100 Years Since Tunguska

Monday, June 30 marks the 100th anniversary of the Tunguska incident in 1908, in which a meteor or comet fragment entered the atmosphere over Tunguska in Siberia producing an enormous explosion.

“We know that a rather massive body flew into the atmosphere of our planet,” said Boris Shustov of the Russian Academy of Sciences.

“It measured 40 to 60 meters in diameter. Clearly, it did not consist of iron, otherwise it would have certainly reached the earth. The body decelerated in the atmosphere, the deceleration being very abrupt, so the whole energy of this body flying with a velocity of more than 20 meters per second [probably should be: kilometers per second] was released, which resulted in a mid-air explosion, very similar to a thermonuclear blast,” he told Tass news agency yesterday.

“The yield of the explosion totaled 10 to 15 megatons, which matches the yields of the largest hydrogen bomb ever tested on the planet [actually, the largest reported test in October 1961 had a yield in excess of 50 megatons]. The explosion felled some 80 million trees [but] it is generally assumed that the blast did not kill any people,” he added.

“The Tunguska phenomenon showed that the asteroid-comet danger is quite real. It happened not in the era of dinosaurs, but in our recent history. Russia was definitely lucky; had the body flown up to the Earth several hours later, it would have hit St.Petersburg. The consequences would have been horrendous,” he said.

“Impacts such as the Tunguska incident are thought to occur about once in one hundred years based on the density of impact craters on the Moon,” according to a White Paper on Planetary Defense attached to the 1994 U.S. Air Force report Spacecast 2020.

A 2007 NASA summary report to Congress on planetary defense is here (pdf). A longer account is here (pdf).

No Responses to “100 Years Since Tunguska”

  1. jhm June 28, 2008 at 8:06 AM #

    The email I received from JPL this morning contradicts some of the items here:

    It is estimated the asteroid entered Earth’s atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs.

    [...]

    Yeomans and his colleagues at JPL’s Near-Earth Object Office are tasked with plotting the orbits of present-day comets and asteroids that cross Earth’s path, and could be potentially hazardous to our planet.

    Yeomans estimates that, on average, a Tunguska-sized asteroid will enter Earth’s atmosphere once every 300 years.

    Little boy , according to Wikipedia, yielded 13 to 16 kilotons, making this less than three megatons, no?

  2. Steven Aftergood June 28, 2008 at 8:18 AM #

    Thanks for this. All of these numbers are estimates, but several of the numbers in the Tass news report are wrong (as indicated above), and the JPL estimates are presumptively more reliable.

  3. Henry M July 2, 2008 at 8:45 AM #

    If explosions like Tunguska “occur about once in one hundred years,” then why have no others been recorded?

    Just asking.

  4. Steven Aftergood July 2, 2008 at 9:03 AM #

    The 1994 Air Force estimate of a Tunguska-like “impact” once every hundred years now appears to be overstated, based on surveys of near-Earth objects that have been performed since that time.

    According to Alan Harris of the Space Science Institute writing in Nature (26 June 2008, p. 1178), “an object that might cause a Tunguska-like event — roughly 50 meters in diameter — should collide with Earth only about every 1,500 years.”

    On the other hand, writes astronomer Duncan Steel in the same issue of Nature (p. 1159), “50 meter objects are too small to spot far in advance of their impact. So although another Tunguska coming out of the blue is not a likely event in any given June, it is not out of the question.”

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