The Tunguska event was a large explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of 30 June 1908 (NS). The explosion over the sparsely populated Eastern Siberian Taiga flattened an estimated 80 million trees over an area of 2,150 km2 (830 sq mi) of forest, and eyewitness reports suggest that at least three people may have died in the event. The explosion is generally attributed to the air burst of a stony meteoroid about 100 metres (330 feet) in size.:p. 178 It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 5 to 10 kilometres (3 to 6 miles) rather than to have hit the surface of the Earth.
Location of the event in Siberia (modern map)
|Event||Explosion in forest area (10–15 megatons of TNT)|
|Time||30 June 1908|
|Place||Podkamennaya Tunguska River in Siberia, Russia|
|Effects||Flattening 2,000 km2 (770 sq mi) of forest|
|Damage||Devastation to local plants and animals|
A few damaged buildings
|Deaths||0 confirmed, 3 possible|
|Cause||Probable air burst of small asteroid or comet|
Due to the remoteness of the site and the limited instrumentation available at the time of the event, modern scientific interpretations of its cause and magnitude have relied chiefly on damage assessments and geological studies conducted many years after the fact. Studies have yielded different estimates of the meteoroid's size, on the order of 50 to 190 metres (160 to 620 feet), depending on whether the body entered with a low or high speed. It is estimated that the shock wave from the air burst would have measured 5.0 on the Richter magnitude scale, and estimates of its energy have ranged from 3–30 megatons of TNT (13–126 petajoules). An explosion of this magnitude would be capable of destroying a large metropolitan area. Since the 1908 event, there have been an estimated 1,000 scholarly papers (most in Russian) published about the Tunguska explosion. In 2013, a team of researchers published the results of an analysis of micro-samples from a peat bog near the center of the affected area which show fragments that may be of meteoritic origin.
The Tunguska event is the largest impact event on Earth in recorded history, though much larger impacts have occurred in prehistoric times. It has been mentioned numerous times in popular culture, and has also inspired real-world discussion of asteroid impact avoidance.
On 30 June 1908 (cited in Russia as 17 June 1908, Julian Calendar, prior to implementation of the Soviet calendar in 1918), at around 07:17 local time, Evenki natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About ten minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported that the source of the sound moved from the east to the north of them. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of kilometres away.
The explosion registered at seismic stations across Eurasia, and air waves from the blast were detected in Germany, Denmark, Croatia, the United Kingdom, and as far away as Batavia, Dutch East Indies and Washington, D.C. It is estimated that, in some places, the resulting shock wave was equivalent to an earthquake measuring 5.0 on the Richter magnitude scale. Over the next few days, night skies in Asia and Europe were aglow, with contemporaneous reports of photographs being successfully taken at midnight in Sweden and Scotland. It has been theorized that this effect was due to light passing through high-altitude ice particles that had formed at extremely low temperatures—a phenomenon that many years later was reproduced by space shuttles. In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.
Selected eyewitness reportsEdit
Though the region of Siberia in which the explosion occurred was extremely sparsely populated in 1908, accounts of the event from eyewitnesses who were in the surrounding area at the time do exist. Regional newspapers also reported the event shortly after it occurred.
At breakfast time I was sitting by the house at Vanavara Trading Post [approximately 65 kilometres (40 mi) south of the explosion], facing north. […] I suddenly saw that directly to the north, over Onkoul's Tunguska Road, the sky split in two and fire appeared high and wide over the forest [as Semenov showed, about 50 degrees up—expedition note]. The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn't bear it as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the Earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn, a part of the iron lock snapped.
Testimony of Chuchan of Shanyagir tribe, as recorded by I. M. Suslov in 1926:
We had a hut by the river with my brother Chekaren. We were sleeping. Suddenly we both woke up at the same time. Somebody shoved us. We heard whistling and felt strong wind. Chekaren said, 'Can you hear all those birds flying overhead?' We were both in the hut, couldn't see what was going on outside. Suddenly, I got shoved again, this time so hard I fell into the fire. I got scared. Chekaren got scared too. We started crying out for father, mother, brother, but no one answered. There was noise beyond the hut, we could hear trees falling down. Chekaren and I got out of our sleeping bags and wanted to run out, but then the thunder struck. This was the first thunder. The Earth began to move and rock, the wind hit our hut and knocked it over. My body was pushed down by sticks, but my head was in the clear. Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them. It was like what the Russians call lightning. And immediately there was a loud thunderclap. This was the second thunder. The morning was sunny, there were no clouds, our Sun was shining brightly as usual, and suddenly there came a second one!
Chekaren and I had some difficulty getting out from under the remains of our hut. Then we saw that above, but in a different place, there was another flash, and loud thunder came. This was the third thunder strike. Wind came again, knocked us off our feet, struck the fallen trees.
We looked at the fallen trees, watched the tree tops get snapped off, watched the fires. Suddenly Chekaren yelled "Look up" and pointed with his hand. I looked there and saw another flash, and it made another thunder. But the noise was less than before. This was the fourth strike, like normal thunder.
Now I remember well there was also one more thunder strike, but it was small, and somewhere far away, where the Sun goes to sleep.
Sibir newspaper, 2 July 1908:
On the morning of 17th of June, around 9:00, we observed an unusual natural occurrence. In the north Karelinski village [200 verst (213 km (132 mi)) north of Kirensk] the peasants saw to the northwest, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a "pipe", i.e., a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world. The author of these lines was meantime in the forest about 6 versts [6.4 km] north of Kirensk and heard to the north east some kind of artillery barrage, that repeated in intervals of 15 minutes at least 10 times. In Kirensk in a few buildings in the walls facing north-east window glass shook.
Siberian Life newspaper, 27 July 1908:
When the meteorite fell, strong tremors in the ground were observed, and near the Lovat village of the Kansk uezd two strong explosions were heard, as if from large-calibre artillery.
Krasnoyaretz newspaper, 13 July 1908:
Kezhemskoe village. On the 17th an unusual atmospheric event was observed. At 7:43 the noise akin to a strong wind was heard. Immediately afterward a horrific thump sounded, followed by an earthquake that literally shook the buildings as if they were hit by a large log or a heavy rock. The first thump was followed by a second, and then a third. Then the interval between the first and the third thumps was accompanied by an unusual underground rattle, similar to a railway upon which dozens of trains are travelling at the same time. Afterward, for 5 to 6 minutes an exact likeness of artillery fire was heard: 50 to 60 salvoes in short, equal intervals, which got progressively weaker. After 1.5–2 minutes after one of the "barrages" six more thumps were heard, like cannon firing, but individual, loud and accompanied by tremors. The sky, at the first sight, appeared to be clear. There was no wind and no clouds. Upon closer inspection to the north, i.e. where most of the thumps were heard, a kind of an ashen cloud was seen near the horizon, which kept getting smaller and more transparent and possibly by around 2–3 p.m. completely disappeared.
It was not until more than a decade after the event that any scientific analysis of the region took place, in part due to the isolation of the area. In 1921, the Russian mineralogist Leonid Kulik led a team to the Podkamennaya Tunguska River basin to conduct a survey for the Soviet Academy of Sciences. Although they never visited the central blast area, the many local accounts of the event led Kulik to believe that the explosion had been caused by a giant meteorite impact. Upon returning, he persuaded the Soviet government to fund an expedition to the suspected impact zone, based on the prospect of salvaging meteoric iron.
Kulik led a scientific expedition to the Tunguska blast site in 1927. He hired local Evenki hunters to guide his team to the centre of the blast area, where they expected to find an impact crater. To their surprise, there was no crater to be found at ground zero. Instead they found a zone, roughly 8 kilometres (5.0 mi) across, where the trees were scorched and devoid of branches, but still standing upright. Trees more distant from the center had been partly scorched and knocked down in a direction away from the center, creating a large radial pattern of downed trees.
In the 1960s, it was established that the zone of leveled forest occupied an area of 2,150 km2 (830 sq mi), its shape resembling a gigantic spread-eagled butterfly with a "wingspan" of 70 km (43 mi) and a "body length" of 55 km (34 mi). Upon closer examination, Kulik located holes that he erroneously concluded were meteorite holes; he did not have the means at that time to excavate the holes.
During the following 10 years, there were three more expeditions to the area. Kulik found several dozens of little "pothole" bogs, each 10 to 50 metres (33 to 164 feet) in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs (the so-called "Suslov's crater", 32 m (105 ft) in diameter), he found an old tree stump on the bottom, ruling out the possibility that it was a meteoric crater. In 1938, Kulik arranged for an aerial photographic survey of the area covering the central part of the leveled forest (250 square kilometres (97 sq mi)). The original negatives of these aerial photographs (1,500 negatives, each 18 by 18 centimetres (7.1 by 7.1 inches)) were burned in 1975 by order of Yevgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences, as part of an initiative to dispose of flammable nitrate film. Positive prints were preserved for further study in the Russian city of Tomsk.
Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteoroid air burst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.
Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of carbon, hydrogen, and nitrogen at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the Cretaceous–Paleogene boundary. These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion.
Researcher John Anfinogenov has suggested that a boulder found at the event site, known as John's stone, is a remnant of the meteorite, but oxygen isotope analysis of the quartzite suggests that it is of hydrothermal origin, and probably related to Permian-Triassic Siberian Traps magmatism.
Earth impactor modelEdit
Meteoroids enter Earth's atmosphere from outer space every day, travelling at a speed of at least 11 km/s (7 mi/s). The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Early estimates of the energy of the Tunguska air burst ranged from 10–15 megatons of TNT (42–63 petajoules) to 30 megatons of TNT (130 PJ), depending on the exact height of the burst as estimated when the scaling laws from the effects of nuclear weapons are employed. More recent calculations that include the effect of the object's momentum find that more of the energy was focused downward than would be the case from a nuclear explosion and estimate that the air burst had an energy range from 3 to 5 megatons of TNT (13 to 21 PJ). The 15-megaton (Mt) estimate represents an energy about 1,000 times greater than that of Hiroshima bomb, and roughly equal to that of the United States' Castle Bravo nuclear test in 1954 (which measured 15.2 Mt) and one-third that of the Soviet Union's Tsar Bomba test in 1961. A 2019 paper suggests the explosive power of the Tunguska event may have been around 20–30 megatons.
Since the second half of the 20th century, close monitoring of Earth's atmosphere through infrasound and satellite observation has shown that asteroid air bursts with energies comparable to those of nuclear weapons routinely occur, although Tunguska-sized events, on the order of 5–15 megatons, are much rarer. Eugene Shoemaker estimated that 20-kiloton events occur annually and that Tunguska-sized events occur about once every 300 years. More recent estimates place Tunguska-sized events at about once every thousand years, with 5-kiloton air bursts averaging about once per year. Most of these air bursts are thought to be caused by asteroid impactors, as opposed to mechanically weaker cometary materials, based on their typical penetration depths into the Earth's atmosphere. The largest asteroid air burst to be observed with modern instrumentation was the 500-kiloton Chelyabinsk meteor in 2013, which shattered windows and produced meteorites.
Glancing impact hypothesisEdit
In 2020 a group of Russian scientists used a range of computer models to calculate the passage of asteroids with diameters of 200, 100 and 50 metres at oblique angles across Earth's atmosphere. They used a range of assumptions about the object's composition as if it was made of iron, rock or ice. The model which most closely matched the observed event was an iron asteroid up to 200 metres in diameter, travelling at 11.2 km per second which glanced off the Earth's atmosphere and returned into solar orbit.
The explosion's effect on the trees near the hypocentre of the explosion was similar to the effects of the conventional Operation Blowdown. These effects are caused by the blast wave produced by large air burst explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, but remain standing upright, while trees farther away are knocked over because the blast wave is travelling closer to horizontal when it reaches them.
Soviet experiments performed in the mid-1960s, with model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.
Asteroid or comet?Edit
In 1930, the British astronomer F. J. W. Whipple suggested that the Tunguska body was a small comet. A comet is composed of dust and volatiles, such as water ice and frozen gases, and could have been completely vaporised by the impact with Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or "skyglows" or "bright nights") observed across Eurasia for several evenings after the impact, which are possibly explained by dust and ice that had been dispersed from the comet's tail across the upper atmosphere. The cometary hypothesis gained a general acceptance among Soviet Tunguska investigators by the 1960s.
In 1978, Slovak astronomer Ľubor Kresák suggested that the body was a fragment of Comet Encke. This is a periodic comet with an extremely short period of just over three years that stays entirely within the orbit of Jupiter. It is also responsible for the Beta Taurids, an annual meteor shower with a maximum activity around 28–29 June. The Tunguska event coincided with the peak activity of that shower, and the approximate trajectory of the Tunguska object is consistent with what would be expected from a fragment of Comet Encke. It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades. During 2019 astronomers searched for hypothesized asteroids ~100 metres in diameter from the Taurid swarm between 5–11 July, and 21 July – 10 August. As of February 2020[update], there have been no reports of discoveries of any such objects.
In 1983, astronomer Zdeněk Sekanina published a paper criticising the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina also argued that the evidence pointed to a dense, rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, et al. released a study calculating the probabilities based on orbital modelling extracted from the atmospheric trajectories of the Tunguska object. They concluded with a probability of 83% that the object moved on an asteroidal path originating from the asteroid belt, rather than on a cometary one (probability of 17%). Proponents of the comet hypothesis have suggested that the object was an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.
The chief difficulty in the asteroid hypothesis is that a stony object should have produced a large crater where it struck the ground, but no such crater has been found. It has been hypothesised that the passage of the asteroid through the atmosphere caused pressures and temperatures to build up to a point where the asteroid abruptly disintegrated in a huge explosion. The destruction would have to have been so complete that no remnants of substantial size survived, and the material scattered into the upper atmosphere during the explosion would have caused the skyglows. Models published in 1993 suggested that the stony body would have been about 60 metres (200 ft) across, with physical properties somewhere between an ordinary chondrite and a carbonaceous chondrite. Typical carbonaceous chondrite substance tends to be dissolved with water rather quickly unless it is frozen.
Christopher Chyba and others have proposed a process whereby a stony meteorite could have exhibited the behaviour of the Tunguska impactor. Their models show that when the forces opposing a body's descent become greater than the cohesive force holding it together, it blows apart, releasing nearly all of its energy at once. The result is no crater, with damage distributed over a fairly wide radius, and all of the damage resulting from the thermal energy released in the blast.
Three-dimensional numerical modelling of the Tunguska impact done by Utyuzhnikov and Rudenko in 2008 supports the comet hypothesis. According to their results, the comet matter dispersed in the atmosphere, while the destruction of the forest was caused by the shock wave.
During the 1990s, Italian researchers, coordinated by the physicist Giuseppe Longo from the University of Bologna, extracted resin from the core of the trees in the area of impact to examine trapped particles that were present during the 1908 event. They found high levels of material commonly found in rocky asteroids and rarely found in comets.
Kelly et al. (2009) contend that the impact was caused by a comet because of the sightings of noctilucent clouds following the impact, a phenomenon caused by massive amounts of water vapor in the upper atmosphere. They compared the noctilucent cloud phenomenon to the exhaust plume from NASA's Endeavour space shuttle. A team of Russian researchers led by Edward Drobyshevski in 2009 have suggested that the near-Earth asteroid 2005 NB56 may be a possible candidate for the parent body of the Tunguska object as the asteroid has made a close approach of 0.06945 AU (27 LD) from Earth on 27 June 1908, three days prior to the Tunguska impact. The team suspected that 2005 NB56's orbit likely fits with the modeled orbit of the Tunguska object, even with the effects of weak non-gravitational forces. In 2013, analysis of fragments from the Tunguska site by a joint US-European team was consistent with an iron meteorite.
The February 2013 Chelyabinsk bolide event provided ample data for scientists to create new models for the Tunguska event. Researchers used data from both Tunguska and Chelyabinsk to perform a statistical study of over 50 million combinations of bolide and entry properties that could produce Tunguska-scale damage when breaking apart or exploding at similar altitudes. Some models focused on combinations of properties which created scenarios with similar effects to the tree fall pattern as well as the atmospheric and seismic pressure waves of Tunguska. Four different computer models produced similar results; they concluded that the likeliest candidate for the Tunguska impactor was a stony body between 50 and 80 m (164 and 262 ft) in diameter, entering the atmosphere at roughly 55,000 km/h (34,000 mph), exploding at 10 to 14 km (6 to 9 mi) altitude, and releasing explosive energy equivalent to between 10 and 30 megatons. This is similar to the blast energy equivalent of the 1980 volcanic eruption of Mount St. Helens. The researchers also concluded impactors of this size only hit the Earth at an average interval scale of millennia.
In June 2007, scientists from the University of Bologna identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded in mid-air, but believe that a 10-metre (33 ft) fragment survived the explosion and struck the ground. Lake Cheko is a small, bowl-shaped lake approximately 8 km (5.0 mi) north-northwest of the hypocentre.
The hypothesis has been disputed by other impact crater specialists. A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres-thick silt deposits at the lake's bed suggests an age of at least 5,000 years, but more recent research suggests that only a metre or so of the sediment layer on the lake bed is "normal lacustrine sedimentation", a depth consistent with an age of about 100 years. Acoustic-echo soundings of the lake floor provide support for the hypothesis that the lake was formed by the Tunguska event. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings indicate a possible metre-sized chunk of rock below the lake's deepest point that may be a fragment of the colliding body. Finally, the lake's long axis points to the hypocentre of the Tunguska explosion, about 7.0 km (4.3 mi) away. Work is still being done at Lake Cheko to determine its origins.
The main points of the study are that:
Cheko, a small lake located in Siberia close to the epicentre [sic] of the 1908 Tunguska explosion, might fill a crater left by the impact of a fragment of a cosmic body. Sediment cores from the lake's bottom were studied to support or reject this hypothesis. A 175-centimetre-long (69 in) core, collected near the center of the lake, consists of an upper c. 1-metre-thick (39 in) sequence of lacustrine deposits overlaying coarser chaotic material. 210Pb and 137Cs indicate that the transition from lower to upper sequence occurred close to the time of the Tunguska event. Pollen analysis reveals that remains of aquatic plants are abundant in the top post-1908 sequence but are absent in the lower pre-1908 portion of the core. These results, including organic C, N and δ13C data, suggest that Lake Cheko formed at the time of the Tunguska event. Pollen assemblages confirm the presence of two different units, above and below the ~100‐cm level (Fig. 4). The upper 100‐cm long section, in addition to pollen of taiga forest trees such as Abies, Betula, Juniperus, Larix, Pinus, Picea, and Populus, contains abundant remains of hydrophytes, i.e., aquatic plants probably deposited under lacustrine conditions similar to those prevailing today. These include both free floating plants and rooted plants, growing usually in water up to 3–4 meters in depth (Callitriche, Hottonia, Lemna, Hydrocharis, Myriophyllum, Nuphar, Nymphaea, Potamogeton, Sagittaria). In contrast, the lower unit (below ~100 cm) contains abundant forest tree pollen, but no hydrophytes, suggesting that no lake existed then, but a taiga forest growing on marshy ground (Fig. 5). Pollen and microcharcoal show a progressive reduction in the taiga forest, from the bottom of the core upward. This reduction may have been caused by fires (two local episodes below ~100 cm), then by the TE and the formation of the lake (between 100 and 90 cm), and again by subsequent fires (one local fire in the upper 40 cm).
In 2017, new research by Russian scientists pointed to a rejection of the theory that Lake Cheko was created by the Tunguska event. They used soil research to determine that the lake is 280 years old or even much older; in any case clearly older than the Tunguska event. In analyzing soils from the bottom of Lake Cheko, they identified a layer of radionuclide contamination from mid-20th century nuclear testing at Novaya Zemlya. The depth of this of layer gave an average annual sedimentation rate of between 3.6 and 4.6 mm a year. These sedimentation values are less than half of the 1 cm/year calculated by Gasperini et al. in their 2009 publication on their analysis of the core they took from Lake Cheko in 1999. The Russian scientists in 2017, counted at least 280 such annual varves in the 1260 mm long core sample pulled from the bottom of the lake, representing an age of the lake that would be older than the Tunguska Event.
Additionally, there are problems with impact physics: It is unlikely that a stony meteorite in the right size range would have the mechanical strength necessary to survive atmospheric passage intact, and yet still retain a velocity large enough to excavate a crater that size on reaching the ground.
Though scientific consensus is that the Tunguska explosion was caused by the impact of a small asteroid, there are some dissenters. Astrophysicist Wolfgang Kundt has proposed that the Tunguska event was caused by the release and subsequent explosion of 10 million tons of natural gas from within the Earth's crust. The basic idea is that natural gas leaked out of the crust and then rose to its equal-density height in the atmosphere; from there, it drifted downwind, in a sort of wick, which eventually found an ignition source such as lightning. Once the gas was ignited, the fire streaked along the wick, and then down to the source of the leak in the ground, whereupon there was an explosion.
The Tunguska event is not the only example of an enormous unobserved explosion event. For example, the 1930 Curuçá River event in Brazil may have been an explosion of a superbolide that left no clear evidence of an impact crater. Modern developments in infrasound detection by the Comprehensive Nuclear-Test-Ban Treaty Organization and infrared DSP satellite technology have reduced the likelihood of undetected air bursts.
A smaller air burst occurred over a populated area on 15 February 2013, at Chelyabinsk in the Ural district of Russia. The exploding meteoroid was determined to have been an asteroid that measured about 17–20 metres (56–66 ft) across, with an estimated initial mass of 11,000 tonnes and which exploded with an energy release of approximately 500 kilotons. The air burst inflicted over 1,200 injuries, mainly from broken glass falling from windows shattered by its shock wave.
In popular cultureEdit
- Asteroid Day
- Patomskiy crater, about 830 kilometres (520 mi) to the east-southeast
- Sikhote-Alin meteorite, 1947 impact
- Tall el-Hammam, a Middle Bronze Age site that one archaeological team has proposed was destroyed by an air burst
- Tunguska Nature Reserve, protected area covering a portion of the site; ongoing scientific study of forest recovery
- Farinella, Paolo; Foschini, L.; Froeschlé, Christiane; Gonczi, R.; Jopek, T. J.; Longo, G.; Michel, Patrick (2001). "Probable asteroidal origin of the Tunguska Cosmic Body" (PDF). Astronomy & Astrophysics. 377 (3): 1081–1097. Bibcode:2001A&A...377.1081F. doi:10.1051/0004-6361:20011054. Retrieved 1 September 2015.
- Trayner, C (1994). "Perplexities of the Tunguska meteorite". The Observatory. 114: 227–231. Bibcode:1994Obs...114..227T.
- Gritzner, C (1997). "Human Casualties in Impact Events". WGN. 25: 222. Bibcode:1997JIMO...25..222G.
- Jay, Paul. "The Tunguska event". CBC News. Retrieved 20 July 2017.
- Coppins, Philip. "The Tunguska explosion: an unexpected loud bang and explosion". philipcoppins.com. Retrieved 8 October 2017.
- "Reported Deaths and Injuries from Meteorite Impact". delong.typepad.com. Retrieved 8 October 2017.
- Jenniskens, P (2019). "Tunguska eyewitness accounts, injuries and casualties". Icarus. 327: 4–18. Bibcode:2019Icar..327....4J. doi:10.1016/j.icarus.2019.01.001.
- de Pater, Imke; Lissauer, Jack (2001). Planetary Sciences. Cambridge University Press. ISBN 0521482194.
- Nemiroff, R.; Bonnell, J., eds. (14 November 2007). "Tunguska: The Largest Recent Impact Event". Astronomy Picture of the Day. NASA. Retrieved 12 September 2011.
- Lyne, J. E.; Tauber, M. (1995). "Origin of the Tunguska Event". Nature. 375 (6533): 638–639. Bibcode:1995Natur.375..638L. doi:10.1038/375638a0.
- Longo, Giuseppe (2007). "18: The Tunguska event" (PDF). In Bobrowsky, Peter T.; Rickman, Hans (eds.). Comet/Asteroid Impacts and Human Society, An Interdisciplinary Approach. Berlin Heidelberg New York: Springer-Verlag. pp. 303–330. Bibcode:2007caih.book.....B. ISBN 978-3-540-32709-7. Archived from the original (PDF) on 14 October 2013.
- Peplow, Mark (10 June 2013). "Rock samples suggest meteor caused Tunguska blast". Nature.
- Kvasnytsya, Victor; R. Wirth; L. Dobrzhinetskaya; J. Matzel; B. Jacobsen; I. Hutcheon; R. Tappero; M. Kovalyukh (2013). "New evidence of meteoritic origin of the Tunguska cosmic body". Planet. Space Sci. 84: 131–140. Bibcode:2013P&SS...84..131K. doi:10.1016/j.pss.2013.05.003.
- Whipple, F. J. W. (1934). "On Phenomena related to the great Siberian meteor". Quarterly Journal of the Royal Meteorological Society. 60 (257): 505–522. Bibcode:1934QJRMS..60..505W. doi:10.1002/qj.49706025709. ISSN 0035-9009.
- Traynor, Chris (1997). "The Tunguska Event". Journal of the British Astronomical Association. 107 (3).
- Watson, Nigel. "The Tunguska Event". History Today 58.1 (July 2008): 7. MAS Ultra-School Edition. EBSCO. 10 February 2009
- Cornell University (24 June 2009). Space Shuttle Science Shows How 1908 Tunguska Explosion Was Caused By A Comet.
- Kelley, M. C.; Seyler, C. E.; Larsen, M. F. (2009). "Two-dimensional Turbulence, Space Shuttle Plume Transport in the Thermosphere, and a Possible Relation to the Great Siberian Impact Event". Geophys. Res. Lett. 36 (14): L14103. Bibcode:2009GeoRL..3614103K. doi:10.1029/2009GL038362.
- Turco, R.P.; et., al. (April 1982). "An Analysis of the Physical, Chemical, Optical and Historical Impacts of the 1908 Tunguska Meteor Fall". Icarus. 50 (1): 1–52. Bibcode:1982Icar...50....1T. doi:10.1016/0019-1035(82)90096-3.
- N. V. Vasiliev, A. F. Kovalevsky, S. A. Razin, L. E. Epiktetova (1981). Eyewitness accounts of Tunguska (Crash). Archived 30 September 2007 at the Wayback Machine, Section 6, Item 4
- Vasiliev, Section 5
- Vasiliev, Section 1, Item 2
- Gregorian calendar: 30 June
- Vasiliev, Section 1, Item 3
- Vasiliev, Section 1, Item 5
- "The Tunguska Impact – 100 Years Later". NASA Science. Retrieved 13 January 2019.
- "This Month in Physics History". American Physical Society. June 2018. Retrieved 22 December 2018.
- Boyarkina, A. P., Demin, D. V., Zotkin, I. T., Fast, W. G. "Estimation of the blast wave of the Tunguska meteorite from the forest destruction". Meteoritika, Vol. 24, 1964, pp. 112–128 (in Russian).
- Longo G. "The 1938 aerophotosurvey". Retrieved 8 October 2017.
- See: Bronshten (2000), p. 56.
- Rubtsov (2009), p. 59.
- Florenskiy, K P (1963). "Preliminary results from the 1961 combined Tunguska meteorite expedition". Meteoritica. 23. Retrieved 26 June 2007.
- Kolesnikov et al. "Finding of probable Tunguska Cosmic Body material: isotopic anomalies of carbon and hydrogen in peat", Planetary and Space Science, Volume 47, Issues 6–7, 1 June 1999, pp. 905–916.
- Hou et al. "Discovery of iridium and other element anomalies near the 1908 Tunguska explosion site", Planetary and Space Science, Volume 46, Issues 2–3, February–March 1998, pp. 179–188.
- Kolesnikov et al. "Isotopic anomaly in peat nitrogen is a probable trace of acid rains caused by 1908 Tunguska bolide", Planetary and Space Science, Volume 46, Issues 2–3, February–March 1998, pp. 163–167.
- Anfinogenov, John; et al. (15 November 2014). "John's Stone: A possible fragment of the 1908 Tunguska meteorite". Icarus. 245: 139–147. Bibcode:2014Icar..243..139A. doi:10.1016/j.icarus.2014.09.006.
- Anfinogenova, Yana; Anfinogenov, John; Polonia, Alina; Gasperini, Luca; Franchi, Fulvio; Rocco, Tommaso Di; Breger, Dee; Bonatti, Enrico (5 September 2015). "Origin of John's Stone: A quartzitic boulder from the site of the 1908 Tunguska (Siberia) explosion". Icarus. 258: 297–308. Bibcode:2015Icar..258..297B. doi:10.1016/j.icarus.2015.06.018. ISSN 0019-1035.
- Shoemaker, Eugene (1983). "Asteroid and Comet Bombardment of the Earth". Annual Review of Earth and Planetary Sciences. 11 (1): 461–494. Bibcode:1983AREPS..11..461S. doi:10.1146/annurev.ea.11.050183.002333.
- "Sandia supercomputers offer new explanation of Tunguska disaster". Sandia National Laboratories. 17 December 2007. Retrieved 22 December 2007.
- Verma (2005), p 1.
- Wheeler, Lorien F.; Mathias, Donovan L. (2019). "Probabilistic assessment of Tunguska-scale asteroid impacts". Icarus. 327: 83–96. Bibcode:2019Icar..327...83W. doi:10.1016/j.icarus.2018.12.017.
- Borovička, Jiří; Spurný, Pavel; Brown, Peter; Wiegert, Paul; Kalenda, Pavel; Clark, David; Shrbený, Lukáš (2013). "The trajectory, structure and origin of the Chelyabinsk asteroidal impactor". Nature. 503 (7475): 235–237. Bibcode:2013Natur.503..235B. doi:10.1038/nature12671. PMID 24196708.
- Wiley, John P. Jr. (January 1995). "Phenomena, Comment & Notes". Smithsonian. Archived from the original on 10 September 2012.
- Brown, P.; Spalding, R.E.; ReVelle, D.O.; et al. (21 November 2002). "The flux of small near-Earth objects colliding with the Earth" (PDF). Nature. 420 (6913): 294–296. Bibcode:2002Natur.420..294B. doi:10.1038/nature01238. PMID 12447433. Retrieved 13 January 2019.
- Khrennikov, Daniil E.; Titov, Andrei K.; Ershov, Alexander E.; Pariev, Vladimir I.; Karpov, Sergei V. (21 March 2020). "On the possibility of through passage of asteroid bodies across the Earth's atmosphere". Monthly Notices of the Royal Astronomical Society. 493 (1): 1344–1351. doi:10.1093/mnras/staa329. ISSN 0035-8711.
- "Most Explosive Meteor Impact: 1908 Tunguska Explosion Caused by Iron Asteroid That Entered Earth Then Bounced Back to Space". Science Times. 6 May 2020. Retrieved 7 May 2020.
- "World's largest 'explosion' could have been caused by iron asteroid entering and leaving atmosphere". siberiantimes.com. Retrieved 7 May 2020.
- Siberian Apocalypse on IMDb
- Kresak, L' (1978). "The Tunguska object – A fragment of Comet Encke". Astronomical Institutes of Czechoslovakia. 29 (3): 129. Bibcode:1978BAICz..29..129K.
- Nemtchinov, I.V.; Jacobs, C.; Tagliaferri, E. (1997). "Analysis of Satellite Observations of Large Meteoroid Impacts". Annals of the New York Academy of Sciences. 822 (1 Near–Earth Ob): 303–317. Bibcode:1997NYASA.822..303N. doi:10.1111/j.1749-6632.1997.tb48348.x.
- Phil Plait (14 May 2019). "Could larger space rocks be hiding in the Beta Taurid Meteor stream? We may find out this summer". Bad Astronomy.
- "Arctic Asteroid!". Science at NASA. Retrieved 8 October 2017.
- Utyuzhnikov, S. V. and Rudenko, D. V. "An adaptive moving mesh method with application to nontstationary hypersonic flows in the atmosphere" Proceedings of the Institution of Mechanical Engineers, Part G, Journal of Aerospace Engineering, 2008, 222 (5): 661–671
- Longo, G.; Serra, R.; Cecchini, S.; Galli, M. (1994). "Search for microremnants of the Tunguska Cosmic Body". Planetary and Space Science. 42 (2): 163–177. Bibcode:1994P&SS...42..163L. doi:10.1016/0032-0633(94)90028-0.
- Serra, R.; Cecchini, S.; Galli, M.; Longo, G. (1994). "Experimental hints on the fragmentation of the Tunguska cosmic body". Planetary and Space Science. 42 (9): 777–783. Bibcode:1994P&SS...42..777S. doi:10.1016/0032-0633(94)90120-1.
- Kelly, M.C.; C. E. Seyler; M. F. Larsen (22 June 2009). "Two-dimensional turbulence, space shuttle plume transport in the thermosphere, and a possible relation to the Great Siberian Impact Event". Geophysical Research Letters (Published online 22 July 2009). 36 (14): L14103. Bibcode:2009GeoRL..3614103K. doi:10.1029/2009GL038362.
- Ju, Anne (24 June 2009). "A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused by comet". Cornell Chronicle. Cornell University. Retrieved 25 June 2009.
- Drobyshevski, E. M.; Galushina, T. Yu; Drobyshevski, M. E. (March 2009). "A search for a present-day candidate for the Comet P/Tunguska-1908". arXiv:0903.3313.
- "Meteoroid, not comet, explains the 1908 Tunguska fireball". DiscoverMagazine.com blog. 1 July 2013.
- Smith, Kimberly Ennico (25 June 2019). "Tunguska Revisited: 111-Year-Old Mystery Impact Inspires New, More Optimistic Asteroid Predictions". NASA. Retrieved 6 July 2019.
- Gasperini, L; Alvisi, F; Biasini, G; Bonatti, E; Longo, G; Pipan, M; Ravaioli, M; Serra, R (2007). "A possible impact crater for the 1908 Tunguska Event". Department of Physics. 19 (4): 245. Bibcode:2007TeNov..19..245G. doi:10.1111/j.1365-3121.2007.00742.x.
- Paul, Rincon (27 June 2007). "Team makes Tunguska crater claim". BBC.
- Gasperini, L.; Bonatti, Enrico; Longo, Giuseppe (April 2008). "Reply – Lake Cheko and the Tunguska Event: impact or non-impact?". Terra Nova. 20 (2): 169–172. Bibcode:2008TeNov..20..169G. doi:10.1111/j.1365-3121.2008.00792.x.
- Gasperini, L.; et al. (June 2008). "The Tunguska Mystery". Scientific American. 298 (6): 80–86. Bibcode:2008SciAm.298f..80G. doi:10.1038/scientificamerican0608-80. PMID 18642546.
- "Crater From 1908 Russian Space Impact Found, Team Says". National Geographic. 7 November 2007. Retrieved 8 October 2017.
- Gasperini, Luca; Bonatti, Enrico; Albertazzi, Sonia; Forlani, Luisa; Accorsi, Carla A.; Longo, Giuseppe; Ravaioli, Mariangela; Alvisi, Francesca; Polonia, Alina; Sacchetti, Fabio (December 2009). "Sediments from Lake Cheko (Siberia), a possible impact crater for the 1908 Tunguska Event". Terra Nova. 21 (6): 489–494. Bibcode:2009TeNov..21..489G. doi:10.1111/j.1365-3121.2009.00906.x.
- "Tunguska Event: Russian Scientists Debunk Meteorite Theory". Sputnik News. 18 January 2017. Retrieved 8 October 2017.
- Лебедева, Юлия. "ОЗЕРО ЧЕКО СТАРШЕ ТУНГУССКОГО МЕТЕОРИТА". Retrieved 17 January 2018.
- Rogozin, D.Y.; Darin, A. V. "Sedimentation Rate in Cheko Lake (Evenkia, Siberia): New Evidence on the Problem of the 1908 Tunguska Event" (PDF). Doklady Earth Sciences. Retrieved 25 April 2020.
- Collins, G.S.; Artemieva, N. (2008). "Evidence that Lake Cheko is not an impact crater". Terra Nova. Retrieved 29 April 2020.
- Kundt, W. (2001). "The 1908 Tunguska catastrophe" (PDF). Current Science. 81: 399–407. Archived from the original (PDF) on 4 March 2016.
- Jones, N. (7 September 2002). "Did blast from below destroy Tunguska?". New Scientist. 2359: 14.
- Kundt, W. (2007). "Tunguska (1908) and its relevance for comet/asteroid impact statistics". In Bobrowsky, P. T.; Rickman, H. (eds.). Comet/Asteroid Impacts and Human Society. Springer. pp. 331–339.
- "100 years on, mystery shrouds massive 'cosmic impact' in Russia", Agence France-Presse, 29 June 2008. Retrieved 8 October 2017.
- Choi, Charles Q., "Massive Tunguska Blast Still Unsolved 100 Years Later", Fox News Channel, 4 July 2008. Retrieved 8 October 2017.
- Phipps Morgan, J.; Reston, T. J.; Ranero, C. R. (2004). "Contemporaneous mass extinctions, continental flood basalts, and 'impact signals': are mantle plume-induced lithospheric gas explosions the causal link?" (PDF). Earth and Planetary Science Letters. 217 (3–4): 263–284. Bibcode:2004E&PSL.217..263P. doi:10.1016/s0012-821x(03)00602-2.
- Vannucchi, P.; Morgan, J. P.; Della Lunga, D.; Andronicos, C. L.; Morgan, W. J. (2015). "Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska event". Earth and Planetary Science Letters. 409: 168–174. Bibcode:2015E&PSL.409..168V. doi:10.1016/j.epsl.2014.11.001.
- Burghardt, David (22 July 2009). "A century later scientists still at odds on Tunguska Event explanation". RIA Novosti. Cite journal requires
- Ol'khovatov, A. Yu. (2003). "Geophysical circumstances of the 1908 Tunguska Event in Siberia, Russia" (PDF). Earth, Moon, and Planets. 93 (3): 163–173. Bibcode:2003EM&P...93..163O. doi:10.1023/B:MOON.0000047474.85788.01.
- Sklublov G. T., Marin Yu. B., Skublov S. G., Bidyukov B. F., Logunova L. N., Gembitsky V. V., Nechaeva E. S. (2010), "Geological and mineralogical-geochemical peculiarities of loose sediments and primary rocks in epicenter of Tunguskaya catastrophe in 1908", Proceedings of the Russian Mineralogical Society, 139(1): 111–135 [in Russian, with English abstract].
- Skublov G. T., Marin Yu. B., Skublov S. G., Logunova L. N., Nechaeva E. S., Savichev A. A. (2011), "Mineralogical-geochemical features of primary rocks, loose sediments and catastrophic mosses in the Northern Swamp area (region of the Tunguska catastrophe in 1908)", Proceedings of the Russian Mineralogical Society, 140(3): 120–138 [in Russian with English abstract].
- Shurmina, Natalia; Kuzmin, Andrey. "Meteorite hits central Russia, more than 500 people hurt". Reuters. Retrieved 8 October 2017.
- Baxter, John; Atkins, Thomas. The Fire Came By: The Riddle of the Great Siberian Explosion, (London) Macdonald and Jane's, 1975. ISBN 978-0-446-89396-1.
- Baxter, John; Atkins, Thomas; introduction by Asimov, Isaac. The Fire Came By: The Riddle of the Great Siberian Explosion, (Garden City, New York (state)) Doubleday, 1976. ISBN 978-0-385-11396-0.
- Baxter, John; Atkins, Thomas; introduction by Asimov, Isaac. The Fire Came By: The Riddle of the Great Siberian Explosion, (New York) Warner Books, 1977. ISBN 978-0-446-89396-1.
- Bronshten, V. A. The Tunguska Meteorite: History of Investigations, (Moscow) A. D. Selyanov 2000 (in Russian). ISBN 978-5-901273-04-3.
- Brown, John C.; Hughes, David. W. "Tunguska's comet and the non-thermal carbon-14 production in the atmosphere", Nature, Vol 268 (May) 1977 pp. 512–514.
- Chaikin, Andrew. "Target: Tunguska", Sky & Telescope, January 1984 pp. 18–21. The Kresak/Sekanina debate, in a very widely available journal. Cited in Verma.
- Christie, William H. "The great Siberian meteorite of 1908", The Griffith Observer, (Los Angeles) The Griffith Observatory, Vol 6 (April) 1942 pp. 38–47. This review is widely cited.
- Crowther, J. G. "More about the Great Siberian Meteorite", Scientific American, May 1931 pp. 314–317. Cited in Verma.
- Furneaux, Rupert. The Tungus Event: The Great Siberian Catastrophe of 1908, (New York) Nordon Publications, 1977. ISBN 978-0-8439-0619-6.
- Furneaux, Rupert. The Tungus Event: The Great Siberian Catastrophe of 1908, (St. Albans) Panther, 1977. ISBN 978-0-586-04423-0.
- Gallant, Roy A. The Day the Sky Split Apart: Investigating a Cosmic Mystery, (New York) Atheneum Books for Children, 1995. ISBN 978-0-689-80323-9.
- Gallant, Roy A. "Journey to Tunguska", Sky & Telescope, June 1994 pp. 38–43. Cover article, with full-page map. Cited in Verma.
- Gasperini, Luca, Bonatti, Enrico and Longo, Giuseppe. The Tunguska Mystery 100 Years Later, Scientific American, June 2008.
- Krinov, E. L. Giant Meteorites, trans. J. S. Romankiewicz (Part III: The Tunguska Meteorite), (Oxford and New York) Pergamon Press, 1966.
- Lerman, J. C.; Mook, W. G.; Vogel, J. C. (1967). "Effect of the Tunguska Meteor and Sunspots on Radiocarbon in Tree Rings". Nature. 216 (5119): 990–991. Bibcode:1967Natur.216..990L. doi:10.1038/216990a0.
- Morgan, J. Phipps; Ranero, C. R.; Reston, T.J. (2004). "Contemporaneous mass extinctions, continental flood basalts, and 'impact signals': are mantle plume-induced lithospheric gas explosions the causal link?" (PDF). Earth and Planetary Science Letters. 217 (3–4): 263–284. Bibcode:2004E&PSL.217..263P. doi:10.1016/s0012-821x(03)00602-2.
- Oliver, Charles P (1928). "The Great Siberian Meteorite". Scientific American. 139 (1): 42–44. Bibcode:1928SciAm.139...42O. doi:10.1038/scientificamerican0728-42. Cited in Baxter and Atkins, also in Verma.
- Ol'khovatov, A. Yu. "Geophysical Circumstances of the 1908 Tunguska Event in Siberia, Russia", Earth, Moon, and Planets, Vol 93 November 2003, pp. 163–173
- Perkins, Sid. "A Century Later, Scientists Still Study Tunguska", Science News, 21 June 2008 pp. 5–6. Includes 11 color photographs.
- Rubtsov, Vladimir. The Tunguska Mystery, (Dordrecht and New York) Springer, 2009. ISBN 978-0-387-76573-0; 2012, ISBN 978-1-4614-2925-8.
- Steel, Duncan (2008). "Tunguska at 100". Nature. 453 (7199): 1157–1159. Bibcode:2008MNSSA..67...75.. doi:10.1038/4531157a. PMID 18580919. This is one of several articles in a special issue, cover title: "Cosmic Cataclysms".
- Stoneley, Jack; with Lawton, A. T. Cauldron of Hell: Tunguska, (New York) Simon & Schuster, 1977. ISBN 978-0-671-22943-6.
- Stoneley, Jack; with Lawton, A. T. Tunguska, Cauldron of Hell, (London) W. H. Allen, 1977. ISBN 978-0-352-39619-8
- Verma, Surendra. The Tunguska Fireball: Solving One of the Great Mysteries of the 20th century, (Cambridge) Icon Books Ltd., 2005. ISBN 978-1-84046-620-1.
- Verma, Surendra. The Mystery of the Tunguska Fireball, (Cambridge) Icon Books Ltd., 2006. ISBN 978-1-84046-728-4, also (Crows Nest, NSW, Australia) Allen & Unwin Pty Ltd., 2006, with same ISBN. Index has "Lake Cheko" as "Ceko, Lake", without "h".
|Wikimedia Commons has media related to Tunguska event.|
- Tunguska pictures Many Tunguska-related pictures with comments in English.
- Evgenii A. Vaganov; Malkolm K. Hughes; Pavel P. Silkin; Valery D. Nesvetailo (2004). "The Tunguska Event in 1908: Evidence from Tree-Ring Anatomy" (PDF). Astrobiology. 4 (3): 391–399. Bibcode:2004AsBio...4..391V. doi:10.1089/ast.2004.4.391. PMID 15383242.
- Preliminary results from the 1961 combined Tunguska meteorite expedition
- 1908 Siberia Explosion. Reconstruction by William K. Hartmann.
- NASA Astronomy Picture of the Day: Tunguska: The Largest Recent Impact Event (14 November 2007)
- Mystery space blast 'solved' (BBC News)
- Sound of the Tunguska event (reconstruction)
- The Tunguska Event 100 Years later NASA
- Hogenboom, Melissa (7 July 2016). "In Siberia in 1908, a huge explosion came out of nowhere". BBC News. Earth. London. Retrieved 8 October 2017.