February 22, 2013
NASA Says Russian Meteorite 1000 Times Larger Than Originally Estimated
Lawrence LeBlond for redOrbit.com - Your Universe Online
Initial reports put last Friday´s (Feb 15) Chebarkul meteorite that exploded over Russia´s Urals region at about 10 tons. But after careful analysis, NASA released new information that puts the meteorite closer to 10,000 tons–1,000 times larger than the estimated size reported by the Russian Academy of Sciences (RAS).
But how there could have been such a huge discrepancy in the size estimation of this object may actually not be the most concerning question. One that may be more pertinent is: Why didn´t scientists see this one coming?
NASA does have a team that tracks such objects, the Near-Earth Object (NEO) Program. But even as that group has in the neighborhood of 10,000 objects in its sights, they simply cannot track every object out there. And there are far more small objects out there that are too small to track.
“If you think about objects the size of the one that came into Russia, you´re probably looking at 100 million up there. Of those likely to intersect Earth, there´s less, maybe 100,000,” said K.T. Ramesh, director of the Hopkins Extreme Materials Institute and a professor of mechanical engineering at Johns Hopkins.
The Russian meteor lumps into this group of meteors that are impossible to track with current technology, he added.
“In general, we're pretty good at seeing things that are [3,000 feet] and up in diameter, and have trouble seeing things that are less than [330 feet] in diameter. The big problem is knowing where they are -- once you know where to look, you can find even the smaller objects,” he told Jeremy Kaplan at Fox News.
“A good analogy is finding a few pennies scattered over an acre of dirt. If you know where to look, you have no trouble seeing them. But if you just look over the whole field, your chance of finding a penny is pretty small,” Ramesh said.
Now that we know what is being tracked and what potential there exists for an unknown meteor to enter Earth´s atmosphere with little to no warning, what are the chances the next one will not explode over the Earth, but rather slam into it head on?
Jamie Gleason, an associate research scientist with University of Michigan´s Department of Earth and Environmental Sciences, explained to redOrbit that there is a statistical frequency when it comes to predicting when meteor events will happen. Something like the Russian event may be “considered to be a 100 year event, and yet happen two days in a row, then not again for a couple hundred years.”
“Blasts like the one in the Urals, or the larger [Tunguska Event] in 1908, can probably be considered once-in-a-lifetime events, but as you scale down to smaller objects, the frequency increases. There are small fireballs over the Earth everyday, with space debris constantly burning up in the atmosphere, and occasionally dropping meteorite fragments over populated areas,” he told redOrbit.
“But it is extremely unusual for people to be injured in these types of events, as happened on Feb. 15 from the blast wave,” he said.
As for meteors making a direct impact on Earth, there are a number of factors that must be met for such impacts to occur. Angle of entry, velocity, and composition all play a role in rather a meteorite makes an impact or not.
“By virtue of its composition alone, [the Russian meteor] may have stood only a very small chance of making it to the surface intact to make a crater. Assuming for now that it was a stony meteorite, these objects are understood to be fairly weak and more likely to blow up high in the atmosphere,” continued Gleason.
“To be sure, though, small stony meteorites do land on the Earth with considerable frequency,” he noted.
But for the most part, it is iron-nickel meteorites that most often make an impact, much like the one that left a giant crater in Arizona–known as Meteor Crater. These meteors are much stronger and more likely to survive interaction with Earth´s atmosphere.
“We do know that stony meteorites are the type most commonly observed to fall from the sky. So it might be lucky that, up to a point, most of the larger objects impacting Earth are more likely to be something that will explode high in the atmosphere first before they reach the surface,” explained Gleason. “The Russian meteorite probably never had a chance of making a sizeable crater, nor for that matter did the Tunguska object.”
“However, if you scale up to huge objects, like those that caused impact-induced mass extinctions on Earth in the geologic past, then all bets are off. The largest known impacts to have happened over the last few millions of years blasted large amounts of Earth material out of craters to produce what are called ℠tektites´, glassy bits of once-molten Earth material that was hurled out into space on ballistic trajectories from the energy of impact, and then re-entered the atmosphere to rain down onto the surface as aerodynamically-shaped glassy blebs,” Gleason told redOrbit.
“So those are pretty energetic events, but quite rare compared to the much more common exploding fireball events of the ℠non-crater forming´ type that occurred over the Urals on Feb. 15,” he said.
There have been some comparisons between last week´s Chebarkul meteorite event and the 1908 Tunguska Event, but the reality is that the Tunguska meteor was probably about 1,000 times larger than the Russian meteor. With the latest event, at least we have something to show for it; some type of material that can be analyzed to learn more about the composition and perhaps origin of the Chebarkul meteorite.
“No such material was ever definitely collected, to my knowledge, in the aftermath of the Tunguska explosion,” said Gleason.