Tunguska

In Brief:

At 7:17 AM on the morning of June 30, 1908, a mysterious explosion occurred in the skies over Siberia. It was caused by the impact and breakup of a large meteorite, at an altitude roughly six kilometers in the atmosphere. Realistic pictures of the even t are unavailable. However, Russian scientists collected eyewitness accounts of the event. I believe that we now know enough about large impacts to “decode” the subjective descriptions of the witnesses and create realistic views of this historic aster oid impact as seen from different distances.

What do we know about the explosion?

You can get a sense of the magnitude of this event by comparing observations made at different distances. Seismic vibrations were recorded by sensitive instruments as much as 1000 km (600 mi) away. At 500 km (300 mi), observers reported “deafening bangs” and a fiery cloud on the horizon. About 170 km (110 mi) from the explosion, the object was seen in the cloudless, daytime sky as a brilliant, sunlike fireball; thunderous noises were heard. At distances around 60 km, people were thrown to the ground or even knocked unconscious; windows were broken and crockery knocked off shelves. Probably the closest observers were some reindeer herdersasleep in their tents in several camps about 30 km (20 mi) from the site. They were blown into the air and knocked unconscious; one man blown into a tree later died. “Everything around was shrouded in smoke and fog from the burning fallen trees.”

My Paintings of the Event

A few years ago, I decided to use eyewitness reports such as the following ones, collected by Russian scientists decades ago, to reconstruct the appearance of the event from various locations, and at various moments. I project a series of four or five p aintings, but so far I have completed only three. Here are descriptions of my work so far.

400 KM Southeast of Ground Zero

View from Kirensk, two seconds before the explosion.Painting copyright William K. Hartmann

Witnesses in the town of Kirensk and nearby towns at the same distance recollected the fireball flashing across the sky in the following terms:

“A ball of fire…coming down obliquely. A few minutes later [we heard] separate deafening crash like peals of thunder…followed by eight loud bangs like gunshots.”

“A ball of fire appeared in the sky… As it approached the ground, it took on a flattened shape…”

“A flying star with a fiery tail; its tail disappeared into the air.”

After this object passed across the sky, it approached the horizon where it was consistently described from this distance of 400 km, as appearing like a “pillar of fire,” then replaced by “a cloud of smoke rising from the ground,” or “a cloud of ash…o n the horizon,” or “a huge cloud of black smoke.”From a closer distance of around 200 km, several witnesses gave a better description of the object itself. It was called diffuse bright ball two or three times larger than the sun but not as bright; the t rail was a “fiery-white band.” Inconsistent colors were mentioned: white, red, flame-like, bluish-white. Perhaps it had a flame-like iridescence. I used these descriptions in this painting, but I compensated for the twice-greater distance. I used a visit to Washington state as an opportunity to find a landscape that generally matched the photos from Siberia, and then I painted this piney-woodland scene from life, adding the fireball from the above descriptions.

60 KM South of Ground Zero

View from Vanavara trading post, at the moment of the explosion.Painting copyright William K. Hartmann

The Russians collected a number of accounts from eyewitnesses at the trading station, which was probably the closest permanent habitation. These included:

“I was sitting on the porch of the house at the trading station, looking north. Suddenly in the north…the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire. I felt a great heat, as if my shir t had caught fire… At that moment there was a bang in the sky, and a mighty crash… I was thrown twenty feet from the porch and lost consciousness for a moment…. The crash was followed by a noise like stones falling from the sky, or guns firing. The earth trembled…. At the moment when the sky opened, a hot wind, as if from a cannon, blew past the huts from the north. It damaged the onion plants. Later, we found that many panes in the windows had been blown out and the iron hasp in the barn door had been broken.”

A second witness said:

“I saw the sky in the north open to the ground and fire poured out. The fire was brighter than the sun. We were terrified, but the sky closed again and immediately afterward, bangs like gunshots were heard. We thought stones were falling… I ran with my head down and covered, because I was afraid stones my fall on it.”

In this painting I tried to show the moment when “the sky opened and with fire.” I used the more distant reports, of the fire ball spreading and flattening at the end of its trajectory, to give the shape of fiery trail and the explosive fireball. I pa inted the basic landscape from life in a Siberian-looking landscape outside of Flagstaff Arizona, basing the structures on old photos from the expeditions to the Siberian impact area.

15 KM from Ground Zero

A few minutes after the explosionPainting copyright William K. Hartmann

Because the object exploded up in the atmosphere, instead of hitting the ground, it left no crater. The effect on the ground was limited to devastation of a large forest areas. At ground zero, tree branches were stripped, leaving trunks standing up. Bu t at distances from roughly 3 out to 10 miles, the trees were blown over, lying with tops pointed away from the blasts. No one was known to have been this close to the blast. The closest humans were probably herders camped in tents roughly 30 km from gro und zero. They related:

“Early in the morning when everyone was asleep in the tent, it was blown up in the air along with its occupants. Some lost consciousness. When they regained consciousness, they heard a great deal of noise and saw the forest burning around them, much of it devastated.”

“The ground shook and incredibly prolonged roaring was heard. Everything round about was shrouded in smoke and fog from burning, falling trees. Eventually the noise died away and the wind dropped, but the forest went on burning. Many reindeer rushed aw ay and were lost.”

One older man at about this distance was reportedly blown about forty feet into a tree, causing a compound fracture of his arm, and he soon died. Hundreds of the herders’ reindeer, in the general area around ground zero, were killed. Many campsites and storage huts scattered in the area were destroyed. During a workshop of the International Association for the Astronomical Arts, I painted this view at Mt. St. Helens, Oregon, where the devastated area bears an uncanny resemblance to the photos of the e xplosion site. At both the Siberian site and Mt. St. Helens are vistas where one sees nothing but felled trees, mile after mile, across distant hillsides. The transient heat flash from the fireball was felt by the witnesses at Vanavara, and apparentl y within about 30 km it was strong enough to ignite small temporary fires in the forest and singe tree bark. I based the view of the cloud in the sky on the distant reports of a ashy-colored cloud of smoke forming at the site of the blast; it was proba bly augmented some minutes later by smoke from the burning forest. Streamers of smoke from fragmented material would soon dissipate in air currents.

What was the explosion?

Because the meteorite did not strike the ground or make a crater, early researchers thought the object might be a weak, icy fragment of a comet, which vaporized explosively in the air, and left no residue on the ground. However, modern planetary scienti sts have much better tools for understanding meteorite explosion in the atmosphere. As a meteorite slams into the atmosphere at speeds around 12 to 20 km/sec or more, itexperiences a strong mechanical shock, like a diver bellyflopping into water. This can break apart stones of a certain size range, which explode instead of hitting the ground. Some of them drop brick-sized fragments on the ground, but others, such as the one that hit Siberia, may produce primarily a fireball and cloud of fine dust and tiny fragments. In 1993 researchers Chris Chyba, Paul Thomas, and Kevin Zahnle studied the Siberian explosion and concluded it was of this type — a stone meteorite that exploded in the atmosphere. This conclusion was supported when Russian research ers found tiny stoney particles embedded in the trees at the collision site, matching the composition of common stone meteorites. The original asteroid fragment may have been roughly 50-60 meters (50-60 yards) in diameter.

If asteroids hit Earth, why don’t we see more such explosions?

Many asteroidal fragments circle the Sun; the Siberian object was merely the largest to hit the Earth in the last century or so. Had it hit a populated area, devastation would have been enormous. If there are many asteroid fragments, why don’t we see mo re hits? We do! The problem is that they have not been understood until recently. Current studies reveal that such explosions may happen every couple of centuries; however, six out of seven happen over the ocean, and few happen over populated land. A key to the phenomenon is: the larger the impact the rarer it is. An Air Force satellite in the 1990s detected a smaller explosion over the Pacific. In 1972, a 1000-ton object skimmed tangentially through Earth’s atmosphere over the Grand Tetons in Wy oming, and then skipped back out into space, like a stone skipping off water. It was photographed by tourists and detected by Air Force satellites. Had it continued on into the atmosphere, it could have caused a Hiroshima-scale explosion over Canada, so mewhat smaller than the Siberian blast. Even larger objects have hit Earth, but they are more rare. For example, an iron asteroid fragment perhaps 100 m across hit Arizona about 20,000 years ago, leaving the kilometer-wide “Arizona Meteor Crater,” which i s open to visitors; and a 10-km asteroid hit Earth 65 million years ago, ending the reign of dinosaurs. Brick-sized interplanetary stones fall from the sky in various locations every year. Several houses and a car have been hit in recent decades. Tiny dust grains are even more common; they can be seen every night if you watch long enough; they are the bright streaks of light sometimes called “shooting stars.” Interplanetary space contains many small bodies of different sizes. All of them move in ell iptical orbits around the sun as prescribed by Kepler. Occasionally their orbits intersect those of planets, leading to a collision. Large enough bodies leave sizable planets or satellites. This explains why impact craters are present on surfaces of p lanets and moons throughout the solar system. If we continue to study asteroids and build more telescopes for detecting and tracking them, we will have better information about the frequency of such asteroid impact-explosions, and more chance to have warn ing about impending impacts.

For further information:

Chyba, C., P. Thomas, and K. Zahnle 1993. “The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid”. Nature 361, p. 40-44. (Calculation of size of the bolide.)

Gallant, Roy A. 1994 “Journey to Tunguska”. Sky and Telescope, June, 38-43. (Description of a modern journey to the site, with photographs.)

Krinov, E. L. 1966 Giant Meteorites (London: Pergamon Press). (Description of the site and interviews with witnesses).