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100th Anniversary of a Cosmic Warning

By Larry Klaes

“Suddenly in the north sky… the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire… At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled.” So wrote a witness — fully forty miles away from the event — of the Tunguska impact of 1908, whose 100th anniversary is today. As Larry Klaes notes, small bodies still undetected by astronomers could pose the threat of another Tunguska, making the hunt for Earth-crossing objects a matter of high importance not just for science but planetary security.

Across the many billions of miles of space that our Solar System occupies in its small piece of the vaster Milky Way galaxy, the most numerous members of our celestial neighborhood by far are the comets, planetoids, and meteoroids.

Although much smaller than the Sun, the major planets, and many of the attending moons of the largest worlds, comets and planetoids can be found by the billions from inside the orbit of Mercury to halfway to the next star system, Alpha Centauri. The vast majority of comets occupy the utterly dark and cold outer realms of the Solar System past Neptune, the last and smallest of the giant planets. Comets are the icy remains of the birth of our Solar System roughly five billion years ago. Most planetoids can be found scattered between the orbits of Mars and Jupiter. These rocky bodies represent worlds that could never form into anything grander due to the massive, disrupting presence of Jupiter from the earliest days of our star system. Meteoroids are the scattered remnants of planetoid collisions, ranging in size from boulders to dust; they can be found in virtually every corner of the Solar System.

Like most things in reality, though, not everything in the Solar System stays in one neat area, especially over the vast stretches of time that our system has existed. Many comets have been perturbed from their distant orbits into new ones which cause them to venture into the inner Solar System. Some comets make briefs visits and then head back out into the blackness, not to be seen again for many centuries. Others plow directly into the Sun, while some get “caught” just right and end up in new, smaller orbits that cause these icy worldlets to return to our region of space in mere decades. Halley’s Comet is perhaps the most famous example of what astronomers call a short-period comet: Last visiting Earth in 1986, Halley’s Comet will come around again in 2061. Planetoids have also been knocked about through the ages, being whipped around by each other and larger worlds, or smashing together and subsequently finding themselves in different shapes and sizes along new circuits about the Sun.

The Tunguska object explodes in the upper atmosphere

Though aware of comets and planetoids for centuries, it has only been in the last few decades that astronomers and others have begun to take seriously the remote but definite possibility that some of these “rogue” objects could one day collide with our planet Earth. While our actual world could survive an impact with these much smaller celestial bodies, it would be another story for most of the life existing on the crust of this planet. The 160 million-year reign of the dinosaurs came to an abrupt end 65 million years ago when a chunk of ancient space debris a mere five miles across slammed into Earth. Earlier epochs may have seen even more organisms rendered extinct by similar celestial impacts.

Image: Space debris is by no means a thing of the past. This artist’s conception shows the moment of explosion of a huge space rock that shattered trees throughout a large section of Siberia in 1908. Events of this magnitude are thought to occur approximately every 300 years. Credit: William Hartmann.

For those who might think that destructive collisions between our planet and a comet or planetoid happen too seldom to be of much concern for humanity and the rest of Earth’s biosphere, one should take note of an important anniversary in astronomical history which took place one century ago.

In the early morning of June 30, 1908, a huge ball of light almost as bright as the Sun was seen moving across the sky over a remote region of Siberia. The strange object was a piece of a comet or planetoid perhaps the size of a ten-story building streaking through Earth’s atmosphere at tremendous speed.

Suddenly the space visitor violently broke apart several miles above the ground near the Podkamennaya Tunguska River with a force estimated at one thousand times greater than the explosion of the atomic bomb dropped on Hiroshima, Japan in 1945. Eighty million trees for 830 square miles around the detonation zone were flattened. The ground shook like an earthquake and the resulting shock wave caused people living miles from ground zero to be knocked off their feet.

Trees knocked over by the Tunguska event

This cosmic impact, which became known as the Tunguska Event, is one of the earliest instances which made people aware of the fact that not everything plunging in from space is either a harmless and pretty meteor shower of particles no bigger than dust or a larger yet still “safe” rock or boulder known as a meteorite. Had the object which created the Tunguska Event landed on a major city instead of a remote forested region of Earth, the unfortunate urban center would have suffered major damage and a high loss of life. If a Tunguska Event happened today in either a rural or urban area, the explosion might be interpreted as a nuclear attack, with potentially deadly consequences for all humanity from the response to such a “message” from the Universe.

Image: A Russian expedition to the Tunguska site showed what the scene looked like fully twenty years after the event. The expedition, led by Leonard Kulik, discovered that the pattern of fallen trees pointed to an explosion caused by a meteorite, but Kulik found no fragments of the object. It is now believed that the Tunguska asteroid, about 36 meters across, exploded some 28,000 feet up in the atmosphere.

Of greatest concern to scientists, political leaders, and others is the random space object miles across with an orbit that may one day intersect our planet on its path around the Sun. Thanks to the diligence of various professional groups over the last decade, thousands of Near Earth Objects (NEOs) have been found and their orbits calculated.

The majority of these bodies pose no threat to us and our planet for the foreseeable future. However, the orbits of a few NEOs are still not known with enough certainty to remove them from the list of potentially deadly space rocks just yet. Additionally, there are quite a few smaller bodies roaming the Solar System that remain undetected by astronomers, any of which could one day cause destruction to our world on a Tunguska level or worse.

Facilities like the Cornell-run Arecibo Radio Observatory in Puerto Rico remain at the forefront in helping scientists to understand the makeup of these NEOs and how to deal with them in case one does have Earth in its sights some day. The continued existence of Arecibo and other science facilities may become the difference between a thriving future for humanity or extinction.

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  • ljk June 30, 2008, 15:45

    The Tunguska Impact–100 Years Later

    NASA Science News for June 30, 2008

    One hundred years after the Tunguska event in Siberia, scientists review what they’ve learned about the mysterious blast from the heavens.



  • ljk June 30, 2008, 15:47

    Huge Tunguska Explosion Remains Mysterious 100 Years Later


  • ljk July 1, 2008, 10:04

    Acid Rain Traces Support Meteor Theory For 1908 Tunguska Blast

    Moscow (RIA Novosti) Jul 01, 2008 – International researchers investigating the Tunguska Event, an explosion exactly 100 years ago in central Siberia, say acid rain traces in the region back up the theory that the blast was caused by a meteorite.

    On June 30, 1908, an explosion equivalent to between 5 and 30 megatons of TNT occurred approximately 7-10 km (3-6 miles) above the Podkamennaya Tunguska River in a remote Siberian …

    Full article here:


  • ljk July 2, 2008, 11:04

    Maverick scientists probe Siberian forest mystery

    Moscow (AFP) July 1, 2008 – Was it a gigantic meteorite? A
    tremendous bolt of lightning? Perhaps the crash of a UFO the
    size of Tokyo? No one is certain of the answer to one of the
    20th century’s greatest scientific mysteries — the “Tunguska
    Event” 100 years ago this week.

    But a group of maverick Russian scientists gathered in Moscow
    this week left no doubt that they share a singular passion to find
    out what cause … more


  • ljk July 2, 2008, 21:43

    The Tunguska Mystery–100 Years Later

    Finding a piece of the elusive cosmic body that devastated a Siberian forest a century ago could help save Earth in the centuries to come

    By Luca Gasperini, Enrico Bonatti and Giuseppe Longo

    Giant fireball in the sky was the first indication that an unknown celestial object had exploded over Siberia. In this artist’s conception, Semen Semenov, who witnessed the blast at a distant trading post, starts to feel the heat.

    James Porto

    Editor’s Note: This story was originally printed in the June 2008 issue of Scientific American.

    June 30, 1908, 7:14 a.m., central Siberia—Semen Semenov, a local farmer, saw “the sky split in two. Fire appeared high and wide over the forest…. From … where the fire was, came strong heat…. Then the sky shut closed, and a strong thump sounded, and I was thrown a few yards…. After that such noise came, as if . . . cannons were firing, the earth shook …”

    Such is the harrowing testimony of one of the closest eyewitnesses to what scientists call the Tunguska event, the largest impact of a cosmic body to occur on the earth during modern human history. Semenov experienced a raging conflagration some 65 kilometers (40 miles) from ground zero, but the effects of the blast rippled out far into northern Europe and Central Asia as well. Some people saw massive, silvery clouds and brilliant, colored sunsets on the horizon, whereas others witnessed luminescent skies at night—Londoners, for instance, could plainly read newsprint at midnight without artificial lights. Geophysical observatories placed the source of the anomalous seismic and pressure waves they had recorded in a remote section of Siberia. The epicenter lay close to the river Podkamennaya Tunguska, an uninhabited area of swampy taiga forest that stays frozen for eight or nine months of the year.

    Full article here:


  • ljk July 7, 2008, 11:03

    Maybe Chicken Little Wasn’t Paranoid After All

    New York Times July 6, 2008

    NASA’s Jet Propulsion Laboratory
    has estimated that a Tunguska-size
    asteroid (just 90 feet across,
    leveling some 800 square miles of
    forest in Siberia) will enter
    Earth’s atmosphere once every 300
    years and says there may be 375,000
    objects of such size out there. NASA
    only tracks potential “civilization
    killers” of 1 kilometer (.62 miles)


  • ljk March 19, 2009, 19:05

    To continue our commemoration of the life and work of Arthur C Clarke I would like to offer for your viewing pleasure the following episode from Arthur C Clarke’s Mysterious World – The Great Siberian Explosion.

    This particular episode concerns the Tunguska impact event of June 30th, 1908 and first aired on British television on October 14th, 1980 and has to rank as one of my all time favourites along side Carl Sagan’s Cosmos Episode 4: “Heaven and Hell”.


  • ljk March 19, 2009, 23:30

    Tunguska-1908 and similar events in light of the New Explosive Cosmogony of minor bodies

    Authors: Edward M. Drobyshevski

    (Submitted on 19 Mar 2009)

    Abstract: The well-known Tunguska-1908 phenomenon (TP) problems (the fast transfer of the kinetic energy of the meteoroid W~10-50 Mt TNT to air, with its heating to T>10^4 K at an altitude of 5-10 km, the final turn of the smoothly sloping, ~0-20^o to horizon, trajectory of the body through ~10^o to the West, the pattern and area of the tree-fall and trees’ scorching by heat radiation, etc.) allow a simple solution within the New Explosive Cosmogony (NEC) of minor bodies, as opposed to other approaches.

    The NEC considers the short-period (SP) comet nuclei, to which the Tunguska body belonged, to be fragments produced in explosions of massive icy envelopes of Ganymede-type bodies saturated by products of bulk electrolysis of ices to the form of a 2H2+O2 solid solution.

    The nearly tangent entry into the Earth’s atmosphere with V~20 km/s of such a nucleus, ~200-500 m in size and ~(5-50)x10^12 g in mass, also saturated by 2H2+O2, initiated detonation of its part of ~10^12 g at an altitude of 5-10 km. This resulted in deflection of this fraction trajectory by 5^o-10^o, and fast expansion with ~2 km/s of its detonation products brought about their fast slowing down by the air, heating of the latter to T>10^4 K and a phenomenon of high-altitude explosion.

    On crossing the Earth’s atmosphere, the main part of the unexploded nucleus escaped into space, and this body moving presently in an SP orbit should eventually be identified in time.

    Its impact with W~250-3000 Mt TNT on the Earth’s surface (which could occur in 1908) would have produced a crater up to ~3.5-8 km in size, with an ejection of dust that would have brought about a climatic catastrophe.

    The processes involved in the TP are resembling those accompanying falling P/Shoemaker-Levy 9 onto Jupiter and, possibly, the impact-caused Younger Dryas cooling ~13 ka ago.

    Comments: 18 pages

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0903.3309v1 [astro-ph.EP]

    Submission history

    From: Edward M. Drobyshevski [view email]

    [v1] Thu, 19 Mar 2009 12:28:09 GMT (256kb)


  • ljk March 19, 2009, 23:41

    A search for a present-day candidate for the Comet P/Tunguska-1908

    Authors: E.M.Drobyshevski, T.Yu.Galushina, M.E.Drobyshevski

    (Submitted on 19 Mar 2009)

    Abstract: The reason for the horizontal turn of the Tunguska-1908 bolide trajectory remains difficult to understand. It finds explanation, however, in the New Explosive Cosmogony of minor bodies as having been caused by an explosion of a part (M up to 10^12 g) of the comet nucleus whose ices contained products of its electrolysis, 2H2+O2.

    In detonation, this part was repelled from the more massive unexploded nucleus remnant, changed the direction of its own motion by ~10^o and imparted its kinetic energy, in expanding and slowing down, to the air in producing an effect of a high-altitude explosion.

    Because there are no traces of a fall of the more massive remnant, one comes to the conclusion that on passing through the Earth’s atmosphere it again entered a heliocentric orbit (the hypothesis of V.Vernadskiy, 1932).

    A search for this comet, P/Tunguska-1908, among the 6077 known NEAs shows the 2005NB56 object to be the most appropriate candidate for a number of its parameters (a size is ~ 170 m, P = 2.106 y, e = 0.473 and i = 6.8^o).

    Back integration of its orbit made without allowing for non-gravitational effects suggests that it had passed the Earth on June 27, 1908 at a distance of 0.06945 AU. It is quite possible that a proper inclusion of even fairly weak non-gravitational forces would make its orbit fit in parameters that of the Tunguska bolide.

    Comments: 8 pages

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0903.3313v1 [astro-ph.EP]

    Submission history

    From: Edward M. Drobyshevski [view email]

    [v1] Thu, 19 Mar 2009 16:15:01 GMT (204kb)


  • ljk June 24, 2009, 14:48

    Space shuttle science shows how 1908 Tunguska explosion was caused by a comet

    Public release date: 24-Jun-2009

    The mysterious 1908 Tunguska explosion that leveled 830 square miles of Siberian forest was almost certainly caused by a comet entering the Earth’s atmosphere, says new Cornell University research. The conclusion is supported by an unlikely source: the exhaust plume from the NASA space shuttle launched a century later.

    The research, accepted for publication (June 24, 2009) by the journal Geophysical Research Letters, published by the American Geophysical Union, connects the two events by what followed each about a day later: brilliant, night-visible clouds, or noctilucent clouds, that are made up of ice particles and only form at very high altitudes and in extremely cold temperatures.

    “It’s almost like putting together a 100-year-old murder mystery,” said Michael Kelley, the James A. Friend Family Distinguished Professor of Engineering at Cornell who led the research team. “The evidence is pretty strong that the Earth was hit by a comet in 1908.” Previous speculation had ranged from comets to meteors.

    The researchers contend that the massive amount of water vapor spewed into the atmosphere by the comet’s icy nucleus was caught up in swirling eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the noctilucent clouds formed a day later many thousands of miles away.

    Noctilucent clouds are the Earth’s highest clouds, forming naturally in the mesosphere at about 55 miles over the polar regions during the summer months when the mesosphere is around minus 180 degrees Fahrenheit (minus 117 degrees Celsius).

    The space shuttle exhaust plume, the researchers say, resembled the comet’s action.

    Full article here:


  • ljk September 28, 2010, 23:37

    Australian Aboriginal Geomythology: Eyewitness Accounts of Cosmic Impacts?

    Authors: Duane W. Hamacher, Ray P. Norris

    (Submitted on 22 Sep 2010)

    Abstract: Descriptions of cosmic impacts and meteorite falls are found throughout Australian Aboriginal oral traditions. In some cases, these texts describe the impact event in detail, sometimes citing the location, suggesting that the events were witnessed.

    We explore whether cosmic impacts and meteorite falls may have been witnessed by Aboriginal Australians and incorporated into their oral traditions. We discuss the complications and bias in recording and analysing oral texts but suggest that these texts may be used both to locate new impact structures or meteorites and model observed impact events.

    We find that, while detailed Aboriginal descriptions of cosmic impacts are abundant in the literature, there is currently no physical evidence connecting these accounts to impact events currently known to Western science.

    Comments: Submitted to “Archaeoastronomy: The Journal of Astronomy in Culture”, 51 pages, 6 Figures, 6 Tables

    Subjects: History of Physics (physics.hist-ph); Popular Physics (physics.pop-ph)

    Cite as: arXiv:1009.4251v1 [physics.hist-ph]

    Submission history

    From: Duane Hamacher [view email]

    [v1] Wed, 22 Sep 2010 02:47:39 GMT (1831kb)