Rain of iron on Earth

By Phil Plait | October 9, 2008 9:27 am

Going through some articles on Digg, I noticed one with the unassuming title of Sikhote-Alin Meteorite. Regular readers here know I am a sucker for meteorites, and Sikhote-Alins are my favorites. I own several small ones.

So I clicked through to find myself on a blog called Black Holes and Stuff. This particular post was about the Sikhote-Alin meteorite fall that occurred in the old Soviet Union back in 1947. They are treasured by collectors for their beauty and unusual shapes.

To my delight, on that blog post I found an embedded YouTube video that’s a condensed version of an old Soviet documentary about the finding of the debris!

This is actually a pretty cool doco. I think I may have groaned out loud when they showed a worker finding a chunk bigger than his hand; it would be worth well over $10,000 today. Sikhote-Alin sells for $2/gram or more, and a chunk that size would easily have massed 2 – 3 kilos. At least.

The coolest part happens starting at 6:10 in the video, where they show a hole created by a falling chunk that’s drilled clear through a standing tree! Wow!

If you go outside at night and look up for an hour, chances are you’ll see the odd meteor or two. Those are usually chunks of rock or ice from comets, and they burn up high in the air. They’re small, maybe the size of a grain of sand or smaller. Pieces as big as Sikhote-Alin are very rare, falling to Earth only every few years or so, and most come in over water. It’s incredibly rare to have one come in over a populated area, so you don’t have to worry too much about meteorite insurance.

On the other hand, one big enough — say, 30 meters across — could wipe out a city. Those are really, really rare, of course, happening on the timescale of many millennia. But that’s a statistical occurrence. The next one might not hit for 100,000 years, or it may come in tonight.

That’s why I advocate keeping our eyes on these guys in the skies. On a small scale, rains of iron are really interesting. On a large scale, they can wipe out whole genera of life on the planet. That’s interesting, too, but more of a bummer.

Wanna learn more? Get my book, Death from the Skies! Chapter 1 is all about asteroid and comet impacts.

Comments (66)

  1. Chapter 1? Are the sources of doom in your book organized by most likely to least likely? If so, I claim credit for that idea ex post facto, and demand royalties!

  2. Jared Lessl

    If my math is correct, the value of a 30-meter wide sphere of Sikhote-Alin (~14000 m^3, mostly iron at 7.8 g/cm^3, $2 per gram => $222B) would just about pay to rebuild a good-sized city. So it all evens out, right?

  3. rob

    hey, if we get his by three 30 meter skhote-alins, at $222B apiece, we could use the proceeds to pay for the wall street bailout!

    another idea is to find an asteroid that is made of a high concentration of carbon. drop it somewhere on earth. the impact will turn the carbon into diamonds. instead of only fire raining down on earth, there will be diamonds! everyone that isn’t vaporized will be rich! rich i tell you!

    do you suppose i could get elected on the “rain diamonds from the sky” platform?

  4. Ummm… I don’t think ‘bummer’ is the right word in that case Phil. ;)

  5. Cheyenne

    Seems like an earmark of a couple more million $’s to bolster NASA’s asteroid/comet monitoring would be a pretty good idea.

  6. Todd W.

    @rob

    Nah…3 of ‘em would glut the Sikhote-Alin market, leading to rapid depreciation of individual chunks. Likewise with diamonds raining down from the heavens, though it would potentially increase the number of colored diamonds, which would be spiffy, since those clear white ones are kinda boring.

  7. SteveG

    BA, have you seen this website?

    http://www.perigeezero.org/

    I stumbled upon it yesterday and thought of death from the Skies. It seems an interesting theory. I have to admit I didn’t read any of it thoroughly so I don’t know how scientific the approach is. Do you have any comments?

  8. madge

    I love meteor watching. Most folks think seeing a “shooting star” is a rare occurrence but, as you point out, if only they would LOOK UP more it is almost guaranteed they would see one every ten minutes or so. And the regular meteor showers are well documented and publicised ( we are just leaving the peak of the Draconid meteor shower and the Orionids peak on the 20th of this month) I never owned one of these hunks of the cosmos but I have held one in my hand and it is some kinda AWESOME.
    :)

  9. Todd W.

    @madge

    It also helps to be in a place with less light pollution. When I was a kid, growing up in the countryside, seeing the night sky was awesome. Could pick out satellites and shooting stars with ease and could even make out the Milky Way on particularly clear nights.

    Then, I moved to an urban area and lost most of the stars and satellites. In college, though, I went to Japan, and the town I was in, though a decent sized city, had far less light pollution than my hometown. One night shortly after arriving, I saw two nice, bright meteors in fairly quick succession. I believe it was part of the Leonids.

  10. gss_000

    Nice. For those into monitoring, over the past weekend not only did scientists discover an incoming impact over Sudan, but predicted when it would hit the atmosphere. They turned out to be off by a few minutes at most, depending on the report. The object was relatively small so it never hit the earth, but it’s a great milestone.

  11. The actual relevant equations are:

    Non-relativistic kinetic energy = K = 0.5 * m * v^2

    m is mass in kilograms
    v is velocity in meters per second
    K is kinetic energy in joules

    m = rho * V

    rho is density in kilograms per cubic meter
    V is volume in cubic meters

    For a spherical object:

    m = (1/6) * pi * rho * d^3

    pi is ratio of circumference of a circle to its diameter (3.14…)
    d is diameter of object in meters

    Putting these two equations together we get:

    K = (1/12) * pi * rho * d^3 * v^2

    If we adjust units so that rho is in grams per cubic centimeter and v is in kilometers per second then:

    K = 2.62E8 * rho * d^3 * v^2

    Now, using the conversion factor 1 kiloton of TNT = 4.184E12 joules, then if we want to measure K in kilotons of TNT we have:

    K = 6.25E-5 * rho * d^3 * v^2

    K is in kilotons of TNT, rho is in grams per cubic centimeter, d is in meters, v is in kilometers per second

    So plugging in some typical values, let v = 10 km/sec which is similar to the impact speed of the recent small asteroid 2008 TC3 which impacted in Sudan (12.8 km/sec). Assume a rocky asteroid with a density of 2.5 gm/cm^3. Then we have the following values:

    d = 1.0 m, K = 0.015 kilotons
    d = 3.0 m, K = 0.42 kilotons
    d = 10.0 m, K = 15 kilotons
    d = 30.0 m, K = 420 kilotons
    d = 100.0 m, K = 15 Megatons

    The atomic bomb dropped on Hiroshima had a yield of 15 kilotons so that would be equivalent to a 10-meter diameter rocky asteroid. A 30-meter rocky asteroid would pack a punch equivalent to a W87 nuclear warhead used on the MX/Peacekeeper missile (300 kiloton yield). And a 100-meter rocky asteroid would pack a punch similar to some of the most powerful nuclear tests ever conducted (e.g., Ivy-Mike conducted at Eniwetok atoll in 1952, yield 10.6 Megatons).

    If you assume an iron-nickel asteroid with a density of 8.0 grams per cubic centimeter then the energies are even higher:

    d = 1.0 m, K = 0.05 kilotons
    d = 3.0 m, K = 1.35 kilotons
    d = 10.0 m, K = 50 kilotons
    d = 30.0 m, K = 1.35 Megatons
    d = 100.0 m, K = 50 Megatons

    And so the BA doesn’t get mad at me, kiloton is pronounced KILo-ton, not ki-LAH-ton like they do in Canada. :)

  12. IVAN3MAN

    Tom Marking, have you noticed how whenever anybody starts talking some serious maths, everybody else then buggers off the thread.

    *Crickets*

  13. That is truly fascinating.

    Straight through the strong chunk of a tree. Wow.

    It shows you the true power of even a very small meteor as it rips through the atmosphere.

  14. @IVAN3MAN “serious maths”

    Serious math? Wow, multiplication, division, and raising to a power. That’s some serious stuff there. :)

    But yes, I have noticed that many of the BA’s “critical thinkers” don’t like to get down and dirty quantitatively speaking while at the same time they will rip him up and down the other side for using kilograms as a unit of weight. Go figure.

    *Crickets*

  15. steve

    I’d never heard of a Sikhote-Alin meteorite until I read this entry by you, then within a half hour I came across this item ( http://www.wired.com/techbiz/people/magazine/16-10/ff_walker?currentPage=all)about Jay Walker’s private library which contains a 15 pound fragment of one. Cool.

  16. AnthonyK

    I posted this question on another another thread, but a little too late. I don’t even know if it’s an interesting one but – I understand that to the ancient Egyptians the most precious metal, and one used in their ceremonial emblaming rituals was…iron. So precious and magical because it came only from meteorites (oids/ites, whatever). So is this true, and if so, how much iron really does fall from the sky? Oh, and has anyone actually been killed by a falling sky rock? I know it’s probably all in the book, but I wanna know now.

  17. Overhead Projector Enthusiast

    The original 18-minute documentary is fascinating. I watched it at meteorites.com.au after I bought a tiny Sikhote-Alin meteorite at a store in Houston. It wasn’t easy terrain for them to investigate and they had to worry about wild animals to boot. Mine looks like a A href=”http://207.56.99.163/sainformationa.htm”>piece of shrapnel. It’s relatively heavy and sticks to magnets.

    The cover of your book reminds me of some of the early 50s sci-fi movies I’ve been watching. In two of them, their rockets meet up with “meteoroid storms”…one being a bunch of whizzing orange blobs that fly by their window with nary a scratch. At least in When Worlds Collide they have a ship ready to go so they can land on the Technicolor planetoid when the asteroid hits Earth.

  18. OPE, I specifically wanted the cover to be a B-movie satire. I’m glad you saw it that way!

  19. IVAN3MAN

    @ AnthonyK

    There are several unconfirmed reports of both people and livestock that had been killed by meteorites, the most infamous fatality being the reported fatality of an Egyptian dog that was killed in 1911, although this report is highly disputed. However, there is substantial evidence that the meteorite known as Valera hit and killed a cow upon impact, nearly dividing the animal in two, and similar unsubstantiated reports of a horse being struck and killed by a stone of the New Concord fall also abound.*

    The first known modern case of a human getting hit by a space rock occurred on 30 November 1954 in Sylacauga, Alabama. There, a 3.86 kg stone chondrite crashed through a roof and hit Ann Elizabeth Hodges (1923-1972) in her living room after it bounced off her large wooden console radio — which was destroyed. The 31-year-old woman was badly bruised on one side of her body, but able to walk. The event received worldwide publicity.*

    The United States Air Force sent a helicopter to take the meteorite. Eugene Hodges, the husband of the woman who was struck, hired a lawyer to get it back. The Hodges’ landlord also claimed it, wanting to sell it to cover the damage to the house. There were offers of up to US$5,000 for the meteorite. By the time the meteorite was returned to Mr. & Mrs. Hodges, over a year later, public attention had diminished and they were unable to find a buyer willing to pay much money.* (Bummer! I’ll wager that the bloody lawyer still got his “fee”!)

    Mrs. Hodges, uncomfortable with the publicity and stress over the disputed ownership of the meteorite, donated it, against her husband’s wishes, to the Alabama Museum of Natural History where it is on display at the University of Alabama in Tuscaloosa.*

    *Source: Wikipedia — Hodges meteorite.

  20. IVAN3MAN

    ERRATUM: The source of the first paragraph in my post above should be: Wikipedia — Meteorite (Meteorites in history).

  21. Don Snow

    @ Anthony K –

    In space, they’re meteroids; in the atmosphere, they’re meteorites; and on the ground, they’re meteors. I think that last two is correct.

    Somebody correct me, if I’m wrong.

    Anyway, they have the most cumbersome naming of any inanimate object that I know.

  22. @ Don Snow: no, in the atmosphere it is a meteor, on the ground a meteorite

    @ Anthony K: yes, several early iron artifacts from the Near East (nut just Egypt) are meteoritic iron. There are also Mesopotamian clay tablets talking about the trade in meteoritic iron. As meteoritic iron is very pure, you can cold-hammer it, like copper. By the way, the iron objects from Tutankamon’s tomb are widely believed to be meteoritic iron (given their age); but have never been tested….

    I am happy to own a piece of the Valera cow killer, along with a photocopy of the afidavit that it hit a cow (detail mentioned in it: it – the cow- was eaten afterwards…). I also have a nice specimen of Sikhote Alin.

    Apart from the Miss Hodges hit in Syllacaugha, a boy was also hit in the 1992 MBale fall in Uganda.

  23. @ Don Snow

    You got the last two definitions transposed. It should be: when entering the atmosphere, they’re are meteors; when found on the ground, they’re meteorites.

    Click on my name for more information.

  24. D’oh! Marko Langbroek beat me to it.

  25. Sorry, Marco Langbroek, for misspelling your first name. Man, I need a coffee break!

  26. AnthonyK

    Ain’t it great when you ask a question and you get brilliant answers from experts? Thanks very much. Now, can anyone tell me – what is the one true faith?
    AnthonyK

  27. AnthonyK: “What is the one true faith?”

    Jedi-ism — may The Force be with you.

  28. There have been several large meteors that have been filmed during the daytime. Probably the most famous one was filmed by Linda Baker on August 10, 1972 in Montana:

    http://www.youtube.com/watch?v=It5EztnIdHc&feature=related

    This is called the Great Daylight 1972 fireball. In this particular case the asteroid entered the earth’s atmosphere at a very shallow angle, reached a minimum altitude of 57 kilometers, and then skipped back out of our atmosphere. The trajectory was atmospheric entry over Utah and atmospheric exit over Alberta, Canada. Linda Baker was tourist at Grand Teton National Park in Wyoming. She managed to capture it using an 8mm movie camera.

    There is some uncertainty as to the size but it’s generally believed to be in the range of 3 to 15 meters in diameter. The thing I notice about the film is that the head of the meteor has a noticeable extent. If the meteor is at least 100 km away from where Linda Baker was standing then a 15 meter object should have an apparent diameter of only 30 seconds of arc. The actual white circle you see on the film must be at least half a degree in angular diameter, or 60 times larger. So that means we’re not actually seeing the object itself, but rather a large envelope of heated gas surrounding it. The object itself would be too small to see at that distance.

    There may not be extraterrestrial spaceships visiting our planet, but that does not mean that we don’t receive extraterrestrial visitors from time to time. This particular one paid us a visit back in 1972, checked out the scenery on the earth, and then headed back into space.

    BTW, I’m not sure the BA’s statistic of a 30-meter asteroid hitting every 100,000 years is at all correct. The 1972 object was awfully big and there have been several more detected since then. The correct number might be something like a 30-meter asteroid collision every 100 years or every 50 years.

  29. Some other near misses courtesy of Wikipedia:

    http://en.wikipedia.org/wiki/List_of_noteworthy_asteroids#Record-setting_close_approaches_to_Earth

    2004 FU162 – 6 meter asteroid, passed within 7,000 km of earth’s surface on March 31, 2004

    2004 FH – 30 meter asteroid, passed within 43,000 km of earth’s surface on March 18, 2004

    And of course, the asteroid Apophis which has a diameter of 350 meters will pass within 5.6 Earth radii (35,000 km) of Earth on Friday the 13th in April of 2029. If Apophis passes through a region of space called the “keyhole” during its closest approach to Earth on April 13, 2029 then it will collide with the earth on April 13, 2036. The size of the “keyhole” is approximately 600 meters. The probability that it will pass through the “keyhole” is approximately 1/45,000. Estimated kinetic energy of impact from Apophis is 880 Megatons of TNT. So you don’t want to be hit with that kind of force.

  30. Slightly off topic, I saw a fairly bright meteor last night around 9 heading SE here in Chicago. Very cool. :)

  31. “Slightly off topic, I saw a fairly bright meteor last night around 9 heading SE here in Chicago.”

    Keep in mind, the typical meteor you might see on a clear night is produced by an object about the size of a grain of sand, say 1 millimeter in diameter or so. How bright was it? What would you estimate its apparent visual magnitude at (Venus is about -4, Sirius is about -1.5, the full moon is about -12.5)? Was it brighter than Venus?

    I once saw a bright meteor (i.e., bolide) during the daytime. It was travelling at about ~5 degrees per second and left a green trail. Probably about as bright as the full moon but it was kind of hard to tell since it was daytime. Like the Linda Baker film it had a noticeable extent on the head which was wider than the trail. The head was also burning green. I wonder what the heck that’s all about since the 1972 fireball was pretty much white. Maybe the altitude determines the color or perhaps the composition of the meteor. Damn, I wish I had had a camera with me at the time. I’ve never seen anything like that since and that was back in the mid 1970’s.

  32. Here is an excellent PDF file about the threat of Near Earth Asteroids (NEAs) written by David Morrison and friends a few years ago.

    http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3043.pdf

    Some highlights of the report:

    Total number of NEAs with diameters greater than 1 kilometer = ~1,000

    The atmosphere protects us from most impactors smaller than a few tens of meters.

    The Tunguska impactor of 1908 released a kinetic energy of 10-15 Megatons of TNT and exploded at an altitude of 8 kilometers. Recent numerical modeling reveals that it was a rocky asteroid, not a comet. Approximate diameter of the impactor was ~60 meters.

    There have been 136 reported atmospheric entries of asteroids between 1975 and 1992. The maximum energy of an impactor expected per year is 10 kilotons of TNT (similar to the atomic bomb dropped on Hiroshima).

    The K/T (Cretaceous/Tertiary) impactor that did in the dinosaurs 65 million years ago had a diameter of 10-15 kilometers. It released ~100 million Megatons of TNT worth of energy.

    “The threshold for atmospheric penetration of impacts, required for the blast effects to reach the ground, is at a few megatons (Chyba et al., 1993; Hills and Goda, 1993; Chyba, 1993). Below this energy, the atmosphere protects us against all but the rare metallic projectiles. For impacts
    above this threshold, the primary effects of both airbursts and ground impacts are local blasts and earthquakes, together with the setting of local fires.”

    “While there is considerable uncertainty in both the height of the open-ocean wave and the run-up as it reaches the shore, Toon et al. (1997) conclude that large tsunami, occurring with average frequency of tens of thousands of years, contribute much more to the hazard than do terrestrial impacts in the same energy range.”

    “Toon et al. (1997) conclude that the energy range between 1.0E5 and 1.0E6 Megatons is transitional between regional and global effects, with a mean value for the threshold of global catastrophe near 1.0E6 Megatons of energy, corresponding to an NEA diameter of ~2 km.”

    “The threshold impact of ~1.0E6 Megatons from Toon et al. (1997) is expected to take place roughly twice per 1 million years. Chapman and Morrison (1994) define the threshold as an event that would kill 25% of Earth’s population — far less than an “extinction level event” but large enough to rank as the worst catastrophe in human history. Adding the lesser casualties from smaller but more frequent impacts, we estimate very
    roughly that an average individual on Earth today runs a risk of death from an impact on the order of 1 in 1,000,000 each year. For comparison, this is about the same level of risk associated with one roundtrip commercial air flight per year. Depending on where a person lives, this impact risk may be either higher or lower than the risk of more familiar natural disasters such as earthquakes or flooding. Although impacts below the threshold for global catastrophe are much more frequent, the total hazard from such impacts is less. At energies between 1.0E4 and 1.0E6 Megatons, the dominant risk is from tsunami created by deep ocean impacts, as discussed in the previous section. From 10 to 1.0E4 Megatons, the blast effects for land impacts dominate. The average risk level from tsunami is roughly an order of magnitude lower than that of the threshold global catastrophe, and that of smaller (blast-dominated) land impacts is down another order of magnitude.”

  33. Dang, there’s a lot of good stuff in that PDF file:

    “We selected only objects with absolute magnitude H < 18.0 and perihelion distance q 1 km according to the usual assumed albedo of 0.11. The number of asteroids satisfying these criteria was 244, compared to a total of 488 NEAs (q < 1.3 AU) in the same size range. We then calculated all approaches to Earth closer than 0.1 AU for a full century, from 2002 to 2102, choosing the dates to avoid the bias of discovery apparitions, using unperturbed osculating elements to provide a statistically valid measure of close approach frequency.

    We found 273 close approaches (2.73/yr) by a total of 100 different NEAs. For each close approach, we tabulated the encounter velocity so we could compute the mean squared impact velocity of the flux to relate that to energy. The radius of Earth is 4.25E–5 AU, so the frequency of actual impacts, not including the effect of gravitational focusing, should be (4.25E–4)^2 times less than the frequency of passes to within 0.1 AU. Since we kept track of encounter velocity for each event, we could correct for the
    effects of gravitational focusing, which enhanced the impact frequency by a factor of 1.66. Combining all these factors yields an impact frequency from the presently known population of 8.2E–7 per year."

    "An additional result that comes out of the close-encounter calculation above is the RMS velocity to relate mass of impactor to energy. We have weighted the individual encounter velocities to account for the higher probability of slower impacts due to gravitational focusing and also added
    in the contribution of Earth’s gravity to obtain an RMS impact velocity of 20.2 km/s, in excellent agreement with a recent calculation by Bottke (personal communication, 2001), in which he also includes a bias correction for discovery selection effects. If we assume a mean density of asteroids
    of 2.5 gm/cm^3, the mass of a 1-km-diameter asteroid is 1.3E12 kg and its kinetic energy at 20 km/s is 2.6E20 joules. In Fig. 3, the impact energy (top) and diameter (bottom) are thus scaled so that the impact energy is 6.25E4 Megatons for a 1-km-diameter NEA."

    Note, using my previous equation:

    K = 6.25E-5 * rho * d^3 * v^2

    K is kinetic energy is kilotons of TNT,
    rho is average density in grams per cubic centimeter,
    d is the diameter of the asteroid in meters,
    v is the impact velocity in kilometers per second

    Now, plug in 2.5 for rho, 1,000 for d, 20.2 for v
    Then K is 6.37E7 kilotons = 6.37E4 Megatons
    which is 2% off from their figure of 6.25E4 Megatons

    I feel so vindicated now! Of course my number is more accurate than theirs. :)

  34. Figure 3 in the PDF contains a chart showing the average time between impacts versus the size of the object. The best power fit to the data is as follows:

    dT-impact = (d / 4) ^ 2.35

    where dT-impact is the average time between impacts on the earth in years
    and d is the diameter of the asteroid in meters

    Combining this equation with the previous equation we arrive at the following statistics:

    1-meter asteroid (0.06 kilotons) impacts 25 times per year
    3-meter asteroid (1.7 kilotons) impacts 2 times per year
    10-meter asteroid (64 kilotons) impacts once every 8 years
    30-meter asteroid (1.7 Megatons) impacts once every 110 years
    60-meter asteroid (14 Megatons, Tunguska event) impacts once every 580 years
    100-meter asteroid (64 Megatons) impacts once every 1,900 years
    300-meter asteroid (1,700 Megatons, Apophis) impacts once every 25,000 years
    1,000-meter asteroid (64,000 Megatons) impacts once every 430,000 years
    3,000-meter asteroid (1.7E6 Megatons) impacts once every 6 million years
    10,000-meter asteroid (6.4E7 Megatons, Chixculub) impacts once every 97 million years

  35. So every two and a half years a 6-meter asteroid impacts somewhere on the earth with a kinectic energy of 14 kilotons of TNT – equivalent to the atomic bomb dropped on Hiroshima. So the equivalent of a Hiroshima bomb explodes somewhere in our skies every 2.5 years. Only we don’t know about it because the atmosphere absorbs almost all of the energy. That’s truly amazing to think about.

  36. @ Tom Marking

    I find your above posts interesting because I, too, like messing about with some serious maths with regard to meteoroid/asteroid/comet impact potential, but, as I stated above, as soon as somebody starts talking serious mathematics, the whole forum room empties — just like when I crack jokes on other threads.

    *Crickets*

    I found an excellent web-site where you can have some fun by sending an asteroid/comet hurtling towards your favourite planet. You can select the size, composition, and velocity of the asteroid/comet as well as the target moon/planet.

    Click on my name for the link.

  37. @ Tom Marking

    P.S. I forgot to mention that that web-site also calculates the kinetic energy released on impact and the approximate size of any crater that results; e.g., a 100 metre diameter iron asteroid with a velocity of 30 km/s releases on impact the energy equivalent to 439 Mega-Tons of TNT, with a crater diameter of 2.5 km and a crater depth of 0.4 km. Woo-hoo!

  38. IVAN3MAN

    *More Crickets*

  39. Damn! Crickets!

    *sprays pesticide*

    *listens intently*

    *silence*

    *Walks away swinging spray gun and whistling*

  40. *Frogs ‘n’ birds*

  41. Thomas Siefert

    *Sounds of frogs ‘n’ birds dying after eating pesticide killed crickets*

  42. @IVAN3MAN: “I found an excellent web-site where you can have some fun by sending an asteroid/comet hurtling towards your favourite planet. You can select the size, composition, and velocity of the asteroid/comet as well as the target moon/planet.”

    Did you write that web site yourself? I’m particularly interested in the cratering sizes. What equations are you or they using for that? It seems that the crater diameter is going in proportion to the cube root of the kinetic energy or in proportion to the diameter of the asteroid. Thus, a 1.0 km wide rocky asteroid leaves a crater 12.5 km in diameter but a 3.0 km wide rocky asteroid leaves a crater 37.1 km in diameter (~3 times wider) for collision velocity = 20 km/sec.

    However, the crater depth only increased from 0.6 km to 0.9 km. It seems to increase by a steady 0.3 km for every increase in diameter by a factor of 3. Not sure why it’s not scaling the same way the crater diameter is.

    Also, I’m not sure if those earthquake magnitudes are correct. They seem like awfully small numbers to me for very large impacts. Thus, a 3.0 km wide asteroid impact has an energy equivalent to 9.8 on the Richter scale, slightly greater than the largest natural earthquake in history. Just a gut feeling tells me it should be much larger, but I would have to do more research to confirm that.

    Finally, I believe there must be some bugs in that URL as well. A 10.0 km wide rocky asteroid impacting on land at 20 km/sec leaves a crater 122 km wide and 1.2 km deep. The same asteroid impacting in the ocean leaves a temporary crater 122 km wide and 36.7 km deep. Since the deepest point in the ocean (i.e., the Challenger Deep) has a depth of 11 km this means the asteroid would dig a hole at least 25.7 km into the earth’s crust if it lands over the ocean whereas it will dig a hole only 1.2 km deep if it lands over the land. That makes no sense at all and MUST be a bug. The ocean should cushion its impact so that the depth of the crater at the bottom of the ocean should be less than 1.2 km deep. Other than that it seems like a fun program.

    Back to the *crickets*

  43. Yeah, I think there’s a problem with the earthquake magnitude calculation as well, or at least it’s not very accurate. Maybe they are rounding it off.

    Anyway, a 4 km rocky asteroid impacting at 20 km/sec has an energy of 4 Teratons and magnitude 10.0 on the Richter scale according to your URL. According to Wikipedia a magnitude 10.0 earthquake releases 1 Teraton of energy. So that’s one fourth the computed value. The actual magnitude should be 10.4 on the Richter scale. That’s not a terrible adjustment, much better than I thought at first, but still off by quite a bit.

  44. IVAN3MAN

    Tom Marking: “Did you write that web site yourself?”

    No, I said that I found the web-site, not ‘founded’ it personally. It was written and created by Dr. Douglas P. Hamilton and students at the University of Maryland, with support from NASA and NSF. His name is at the bottom of the page on that web-site, and there is a link — “Astronomy Workshop” — just above his name which leads to his main web-site page; at the bottom of that page, there is another link on his name which leads to his “Home Page” with further links to his Research Interests, Scientific Publications, CV, etc.

    Tom Marking: “What equations are you or they using for that?”

    I would like to know, too! Dr. Hamilton has an e-mail address at the top of his CV, which you can download as a PDF file. Maybe the both of us can badger him for the equations! :-)

  45. Tom Marking, there is another interesting web-site called Down 2 Earth — Impact Calculator (click on my name for the new link), which was mentioned by Dr. Phil Plait on this blog some time ago. It has been improved since then; previously, one was only able to select Wales as the “target”, but now you can select: Cardiff; London; Paris; New York; Barringer Meteor Crater, Arizona; Aorounga, Afica; Roter Kamm, Namibia; Mistastin Lake, Canada; Bosumtwi, Ghana; Kara-Kul, Tajikistan; last, but not least, bloody Wolf Creek, Australia! Hey, shane, you will enjoy this, mate! :)

  46. Tom Marking, and anybody else who is interested, there is yet another web-site (click on my name just above this post for the link) that calculates impact energy and effects of an asteroid/comet strike called Earth Impact Effects Program — at that web-site is a PDF file download link at the bottom of the page with details of the observations, assumptions, and equations upon which that program is based.

    All three of the above programs appear to give slightly different results! Ah, this is all theoretical; the only way to determine which program is the most accurate would be to do an actual test experiment by manoeuvring an actual asteroid in space to smash into the Moon. That would be fun!

  47. IVAN3MAN

    *Tumbleweed blows across the scene in Howling Wind*

  48. IVAN3MAN, I always wanted to go to Wolf Creek Crater. Got to within 70 or so kays of the place once. It is pretty remote. Didn’t have a four wheel drive. We asked a local if we could get there without a 4WD and he thought a bit and said “maybe…”. We asked if we could get back too and he just smiled and said “probably not”. We took a chopper over the Bungle Bungles instead. Pictures of Bungle Bungles here… http://outback.nixons.net
    We’re saving the big hole till next time.

    Lots of tumbleweeds up the top end… and goats. Millions of goats. Who new?

  49. “IVAN3MAN, I always wanted to go to Wolf Creek Crater. Got to within 70 or so kays of the place once. It is pretty remote. Didn’t have a four wheel drive.”

    I’ve been to Meteor Crater in Arizona in the 1980’s (previously called the Barringer crater). I’ve also been to Sedan crater in the Nevada Test Site which was made by a nuclear weapon in 1962. Meteor Crater is much, much bigger which puts the puny powers of Homo sapiens into perspective.

    Back in 1992 I wanted to go to the Wolf Creek Meteor crater in Western Australia but it was the rainy season (I think November to February if I remember correctly) and all the roads were washed out and private planes weren’t flying at the time. So I missed out on that one. I hear it’s very similar to Meteor Crater in Arizona but it’s still on my to-do list if I ever get back down under.

  50. @IVAN3MAN: “there is yet another web-site (click on my name just above this post for the link) that calculates impact energy and effects of an asteroid/comet strike called Earth Impact Effects Program — at that web-site is a PDF file download link at the bottom of the page with details of the observations, assumptions, and equations upon which that program is based.”

    Wow, I downloaded the PDF. It’s chock full of equations. It will take me some time to go through all that and understand it. Thanks so much. I particularly like the air blast calculations. My only gripe is there are still not tsunami calculations for ocean strikes even though it does compute the size of the crater at the bottom of the ocean. For asteroids in the range 500 m – 2,000 m it is probably the tsunami that will kill the most number of humans. Still, it’s an impressive program. Thanks again.

  51. Considering green colored bolides, I found the following event which happened way back in 1999. I guess I’m not the only person to have seen a green bolide. One of the commenters on the URL speculates that the green color has to do with the composition of the meteor, but I’m wondering if it is not caused by the same effect that makes the aurora borealis typically shine in the green part of the spectrum (i.e., spectral line at 557.7 nanometers of atomic oxygen).

    http://www.oregonstarparty.org/spacedebris.htm

    There were many sightings of the OSP Bolide that evening, gasping plenty of “oohs and ahs”. The following observations were made: “It was like the Sky was falling at OSP.” “It was a green flame, the size of a nickel at arms length.”

    .
    .
    .

    A shortened report by John Gillis of Rose City Astronomers is as follows: “I saw it at OSP, and it was amazing. It was not a point or a disk, but it was definitely elongated and I clearly saw it tumbling. It was a bright green color and left a smoke trail.” Paul Schmidt of RCA commented jokingly about the OPHI/SATT bolide on the night of the 10th, “It looked like a flaming potato.”

    .
    .
    .

    Richard Pugh, meteorological scientist, had three sightings of bolides. He commented: “The bright green color was common due to temperature and elemental content, indicating either a meteoritical nickel- iron or space debris. A steep descent angle usually was associated with meteorites.”

  52. Also, the Peekskill meteor was also green in color. Apparently green meteors are not that uncommon. I feel so cheated now! :) From Wikipedia:

    “The Peekskill meteorite broke up over the United States on October 9, 1992, an event witnessed by thousands across the East Coast. The meteorite broke up over Kentucky and landed on a parked car in Peekskill, New York. Major cities like Pittsburgh witnessed the bright meteorite. The meteorite traveled northeast and had a pronounced greenish color. The meteorite has been captured on 16 different videos and remains as one of the most famous meteorite sightings.”

    Here’s a good video of it:

    http://video.google.com/videosearch?q=Peekskill+metorite&emb=0&aq=f#

    That sucker is DEFINITELY green. I guess I’m not crazy after all. Green meteors really do exist. Apparently the Peekskill meteor actually impacted someone’s car and thus totally that. I hope his insurance paid for the damage. Anyway, enjoy!

    Still not clear what the cause of the green color is. Is it the nickel in the meteorite or the atomic oxygen surrounding it as it passes through the atmosphere. BA, any thoughts on that one? Yeah, right, the BA has moved on about a week ago on this post.

    Back to the *crickets*

  53. I did some additional research. Nickel has a bright spectral line at 5,476.91 angstroms and iron has a bright spectral line at 5,167.487 angstroms. Both of those wavelengths are in the green part of the spectrum so perhaps it is the iron-nickel composition that is causing the green glow of certain meteors.

    I’m wondering if a spectrograph has ever been used on one of these falling bolides. Probably not but that would certainly clinch it if you see emission at these specific wavelengths.

  54. IVAN3MAN

    Tom Marking:

    Nickel has a bright spectral line at 5,476.91 angstroms and iron has a bright spectral line at 5,167.487 angstroms. Both of those wavelengths are in the green part of the spectrum so perhaps it is the iron-nickel composition that is causing the green glow of certain meteors.

    I was going to post the same conclusion after doing some research, too.

  55. Click on my name for the APOD link featuring a “Bright [Green] Bolide.”

  56. Click on my name again for another picture.

  57. IVAN3MAN

    Note that in the last two pictures, the tails’ of the fireballs are green — which would suggest that it is due to ionized oxygen immediately after the meteor, rather than due to the meteors’ mineral composition.

  58. IVAN3MAN

    *Wolf howls in the distance*

  59. “It’s uh … It’s green.” – Montgomery Scott

  60. IVAN3MAN

    shane, I checked out the link you provided above. Bonzer pictures! I also signed your guestbook. :-)

  61. IVAN3MAN, just approved you post. And Thanks. The whole Red Centre, Top End and West is totally awesome. What amazed us on our drive is how few Aussies below retirement age visit. The Grey Nomads are everywhere in their RVs but if you see someone below 60 that doesn’t live there they are probably euro trash on holiday.

  62. IVAN3MAN

    British binge-drinkers plague European resorts, while Euro-trash plague Australia; there seems to be some sort of domino effect.

  63. IVAN3MAN

    *Sound of Pin dropping*

  64. British person

    “British binge-drinkers plague European resorts, while Euro-trash plague Australia; there seems to be some sort of domino effect.”
    The British contingent, at least, can at least partly be accounted for by the popularity of the following:

    http://www.youtube.com/watch?v=TZ0wB72TTg0
    http://www.youtube.com/watch?v=U8Qp24UDGjw

    I don’t know if they watch them on the continent.

  65. Finchcliff

    http://www.theimpactandexitevent.com cantains a brand new, 2009 impact hypothesis….

    The site also provides a 220+ page free ebook on the hypothesis.

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