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Chelyabinsk-Sized Asteroid Impacts May Be More Common Than We Thought
The Bad Astronomer writes "Using data from the Feb. 15, 2013 asteroid impact over Russia, scientists have determined that we may be hit by objects in this size range (10 — 50 meters across) more often than we previously thought, something like once every 20 years (abstract). They also found the Chelyabinsk asteroid was likely a single rock about 19 meters (60 feet) across, had a mass of 12,000 tons, and was criss-crossed with internal fractures which aided in its breakup as it rammed through the Earth's atmosphere."
It's nice to see that the author didn't buy into the myth that it's friction which causes the increase in temperature as a fast moving body move through the atmosphere.
"As this main mass plummeted through our atmosphere at a speed of 20 kilometers per second â" dozens of times faster than a rifle bullet â" the huge pressure it generated compressed the air in front of it, heating it up."
That kind of journalistic competency it worth noting.
Well, it's less than half the density of water.
Wikipedia tells me that iron has a density of 7.874 g/cm^3. A 60-foot-diameter asteroid has a volume of 4/3*pi*30^3 cubic feet, or about 113100 ft^3. So, units tells me that an iron sphere that size would be about 27,800 tons. So it's not as dense as a solid iron asteroid would be.
(T>t && O(n)--) == sqrt(666)
Not really. If you do the math, it works out to about 113,000 cubic feet (assuming a perfect sphere), meaning a roughly 203 lb/cubic foot of material, which is roughly half the density of cast iron.
You know what, that's 4 times too big, used diameter for radius. I accept any shame heaped upon me.
Eight times. The shame shall be heaped upon you because it is eight times too big, not four. Flatlander chauvinist pig!
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You know, I just default in my head to nice little 2d pictures.
You know, the entire planet of Saturn would float if there were an ocean big enough to put it in ;-)
But yea, he's wrong anyway.
The Slate article mentions there were two Nature papers, but the article summary above only gives a link to one. The papers are:
This one came up with 20 year frequency for these sized events: A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors
This one looked a bunch of YouTube videos and analyzed how it broke up as it went through the atmosphere:The trajectory, structure and origin of the Chelyabinsk asteroidal impactor
Using our best estimate for the Chelyabinsk airburst energy, of about 500 kt, we have estimated the bolide flux at the Earth over the period from 1994 to mid-2013. This estimate is based on 20 years of total global coverage by the US government or infrasound sensors, more than doubling the earlier time coverage.
I have a small piece of Chelyabinsk in my meteorite collection, It's a stony, not an iron, although it does have enough iron-nickel chondrules in it to be attracted by a magnet.
Yeah, density of about 3.4 gm/cm^3 is about right. (My piece, at just over 10gms, is about 3 cc.)
A 60-foot diameter iron is about what carved out the mile-wide Barringer (aka Meteor) Crater in Arizona. Irons tend not to airburst, or if they do, do it lower down in the atmosphere. It may also have been travelling faster.