Robert Roy Britt has a great follow-up over on Space.com, covering the large meteorite that hit Chicago on the evening of Wed 26 Mar 03. In my posting over on Arcturus , I estimated its kinetic energy at seven-tenths of a kiloton. Now, thanks to the U of C’s own Steven Simon, I can perform the calculation more accurately.
From the Space.com article, the inputs:
The Chicago rock was stony and about 6 feet in diameter, the researchers conclude.
“It hit the atmosphere at about 40,000 mph,” Simon said.
He said the original rock weighed at least 1,980 pounds as it entered the atmosphere.
A spherical rock of radius r = 1 meter and mass m = 900 kg has volume V = 4pr³/3 = 4.2 m³ and density r = m/V = 210 kg/m³. Since a solid chunk of water ice of this size would mass around 3,800 kg, either the meteor was nonspherical and its longest dimension was 2 meters, reducing its volume, or — more likely — it had the consistency of a loose pile of rocks and was mostly empty space.
In any case, the U of C news release explains that the meteorite has been classified as “an L5 chondrite, a type of stony meteorite, one low in iron” — a good thing for Chicago; a solid chunk of nickel-iron two meters across would have massed around 30 metric tons, would certainly have penetrated the atmosphere intact, and would have hit with a kinetic energy equivalent to a small tactical nuclear weapon, a little over 1 kiloton of TNT.
For the actual event, applying KE = ½mv², with m = 900 kg and v = 40,000 mph = 18,000 m/sec, we get ~150 billion joules. Dividing by 4.2 MJ, that’s 36,000 kg TNT equivalent, a measly one-twenty-eighth of a kiloton. On the other hand, Chicago’s population density is about 13,000 persons per square mile — just under 20 persons per acre! Fling nearly 80,000 pounds of high explosives at a target like that, and people are going to get hurt. So it’s a good thing the meteor blew apart into small pieces in the upper atmosphere, rather than just above the ground.
As things were, it was in some ways the closest call of our time. Quoting again from the U of C news release:
Local residents collected hundreds of meteorite fragments totaling approximately 65 pounds from an area extending from Crete in the south to the southern end of Olympia Fields in the north. Located in Chicago’s south suburbs, “this is the most densely populated region to be hit by a meteorite shower in modern times,” the authors write [in the April issue of the journal Meteoritics and Planetary Science].
One meteorite narrowly missed striking a sleeping Park Forest resident after it burst through the ceiling of a bedroom. The meteorite sliced through some window blinds, cratered the windowsill, then bounced across the room and broke a mirror before coming to rest.
Amateur astronomers everywhere are turning green with envy reading about that person.
Shameless plug: see my A Modest Proposal … and Asteroid Detection, Again for some risk-management ideas in this area.