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    Shooting Star Brighter Than The Sun
    By Johannes Koelman | February 17th 2013 07:28 AM | 8 comments | Print | E-mail | Track Comments
    About Johannes

    I am a Dutchman, currently living in India. Following a PhD in theoretical physics (spin-polarized quantum systems*) I entered a Global Fortune

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    A shooting star brighter than the sun on the very day an asteroid flyby is predicted. A most remarkable coincidence. And to add to our amazement, the rock whizzing through the skies didn't illuminate a distant ocean or some remote desert, but rather showed its spectacle above one of the very few areas in the world populated by people driving dashcams around 24 hrs a day. No screenwriter would get away with such an unlikely scenario.

    But how likely are meteor event like the one in Russia? What are the chances a similar event will occur this century? And what about even bigger events? What was the power of the blast from the Russian meteor? How does it compare to the explosions we humans create?

    The physics of meteor air bursts is straightforward. Meteors enter earth's atmosphere at velocities centered around 20 km/s (12 mi/s). As the kinetic energy carried by a massive object scales with the square of the velocity, meteors are objects densely packed with energy waiting to be released. A quick calculation shows that every kilogram of rock entering at about 20 km/s carries the explosive energy of 50 kilograms of TNT. The meteor that exploded over the region of Chelyabinsk, was approximately 17 meters (55 ft) across and weighted some 10,000 metric tonnes. That corresponds to an energy release of 50 times 10,000 ton or 500 kiloton of TNT, which is 30 times the energy released by the nuclear bomb on Hiroshima.

    These are shocking figures, and it seems amazing that no more than two cases of serious injuries have been reported. The key point to note is that for the Hiroshima bomb an air burst 600 m (1,900 feet) above the ground was chosen to achieve maximum blast effects at ground level. Nature isn't anywhere that mean, and the Russian meteor air burst occurred at an altitude of about 40 km (25 mi). Explosions at such altitudes spread their shockwave out over a much larger ground area at correspondingly weaker intensities. The abundance of dashcam footage of the Chelyabinsk event has made us all familiar with the typical effects to be expected at sea level from a '30 Hiroshimas' event in the upper atmosphere.

    What more does our universe have in store? Can we expect even bigger events during our lifetime?

    The energy content of asteroids impacting earth follows a spectacular power-law distribution over an amazingly wide range. A 2002 article in Nature combines data from many different sources and specifies that over an energy range of up to a million times Hiroshima the annual frequency of meteor events exceeding an energy E measured in "Hiroshimas" is given by

    N = E-0.9/3

    So one should expect an explosive event exceeding 'one Hiroshima' on average once every 3 years. And an event at least as strong as last week's Chelyabinsk airblast (30 Hiroshimas) will occur with a frequency of roughly once every 65 years. As no '30+ Hiroshimas' event has been reported since 1908, one could declare last week's event 'long overdue'.

    What about even stronger events? Doing the math, the power law relation leads to the frequency distribution described in below table.



    The first thing to note is that, contrary to reports in the popular media, the energy scale of the Russian event does not represent the much discussed 'once in a century event'. That title goes to events at a scale of 50+ Hiroshimas. We haven't seen such an event since 1908.

    Many people seem amazed that astronomers specialized in NEO (near earth object) tracking could not give an early warning of a '30 Hiroshimas' event. Fact is giving a one day warning for a 17 m object approaching earth at 20 km/s requires spotting an object at a distance of 100,000,000 times its size. That equates to spotting a 1 mm grain of sand at 100 km distance. No easy task. Larger (typically 100+ m diameter) near earth objects (NEOs) are actively being monitored, but even that effort is far from complete.

    Russia's deputy prime minister Dmitry Rogozin has called for international efforts to develop a system to intercept shooting stars. This call falls straight in the category knee-jerk reactions. You might not like it, but we probably have to live with the threat of a '30 Hiroshimas shooting star' occurring once in a human lifetime somewhere in earth's upper atmosphere. The good news is that such events tend to give a few minute warning prior to the airblast arriving at ground level, and that future victims of multi-Hiroshima shooting stars have had exposure to some relevant video footage. When a star brighter than the sun shoots far above my head, I surely will avoid the vicinity of any windows or other vulnerable structures for at least the next few minutes.

    Comments

    Stellare
    So, I live in a place where it is in principle possible to see shooting stars - definitely when there is swarm. Only thing is, I hardly do because I have been out of luck. My family members on the other hand see them all the time.

    I was delighted to see the bright - as you say brighter than the Sun - meteor on Friday. Like you I cannot believe the luck we all had with the Russian carcams! I am now diving into meteor footage! :-)

    Great article with some proper numbers. All physics-style! :-)
    —
    Bente Lilja Bye is the author of Lilja - A bouquet of stories about the Earth
    Johannes Koelman
    Finally, finally, your time has come: Now you can wish upon a shooting star (albeit in front of a computer screen). ;-)
    Stellare
    Exactly! You got it! hahaha
    —
    Bente Lilja Bye is the author of Lilja - A bouquet of stories about the Earth
    Johannes Koelman
    A note to the above blog post:
    Emeritus chair of the board of directors of the B612 foundation and former astronaut Rusty Schweickart made the following remark in an e-mail reaction:

    "Tunguska was thought to be about 45 meters diameter and ~10 MT… however the work a few years ago by Mark Boslough (Sandia Labs) now shows that it was more likely ~30 meters and 3-5 MT. The import of this is that a 30 meter object impacts ~ once/300 years vs. once/1000 years for a 45 meter object."

    I was not aware of this recent new insight which places Tunguska in the 200-300 Hiroshimas category. Found the following webpage giving lots of background material to the work:
    https://share.sandia.gov/news/resources/releases/2007/asteroid.html
    Have updated the graphics in above blog post. Thanks Rusty!
    MikeCrow
    4-5 years ago, I was driving back from a long trip, it was near midnight, it had been dark for a number of hours, and we were a ways from any large cities.
    When the sky lit up behind me, almost day light levels of light, but only for a moment, and I didn't see the source of the light. For a couple of minutes I waited for a blast wave, that never came. I did a few google searches the next day, and didn't find anything.
    I figured it was a bolide, and it was a lot brighter than the Peekskill Meteorite that I saw pass overhead.
    —
    Never is a long time.
    The Chelyabinsk rock seemed to enter at a very shallow angle. Is this why it exploded so high? Would it have caused more damage entering at a steeper angle?

    A remote dessert? I suppose if the dessert were sufficiently remote, it might take a meteor to light it sufficiently to find it and eat it.

    Johannes Koelman
    Lol, thanks. Corrected.