Early on the morning of June 30, 1908, a massive explosion
shook central Siberia. Witnesses told of a
fireball that streaked in from the southeast and then detonated in the sky
above the desolate, forested region. At the nearest trading post, about 70
kilometers away from the blast, people were reportedly knocked from their feet.
Seismic instruments in the area registered ground motions equivalent to those
of a magnitude-5 earthquake.
Effects of the event—often called the Tunguska blast, after
a major river running through the area—weren’t restricted to Siberia.
Sensitive barometers in England
detected an atmospheric shock wave as it raced westward and then detected it
again after it traveled around the world. High-altitude clouds that formed over
the region after the event were so lofty that they caught light from beyond the
horizon, illuminating the sky so much that people at locales in Europe and Asia could read newspapers outdoors at midnight.
A number of factors—including the site’s remote location,
World War I and the Russian Revolution—prevented scientists from mounting an
expedition to the blast zone for almost two decades, says physicist Giuseppe
Longo of the University of Bologna in Italy. When researchers eventually
reached the region, they found that a 2,150-square-kilometer patch of forest
had been flattened, with most of the 80 million trees lying in a radial
pattern. What the researchers didn’t find, however, was an obvious crater.
A century later, scientists are still debating the cause of
the Tunguska blast. Through the years, Longo
notes, a variety of scenarios have been proposed, many of them involving the
explosion of an unusual extraterrestrial object—everything from a small black
hole or a chunk of antimatter to a UFO. Most researchers, however, now pin the
blame on the mid-air explosion of a small comet or asteroid, which typically
can’t stand up to the pummeling received while blazing through the atmosphere (SN: 7/19/03, p. 36).
The damage in Siberia suggests that the Tunguska detonation
happened at an altitude of between 6 and 8 kilometers and released the energy
of about 15 megatons of TNT, about a thousand times more than the bomb that
Data gathered by military satellites—including those
designed to detect clandestine nuclear explosions—suggest that tiny versions of
the Tunguska blast occur rather frequently,
says Philip A. Bland, a planetary scientist at Imperial College London. The
largest airburst detected during the 1990s measured only a few tens of
kilotons, the energy release expected from the explosion of an asteroid
measuring about 7 or 8 meters across. Impacts of objects measuring at least 1
meter in diameter occur, on average, about once a week, the data suggest.
Tunguska-sized airbursts would be expected to happen about once every 500
years, says Bland.
Previous studies have estimated that the Tunguska asteroid
was between 50 and 80 meters in diameter, says Mark Boslough, a physicist at
Sandia National Laboratories in Albuquerque,
N.M. However, new supercomputer
simulations by Boslough and his colleagues hint that a much smaller object
could have produced the damage. New models account for the downward momentum of
the air compressed beneath the incoming object, a component of the process
“that previously had been thrown away,” says Boslough.
So the Tunguska asteroid
may have been only 30 to 50 meters across, he notes. Simulations suggest that
it entered the atmosphere traveling about 15 kilometers per second at an angle
about 35 degrees above the horizon. The shock wave produced by the airburst
could have slammed into the ground at 180 kilometers per hour, a gust with the
wind speed of a category-3 hurricane, the team reports in an upcoming issue of
the International Journal of Impact
Late last year, Longo and his colleagues reported that LakeCheko,
a 400-meter–wide lake about eight kilometers northwest of the Tunguska
blast’s epicenter, could be the long-sought crater produced by a chunk of
asteroid that actually reached the ground. The lake is about 50 meters deep,
has a cone-shaped bottom unlike other lakes in the region and—possibly most
important—lies directly along the estimated path of the fireball. Sonar studies
reveal a buried object or a densely compacted layer of sediment about 10 meters
below the center of the lake bottom, the researchers reported in the August
2007 Terra Nova.
But other factors suggest that LakeCheko
isn’t a water-filled impact crater, says Gareth Collins of Imperial College
London. For one thing, he notes, a hole the size of this lake typically would
be one in a series of holes excavated by pieces of the disintegrating object,
whereas LakeCheko apparently has no companions. And
the area around the lake isn’t covered with a layer of material that would have
been thrown out of a crater during the impact. Also, pictures of the lake from
an aerial survey in 1938 show mature trees (more than 30 years old) on the
lakeshore—a sure sign that this body of water has a more benign provenance,
Collins and colleagues write in the April Terra
The lack of an impact crater, along with the dearth of
geochemical anomalies in rock in the region, has spurred some scientists to
seek an alternate explanation for the blast. One favorite down-to-Earth idea
points to the modern-day formation of a kimberlite deposit, an eruption that
brings diamonds to Earth’s surface (SN:
6/30/07, p. 412). Such an eruption could have injected about 10 million
tons of methane into the atmosphere, a plume that if detonated would have
released a forest-flattening burst of energy.
Scientists will attend conferences in Moscow at the end of June to commemorate the
blast and discuss the latest findings. Those proposing an extraterrestrial
cause will be meeting at the RussianAcademy of Sciences.
Those who favor an Earth-based origin of the blast will gather across town, at
Explanations for the Tunguska event have
spanned science and fiction
1925-A Russian scientist says the explosion’s recorded seismic
and air waves suggest a meteorite as the culprit.
1934-A British scientist and a Russian scientist say the blast
must have been an in-air comet explosion.
1941-An American researcher claims an antimatter meteoroid was
annihilated when it encountered matter.
1946-Russian sci-fi writer Alexander Kazantsev tells the tale of Tunguska as a UFO that vaporized.
1973-Two theorists suggest a black hole as small as a comma
passed through Earth and exited via the North Atlantic.
2001-Ideas go down: The blast perhaps was a kimberlite eruption,
which lifts diamonds and methane from Earth’s depths.