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Piecing Together the Chelyabinsk Event

We’re still trying to learn how frequently asteroid events like the spectacular fireball over Chelyabinsk occur. The Chelyabinsk object was the largest to fall to Earth since the Tunguska explosion in 1908, which leveled thousands of acres of forest in Siberia. This BBC story discusses Peter Brown (University of Western Ontario) and colleagues’ recent paper in Nature and goes on to quote Brown as saying that a few days’ to a week’s warning would have been valuable so that we would have been prepared for what happened near the Siberian city. True enough, but what’s significant here is that the Brown team studied 20 years of data from sensors positioned around the world to estimate the frequency of such events.

The upshot: About sixty asteroids up to 20 meters in size entered Earth’s atmosphere during this period, a significantly higher number than was previously assumed. Brown’s team reports we’ve been underestimating the strike rate of asteroids between 10 and 20 meters in size by between two and ten times. That would make a Tunguska-class impact likely every few hundred years rather than every few thousand. As for the most recent event, Brown says this:

“Something like Chelyabinsk, you would only expect every 150 years on the basis of the telescopic information. But when you look at our data and extrapolate from that, we see that these things seem to be happening every 30 years or so.”

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Image: The flash above Chelyabinsk, Russia, from the fireball streaking through the sky on Feb. 15, 2013. The picture was taken by a local, M. Ahmetvaleev. Credit: Copyright M. Ahmetvaleev.

Meanwhile, a new paper in Science gives us more information about the Chelyabinsk object itself. In sharp distinction to what happened in Tunguska in 1908, the Chelyabinsk incident was well reported and photographed, allowing teams under Peter Jenniskens (NASA Ames) and Olga Popova (Russian Academy of Sciences, Moscow) to follow up the story with first-hand accounts from witnesses and to study numerous videos that captured the event, helping them to reconstruct the asteroid’s properties and trajectory.

We learn the following: The impact speed of the meteor, as determined by calibrating video images, was roughly 19 kilometers per second. The object’s passage into the atmosphere caused it to fragment into pieces about 30 kilometers above the surface, at which point its light appeared brighter than the Sun even for people as far as 100 kilometers away. Between 4000 and 6000 kilograms of material fell to the ground, with much of the debris being vaporized. One 650 kilogram fragment was recovered from Lake Chebarkul on October 16, 2013. A 3.4 kilogram rock fell near the town of Timiryazevskiy and another chunk hit a house in Deputatskiy.

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Image: A photograph of the meteor streaking through the sky above Chelyabinsk. Credit: Copyright M. Ahmetvaleev.

Evidently shock fractures in the rock, produced by an impact that may have occurred over four billion years ago, helped cause its breakup in the upper atmosphere. This NASA news release quotes Mike Zolensky (NASA JSC) on the small iron grains found just inside the veins as a contributing factor. “There are cases where impact melt increases a meteorite’s mechanical strength, but Chelyabinsk was weakened by it,” said Zolensky. In further Chelyabinsk investigations, Jiri Borovička (Academy of Sciences of the Czech Republic) and colleagues report in a second paper in Nature that the Chelyabinsk object may well be a fragment from asteroid 86039, a two-kilometer object whose orbit is strikingly similar.

The papers are Brown, et al., “A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors,” published online in Nature 6 November 2013 (abstract) and Borovička et al., “The trajectory, structure and origin of the Chelyabinsk asteroidal impactor,” published online in Nature 6 November 2013 (abstract). See also Popova et al., “Chelyabinsk Airburst, Damage Assessment, Meteorite Recovery, and Characterization,” published online in Science 7 November 2013 (abstract).

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Comments on this entry are closed.

  • Rob Flores November 13, 2013, 16:20

    This rock came in shallow, if it had been steeper angle and detonated at 9-10 km from the ground the devastation on the ground would have been appaling.

    So How many times in Human Racial memory have larger more devastating
    events occured. We can add this to the list of causes of proto-civilization collapse, either through direct effects or long term damage to regional ecology or even weather effects. This is probably a component of the FILTER restricting the rise ETI. Even these small events can have
    consequences just to due to their frequency.

    The news media fail to enphasize that although agencies are searching for
    Threatening bodies out there, there is a certain class that can arrive without
    much warning. Obcourse I mean those that are coming in from direction of our Sun. I don’t think we have a blind spot for object coming in at a high
    angle relative to our ecliptic. That is not the type of orbit most solar system bound objects tend to follow, but I am unsure, ask an object seeking Astronomer.

  • Alex Tolley November 13, 2013, 18:15

    Another way to determine likely frequencies is to measure hits on the moon by observing flashes on the unlit hemisphere. Earth observers are doing that now, although a dedicated satellite might be worth spending money on.

    What I am not clear about is how we mitigate. These meteors give very little, if any, warning, so we cannot gently push them away with various schemes. I don’t even see how attempts to obliterate them with nukes would work without sufficient warning. Does anyone have any thoughts about how we gain earlier warning and how to remove the danger from larger ones?

    While it has been interesting reading about existential risks from ETC’s with next to no data to assess risks, the meteor strike problem is real, the risks computable, as is the economic damage (let alone cultural), yet solid work on solving this problem still seems, AFAICT, still at the very earliest of stages.

    I would have thought that even if we cannot get things moving on an international basis, there is a clear and present danger for national security and would warrant diverting some defense spending to meet this need. The US certainly has enough DoD spending that could be used. Related to this, didn’t the provision of current radar data from NORAD to scientists get canceled a few years ago, or was this decision reversed?

  • Jim Early November 13, 2013, 18:46

    Rob is correct about the shallow entry angle. At a more probable near vertical entry the momentum from the object would have carried the plasma from the explosion to the ground even if the object detonated in the atmosphere.
    At the DPS meeting in Denver the most spectacular video shown of the entry was take through the windshield of a truck driving exactly in the opposite direction of the asteroid. The object starts as a small bright spot and continues to grow larger. Fortunately for the truck driver, the asteroid explodes at a safe distance.

  • Eniac November 13, 2013, 22:26

    I was struck by this little tidbit: “There are cases where impact melt increases a meteorite’s mechanical strength, but Chelyabinsk was weakened by it.” It makes me think that if we wanted to bring platinum group metals from space to the ground, cheaply, a carefully calibrated drop into the atmosphere of a solid lump would probably work quite well.

    Compared to this meteorite, the material of such a lump would be much stronger and more refractory, the velocity would be lower, and the entry angle would be chosen to be optimal.

  • ljk November 13, 2013, 23:40

    Video of the Large Chunk of Chelyabinsk Asteroid Hitting a Frozen Lake!

    By Phil Plait

    I don’t mean this blog to be all asteroids all the time, but a new video just came out (via Universe Today) that is pretty dang cool: It’s blink-and-you’ll-miss-it, but it shows the half-ton chunk of Chelyabinsk meteorite slamming into the Lake Chebarkul on the morning of Feb 15, 2013:

    http://www.slate.com/blogs/bad_astronomy/2013/11/07/video_chunk_of_chelyabinsk_meteorite_hitting_frozen_lake.html

  • Doug M. November 14, 2013, 6:23

    “although a dedicated satellite might be worth spending money on.”

    — A dedicated satellite in Earth orbit would be a bit tricky; in order to observe the Moon continuously, it would have to be in a fairly high orbit and also inclined so that the Earth wasn’t periodically eclipsing it. In a perfect world you’d have two of them in Lunar orbits, so that the unlit hemisphere was always being watched. (Of course, Lunar orbits aren’t stable over decades. But a few years of good observations would tell a lot.)

    “What I am not clear about is how we mitigate. These meteors give very little, if any, warning, so we cannot gently push them away with various schemes… Does anyone have any thoughts about how we gain earlier warning and how to remove the danger from larger ones?”

    — We’re still about 20 years away from being able to detect most impactors of this size that are on NEO tracks. On the other hand, we *are* about 20 years away from that — the next generation of giant telescopes, coming online in the 2020s, will be able to spot Chelyabinsk-size impactors up to 0.5 AU away. To give just one example, the European Extremely Large Telescope, with a 39 (!) meter primary mirror, is scheduled for first light in 2022; if it works as planned, it will be able to capture every NEO down to less than 100m diameter.

    It won’t happen overnight, because the EELT is a general purpose scope, and it will only be devoting ~5% of its time to dedicated asteroid hunting. But OTOH, EELT is just one of a bunch of powerful new scopes, both on the ground and in orbit. So, what with one thing and another, by the early 2030s we’ll be well along the way to surveying all of these guys. Which means, yes, we’ll be alerted to potential impacts decades in advance.

    Doug M.

  • A. A. Jackson November 14, 2013, 13:04

    When I was at the Planetary Defense Meeting in Flagstaff this year.
    Don Yeomans, of JPL, noted that there have been two Congressional Directives, one in 1998 and updated in 2005, that NASA totally catalog all potentially hazardous NEO’s by 2020.
    It was noted that funding for this since 2005 has been such that the 2020 date cannot be met.

  • David A. Czuba November 15, 2013, 20:48

    I keep returning to the idea of protection for light speed spacecraft, especially in light of the new extrapolation of data on the number of Chelyabinsk-sized object hitting the Earth’s atmosphere. Even after 4.5 billion years, we’re living in a shooting gallery. And the empty space beyond the heliopause could be more cluttered than we suspect with dark matter too small to be detectable by reflected light or capable of fusion. There’s a lot we still don’t know.

    To protect starships, should they be built behind shields of ‘sandpile’ asteroids or those comprised of more solid material? Propelling these rocks to overcome orbital inertia to solar escape velocity would be slow, but provide a larger surface platform to mount a solar sail to receive collimated light. Perhaps starship engineers will use hollowed out shells of mined asteroids as relatively lightweight protective shields from which to mount a hybrid propulsion platform with a ramjet scoop in front with light sails and living quarters behind.

  • Eniac November 15, 2013, 21:29

    David A. Czuba:

    And the empty space beyond the heliopause could be more cluttered than we suspect

    I think we have a pretty good idea about how empty interstellar space is, beginning with the fact that we can see through it for many thousands of light years. It is REALLY empty. Encountering pebbles or even dust grains is not really a concern. 99% of the matter is gas, which starts to become a problem around 10% of light speed, as I recall.

  • Rob Flores November 15, 2013, 22:31

    David A.C.
    Best protection is to avoid them IMO. If you travel at 45%c. you can still use radar to evade your larger threats. Once you are over or close to 50%c that option closes. Said radar will use allot of power if your ship
    needs to delect obstacles below microwave sensitivity. It’s probably more efficient to use the ships power for scanning for the smallest threats rather than having to accelerate a massive forwardshield to “accept” an impact.
    Obviously you will still need a modest shield to deal with dust grains.
    If your voyage is dependent on relativistic speeds to ease the effects of
    a long journey you would have to have a daisy chain of leading “radar pods”
    Each one warning the other to evade. (obcourse the leading pod is the point
    man and will get vaporized at some point. Well if theyre relatively cheap and light what’s a few score of radar pods getting zapped compared to the main precious cargo.

  • JoeP November 18, 2013, 13:00

    @Rob Flores,

    I always appreciated the idea of a vanguard of defending “pods”, to use your term, ahead of the spacecraft. I think a variant was introduced with Project Daedalus, of having autonomous “dust-bug” robotic vehicles ahead of the main spacecraft. These robots generate and maintain a dust cloud that acts as a proactive shield for the ship behind; the cloud automatically vaporizes any small objects or deflects them — clears the path.

    Of course there are problems with course correction and replacement mass for the cloud, but it may be manageable for the main and fastest part of the flight. I just do not know how much dust would be lost due to ongoing interactions with gas molecules, cosmic rays, and so on.

  • Eniac November 22, 2013, 21:17

    In my view the biggest obstacle to approaching light speed is the gas, not the dust. Besides appearing as destructive high energy proton radiation, it also generates a large amount of heat when blocked, to the point where at ~0.5 c you would need a solid block of white hot tungsten or carbon for an effective shield. Anything flimsier like a cloud or foil is not likely to hold up for long.

  • RobFlores November 23, 2013, 14:01

    I always thought using something like a viscous fluid pool (protected from
    freezing) mounted up front would do the trick for dust and high energy protons. While a 5 m high pool wide enought to protect the ship would weigh a fair amount it would not weigh as much as metals. Also your fluid should be renewable as some of it’s molecules will be converted to energy or will transmutate and will have to cleaned out by robotic sweeper at the bottom of the ‘POOL’. This obcourse will induce a braking force, but I am sure some clever fellow will be able to use the energy from the gas collision
    onto said pool to help with the propulsive effort to maintain speed.
    Turbo-Compound in space (hopefully w/o throwing cylinders)
    For the record I think using relativistic effects for space travel is not
    going to be practical. I think Sub 45% of C, with “hybernation” is what will
    predominate especially in attempting to colonize nearby stars.