. Edwards12, E. Tagliaferri13, M. B. Boslough14, R. E. Spalding14, R. Dantowitz15, M. Kozubal15, P. Pravec16, J. Borovicka16, Z. Charvat17, J. Vaubaillon18, J. Kuiper19, J. Albers1, J. L. Bishop1, R. L. Mancinelli1, S. A. Sandford20, S. N. Milam20, M. Nuevo20 & S. P. Worden20

In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes1, 2, 3. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995 nm wavelength range, and designated 2008 TC3 (refs 4-6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class3, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.

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SKY FALLS, WHO CARES?
By John Tierney
New York Times: March 26, 2009

Now that we've tracked an asteroid on a collision course with Earth and recovered its debris, might we start taking these close encounters more seriously?

For the first time, scientists have tracked an asteroid headed to Earth, watched it explode in the atmosphere and picked up some of its remnants on the ground, as my colleague Ken Chang reports. This one, which disintegrated in about 23 miles over the Nubian desert of northern Sudan in October, was about the size of a car and weighed 70 tons - not enough to do much damage. But what happens when a bigger one arrives?

NASA has estimated that that there are close to 1,000 near-Earth space rocks at least a kilometer in diameter, my colleague Andy Revkin noted. And scientists have been finding evidence of more frequent catastrophic collisions in the past, as my colleague Sandra Blakeslee reported. Writing in the Atlantic Monthly last year, Gregg Easterbrook summarized the odds:

A generation ago, the standard assumption was that a dangerous object would strike Earth perhaps once in a million years. By the mid-1990s, researchers began to say that the threat was greater: perhaps a strike every 300,000 years. This winter, I asked William Ailor, an asteroid specialist at The Aerospace Corporation, a think tank for the Air Force, what he thought the risk was. Ailor's answer: a one-in-10 chance per century of a dangerous space-object strike.
[Morrison note: As reported here earlier, these impact statistics from Easterbrook could be highly misleading. The "dangerous object" hitting at million year intervals is an asteroid 1-2 km in diameter, enough to cause a global disaster. The "dangerous space-object crash" every thousand years is a Tunguska-class impact, which would do only local damage. The difference in mass (and energy) is about a factor of 100,000. In fact, our estimates of the probability of a hit at any particular size have gone down slightly over the past 20 years, not risen.]

Mr. Easterbrook argues that Congress should force NASA to switch its priorities from building a moon base to scanning the skies for asteroids and developing diversion techniques (like the gravity tractor discussed here at the Lab). I'm as enthusiastic about space travel as anyone, but I can see his point. Do you think that protecting the planet against asteroids is a more important role for the government than building a moon base? Would it be more important - and cost-effective - than most of the programs designed to deal with other environmental dangers?

If you're tired of waiting for governments to act, you can check out a group called Future Asteroid Interception Research, which is calling for citizens to take the financing initiative. Complaining that the White House is spending only $4 million per year on near-Earth object (NEO) research, the group argues:

Despite growing and alarming evidence of the dangers NEOs pose, this area of astronomy is still largely ignored. In 2004, when the Asian tsunami struck, "authorities" in the region deemed the relatively small cost of an early warning system not worth the investment as they believed such a catastrophe was not due for another 70 years. If the citizens of the affected countries had had foreknowledge of the impending disaster, don't you think the private sector would have done something about it? It's up to us to lend a hand in all areas of global concern when bureaucratically hamstrung governments dither. Fixing global warming will entail a monstrous effort. Hunting dangerous asteroids only costs ¤15.

Fifteen Euros works out to about $20. Does that seem worth it to you?

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SURPRISE RECOVERY OF METEORITES FOLLOWING ASTEROID IMPACT
By Seth Shostak
Space News 26 March 2009

Fortunately, it wasn't large enough to require intervention by Bruce Willis, but asteroid 2008 TC3 is the first space rock to have been spotted before it crashed to Earth. It streaked into the skies over northern Sudan in the early morning of October 7, 2008, and then exploded at a high 37 km above the Nubian Desert, before the atmosphere could slow it appreciably. It was believed that the asteroid had fully disintegrated into dust.

A meteor astronomer with the SETI Institute's Carl Sagan Center, Peter Jenniskens, thought otherwise. After establishing a collaboration with Mauwia Shaddad of the Physics Department and Faculty of Sciences of the University of Khartoum, he traveled to the Sudan. The two researchers, together with students and staff from the university, collected nearly 280 pieces of the asteroid, strewn over 29 km of the Nubian Desert. Never before had meteorites been collected from such a high altitude explosion. As it turns out, the assembled remnants are unlike anything in our meteorite collections, and may be an important clue in unraveling the early history of the solar system.

"This was an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space," comments Jenniskens, the lead author on a cover story article in the journal Nature that describes the recovery and analysis of 2008 TC3.

Picked up by Arizona's Catalina Sky Survey telescope on 6 October, 2008, the truck-sized asteroid abruptly ended its 4.5 billion year solar-system odyssey only 20 hours after discovery, when it broke apart in the African skies. The incoming asteroid was tracked by several groups of astronomers, including a team at the La Palma Observatory in the Canary Islands that was able to measure sunlight reflected by the object. Studying the reflected sunlight gives clues to the minerals at the surface of these objects. Astronomers group the asteroids into classes, and attempt to assign meteorite types to each class. But their ability to do this is often frustrated by layers of dust on the asteroid surfaces that scatter light in unpredictable ways.

Jenniskens teamed with planetary spectroscopist Janice Bishop of the SETI Institute to measure the reflection properties of the meteorite, and discovered that both the asteroid and its meteoritic remains reflected light in much the same way - similar to the known behavior of so-called F-class asteroids.

"F-class asteroids were long a mystery," Bishop notes. "Astronomers have measured their unique spectral properties with telescopes, but prior to 2008 TC3 there was no corresponding meteorite class, no rocks we could look at in the lab."

The good correspondence between telescopic and laboratory measurements for 2008 TC3 suggests that small asteroids don't have the troublesome dust layers, and may therefore be more suitable objects for establishing the link between asteroid type and meteorite properties. That would allow us to characterize asteroids from afar.

Rocco Mancinelli, a microbial ecologist at the SETI Institute's Carl Sagan Center, and a member of the research team, says that "2008 TC3 could serve as a Rosetta Stone, providing us with essential clues to the processes that built Earth and its planetary siblings."

In the dim past, as the solar system was taking shape, small dust particles stuck together to form larger bodies, a process of accumulation that eventually produced the asteroids. Some of these bodies collided so violently that they melted throughout.

2008 TC3 turns out to be an intermediate case, having been only partially melted. The resulting material produced what's called a polymict ureilite meteorite. The meteorites from 2008 TC3, now called "Almahata Sitta," are anomalous ureilites: very dark, porous, and rich in highly cooked carbon. This new material may serve to rule out many theories about the origin of ureilites.

In addition, knowing the nature of F-class asteroids could conceivably pay off in protecting Earth from dangerous impactors. The explosion of 2008 TC3 at high altitude indicates that it was of highly fragile construction. Its estimated mass was about 80 tons, of which only some 5 kg has been recovered on the ground. If at some future time we discover an F-class asteroid that's, say, several kilometers in size - one that could wipe out entire species - then we'll know its composition and can devise appropriate strategies to ward it off. Hitting such a fragile asteroid with an atomic bomb, as Bruce Willis might do, would merely turn it into a deadly swarm of shotgun pellets.
As efforts such as the Pan-STARRS project uncover smaller near-Earth asteroids, Jenniskens expects more incidents similar to 2008 TC3. "I look forward to getting a call from the next person to spot one of these," he says. "I would love to travel to the impact area in time to see the fireball in the sky, study its breakup and recover the pieces. If it's big enough, we may well find other fragile materials not yet in our meteorite collections."

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NEO News (now in its fourteenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, Ames Research Center, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. For additional information, please see the website http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.