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updated 5/7/2012 9:19:47 AM ET 2012-05-07T13:19:47

On April 22, sonic booms from an incoming daytime fireball rattled Nevada and central California.

After analysis of signals detected by infrasound stations in the region, it was found that the booms had been generated by a minivan-sized asteroid slamming into the atmosphere. The space rock ended its violent journey over the Sierra Nevada mountain range near Sacramento, CA.

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In the days following the dramatic cosmic encounter, small pieces of the space rock have been recovered from the fall site, but scientists from the SETI Institute and NASA believe larger chunks of asteroid made it to the ground.

"So far, only pieces have been found -- up to maybe 19 grams," SETI Institute astronomer Peter Jenniskens told Discovery News. "However we suspect there are larger pieces, so the question becomes: how do you find them?"

Chartering a zeppelin from the Mountain View, CA, based company Airship Ventures on May 3, Jenniskens led a joint SETI/NASA team on an unprecedented search for any large pieces of asteroid that may have survived to the ground.

21st Century California Asteroid Rush

Pieces of the asteroid were recovered from Henningsen-Lotus state park near Sutters Mill, a region synonymous with the 19th Century California Gold Rush. Astronomers, not prospectors, are now rushing to the region to look for bits of what has now been dubbed the Sutters Mill Meteorite. But time is of the essence.


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After close analysis of the fragments recovered so far, scientists have revealed that this meteorite is a rare CM type carbonaceous chondrite.

"This is the meteorite you want to find!" said Jenniskens. "This particular type of meteorite -- that forms only 1.5 percent of all falls -- is really interesting for research at the SETI Institute and NASA. The material carried in this type of meteorite is what made life possible on Earth.

"The carbon atoms that your body is made out of, and all life on Earth is made out of, arrived on our planet shortly after its formation via meteorite impacts and comet impacts."

To preserve the chemical composition of the meteorite debris, Jenniskens' team had to move quickly to mount the search. The longer these little chunks of primordial rock lay exposed to the elements, the more they will degrade.

Jenniskens estimates that at least two dozen fragments have been recovered from the fall area, but there were no fragments larger than 19 grams (0.7 oz). For such a large daytime meteor, he speculates, there must be some bigger pieces that fell somewhere in the region.

"A zeppelin turns out to be the perfect platform to carry out these searches," he said. "(During the search) we had cloudy skies which was excellent for us because we had no shadows; very nice, perfect conditions to do the search. We traveled a region at least 40 kilometers (25 miles) long and several kilometers wide to look for things that have fallen.

"A one kilogram rock coming down with some residual cosmic speed would leave an impact dent -- a crater if you like -- and that should be many times bigger than the stone itself. We were looking for crash sites, basically."

Although the zeppelin search used the location of the recovered meteorite fragments as a starting point, the uncertainties multiplied the further the airship drifted from those coordinates.

Depending on the meteor's trajectory, the smaller debris would have slowed very quickly due to atmospheric drag before making landfall, whereas the larger chunks would have traveled faster, slamming into the ground at high speed. This means the location of the smaller debris may be wildly different from the fall area of the larger pieces.

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"We had a number of spotters on the zeppelin, including myself, who looked at the landscape and when we saw something that was interesting we would focus on it and zoom in on the detail," Jenniskens added.

Using a powerful camera system, each potential crater could be zoomed-in on and evaluated. If a suspected impact site was found, instructions were sent to ground teams to take a closer look.

"At the end of the day we had 12 or so things that we need to check out. We've checked out 2 and neither were meteorite falls. It's very likely that none are meteorite falls. The area is so big; you can't very quickly check these things."

No asteroid debris larger 19 grams has been recovered so far, but airships seem to be the perfect platform to mount a search in meteorite fall zones.

"There's not many zeppelins around these days so I'd be very surprised if it's been used before -- but it really is ideal," he said. "It's a very slow moving platform, large windows, with an excellent view of the landscape. We were flying at about 1,000 feet. At that height, with binoculars, you can see very small details in the landscape. We were looking for features maybe one or two meters in size -- we could easily spot features that size one or two kilometers either side of the zeppelin."

"Little Worlds"

Jenniskens is no stranger to hunting for meteorite debris. In 2008, he led a team to North Africa to recover debris from the small asteroid 2008 TC3 that slammed into the atmosphere over Sudan.

"What was really interesting about that fall was that it was a change in paradigm -- that little asteroid turned out to be a 'rubble pile' with lots of different types of meteoroids in it. Not just one, but many different types," he said.

2008 TC3 was also notable in that it was the first asteroid to be detected before it hit Earth. Astronomers were then able to pinpoint where it would hit the planet, and then infrasound stations detected the fireball hit the atmosphere in the exact location as predicted. Ground teams then moved in to collect the debris.

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Although the asteroid that formed the Sutters Mill Meteorite wasn't spotted before it hit the atmosphere, it made a grand enough entrance to be noticed.

"It's really a small asteroid coming in, a small little world. So what I'm hoping to do is have as much material recovered from (the California fall) so we can hopefully find diversity in these objects."

Although this zeppelin hunt hasn't identified any larger chunks of meteorite yet, Jenniskens is upbeat about the use of air ships for suture meteorite hunts and the impact the Airship Ventures zeppelin would have had on people living in the area.

"It certainly raised people's awareness, so I'm hoping this sends a signal to people living in the area to keep an eye on their property and to let us know if they see anything as it might be a part of this asteroid coming down."

© 2012 Discovery Channel

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