Meteorite hunters may have found the largest known chunk of the Bronze Age Kaali Meteorite.

Two Polish meteorite hunters, Filip Nikodem and Andrzej Owczarzak, have recovered what they believe to be a fragment of the Kaali Meteorite, a large body which impacted the Estonian island of Saaremaa about 3500 years ago. The piece is reported to weigh about 40 kg, with a second fragment weighing 15 kg also found. The largest potential fragment of the meteorite discovered to date weighed 5.7 kg, and was found by Filip Nikodem in the spring of 2024.

A 40 kg potential fragment of the Kaali Meteorite found in Estonia by two Polish Meteorite Hunters. Z głową w gwiazdach/Facebook.

The Kaali Impact Structures are a series of nine circular crater lakes on the island of Saaremaa, the largest of which has a diameter of 110 m and a depth of 22 m. These are thought to have been caused by an object with a mass of between 20 and 80 tonnes entering the atmosphere at a velocity of somewhere between 10 and 20 km per second, and exploding in a fireball between 10 and 5 km above the ground. This explosion is thought to have largely vaporised the bolide, as well as removing about 81 000 m³ of rock on the ground, and incinerating vegetation up to 6 km from the impact site.  

The lake in the main Kaali Meteorite Crater on Saaremaa Island. The crater is about 110 m across, but the lake only occupies the central part of it. Wikimedia Commons.

This is thought to have happened about 3500 years ago, when the island was occupied by a Bronze Age population, although attempts to date the site and meteorite fragments from it have produced dates of between 5600 and 400 BC. A Bronze Age arrowhead made from meteoric iron which was found in Switzerland in the 1870s has been linked to the Kaali meteorites on the basis of its metallurgy, making a date of around 1500 BC more plausible.

(a) Overview of the Mörigen arrowhead (A/7396). Note adhering bright sediment material. Remnants of an older label on the left of the sample number. Total length is 39.3 mm. ( b) Side view of the Mörigen arrowhead. Layered texture is well visible. Point is to the right. Thomas Schüpbach in Hofmann et al. (2023).

Prior to 2023, the largest fragment of meteorite found at the Kaali sites weighed 6.21 g. However, in the autumn of that year Filip Nikodem obtained a search permit for the area, and, using a metal detector, found a series of pieces of iron which he believed to be meteoric in origin. These were handed to the Estonian National Heritage Board, who sent them to the University of Tartu, where a rapid X-ray fluorescence analysis found that the iron pieces contained between 0.6% and 4.8% nickel, as well as traces of titanium and vanadium, which is typical of meteoric iron.

The four pieces of meteoric iron handed to the Estonian National Heritage Board  by Filip Nikodem in 2023. Kristo Oks/University of Tartu.

In the spring of 2024, Filip Kikodem returned to the Kaali Lakes, collecting several more fragments with a total mass of 10.5 kg, the largest of which weighed 5.7 kg, making it (at that time) potentially the largest chunk of the Kaali bolide ever discovered. However, despite requests from the Estonian National Heritage Board, these pieces have never been surrendered to them. Instead, the Estonian authorities believe that the pieces were taken to Poland, where they are being analysed at a Polish university. Furthermore, the National Heritage Board received a letter from a lawyer in Poland, asserting Nikodem's ownership rights over the meteorite fragments.

A 5.7 kg possible meteorite fragment found by Filip Nikodem at Kaali Lakes in the spring of 2024. Filip Nikodem.

In the autumn of 2024 Filip Nikodem returned to Kaali Lakes, this time accompanied by meteorite hunter Andrzej Owczarzak, who did not posses a permit to search in Estonia, and was therefore in breach of Estonian heritage laws. At this time concerns were raised by the Estonian newspaper Saarte Hääl, which observed that the original search permit had been given to Filip Nikodem in relation to a two week project run by the University of Tartu in 2017, in which metal detectorists were given a short training course by the university then allowed to collect fragments for a display at a visitor centre, with the collectors being allowed to keep some fragments as a reward.

Saarte Hääl also noted that suspiciously large chunks of 'Kaali Meteorite' had begun to be offered for sale in Poland, and that some Polish enthusiasts had raised concerns that these might be fragments of the more common Morasko Meteorite, which fell near Potsdam about 5000 years ago. The newspaper also noted that Estonian law provides no specific protection for meteorites found in the country, although there are some restrictions upon where a metal detector can be used. 

Following the announcement of these discoveries, Jüri Plado of the University of Tartu applied for funding for a study on the feasibility of changing the law to protect meteorites found in Estonia, reasoning that such large objects would be a significant piece of national heritage and ought to be studied at an institute within the country, but this application was rejected. 

Local farmers talked to by Saarte Hääl stated that the Poles had shown them permits and promised to take any material found to the University of Tartu. However, several expressed doubts that the iron found was in fact meteoric, as it came from shallow depths on farmland, likely to have been disturbed since the Bronze Age, and resembled bog iron (impure iron deposits which precipitate out of solution in boggy soils). The newspaper contacted Kristo Oks of the Estonian National Heritage Board, who confirmed that it would be impossible to tell the difference between bog iron and meteorites without laboratory analysis. 

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Looking for the Chinguetti Meteorite.

In 1916 a young French army officer called Captain Gaston Ripert reported being taken to see a giant meteorite in the Mauritanian desert, south of Chinguetti. The story is a strange one, with Ripert claiming he was taken blindfolded, at night, on a ten hour Camel ride into the desert, where he observed a huge iron structure 100 m long and 40 m wide, recovering a smaller, 4.5 kg meteorite from its surface. Shortly after returning to Chinguetti, where he was commander of the local Camel corps, Ripert reported that his guide, a local chief, was poisoned, leaving him unable to relocate the site.

The eccentric nature of this story led many people to dismiss it out of hand. It was not unusual for western travellers of the time to make up tales of wild adventure; some even paid ghostwriters to create particularly entertaining tales. However, officers in colonial armies were supposed to refrain from such nonsense, and some aspects of Ripert's story were hard to rectify with the story being complete fiction. 

During the past century a number of expeditions have sought to locate Ripert's meteorite, with the first in 1924, although by this time Ripert was stationed in Cameroon, and could only be communicated with by letters. This meant that the early searches concentrated on the area to the southwest of Chinguetti, although Ripert later clarified that the area he was taken to was probably to the southeast. The French naturalist and explorer Théodore Monod mounted a number of expeditions to find the meteorite, starting in 1934, but was unable to locate it. In the 1950s an expedition by the French army used a declinometer  (instrument for measuring magnetic declination) in a search for the meteorite, without success, and in the 1990s a team from the British TV station Channel 4 used a magnetometer during a search for the meteorite, but took only a few measurements.

Despite all this, there are a number of elements of the story suggest that it was not complete fiction, not the least of this being Ripert's willingness to talk to experts about his journey for the rest of his life. The smaller rock which Ripert recovered did prove to be a meteorite, albeit one which, when subjected to radionuclide analysis in 2001 was shown not to have been part of a larger body (radionuclides form near the surface of asteroids due to a constant bombardment by cosmic rays, but these can only penetrate a little way, so the radionuclides they form are absent from the interior of large bodies). Finally, Ripert reported observing metallic needles protruding from the large meteorite, which he tried unsuccessfully to break off, finding that they were too ductile (able to be deformed without losing toughness) for the tools he had at hand). In 2003, the American geologist and meteorite specialist William Cassidy reported similar ductile metal needles protruding from nickel-rich zones of iron meteorites, but this was clearly unknown to science in 1916.

A fragment of the smaller meteorite brought back by Gaston Ripert in the collection of the Smithsonian National Museum of Natural History. Wikimedia Commons.

In a paper published on the arXiv database at Cornell University on 21 February 2024, Robert Warren  of Salisbury in England, Stephen Warren of the Astrophysics Group at Imperial College London, and Ekaterini Protopapa of the Department of Physics at the University of Oxford, describe the results of a more recent search for the Chinguetti Meteorite, and the prospects for either discovering its existence or proving its non-existence in the future.

Warren et al. began by collating remote-sensing data covering the region from multiple sources; they are reasonably confident that other researchers will have searched Google Earth for signs of the meteorite,  but they also accessed data from other sources, including the Shuttle Radar Topography Mission, the Advanced Land Observing Satellite (ALOS), and Landsat.

Using the reasoning that the only way a 40 m high meteorite could have disappeared in the deserts of Mauritania is for it to have been covered by a sand dune, Warren et al. began by searching for a region of high dunes which could be reached from Chinguetti by Camel in under 10 hours. There are two bands of dunes close to Chinguetti; the Les Boucles field, most of which is within 20 km of the city, and the Batraz field, which is between 40 and 60 km to the southeast. Much of the intervening area is also covered by sand dunes, but these are not large enough to describe an object as that described by Ripert.

Map showing the high sand dunes, greater than 30 m height, to the south of Chinguetti. Warren et al. (2024).

Warren et al. made two trips into the desert from Chinguetta, in the company of experienced local chameliers, one lasting eleven days and one lasting six. They found that Camels typically travel at speeds of between 2.0 and 3.6 km per hour, assuming good terrain, with the maximum speed achieved by unburdened Camels being about 5.0 km per hour. 

However, even assuming that Ripert and his guide were riding Camels unburdened by anything other than themselves, it is unlikely that this maximum speed would have been achieved for 10 hours, because the primary concern of the chameliers is for the welfare of their Camels, which are not only the most important assets they own, but also their only way of getting back to safety should a problem arise. This meant that if Warren et al.'s chameliers expected a journey to take four hours, they would travel for two hours, then give the Camels a three hour break to rest and feed, before completing the journey, something they were quite inflexible about. Neither would they travel in a straight line on anything other than the flattest terrain, but instead would zig-zag to avoid taking the Camels over steps and ledges, and would never take their Camels over the tops of dunes.

Ripert himself mentioned taking several detours during his journey, which makes a journey 50 km in a straight line from Chinguetta even less plausible. However, for the sake of convenience, Warren et al. take the area within 50 km of the city as a search area. This includes the more distant Batraz Dune Field, which Warren et al. consider unlikely, although they do concede that there is a route along a dry river bey which could bring a determined Camel rider this far in 10 hours if breaks were neglected. They also rule out the area of the Les Boucles Dune Field which lies within 10 km of the city, reasoning that Ripert, who was in charge of the local Camel Corps, would have recognised a location in this area. 

A sand dune in the Les Boucles Dune Field to the south of Chinguetta. Bruno Locatelli/Google Maps.

Having defined their search area, Warren et al. then searched their dataset for dunes large enough to have covered the meteorite described by Ripert. According to Ripert's description, the northeastern side of the meteorite was already covered by a dune at the time when he visited. The area is noted for its strong, prevailing winds, which blow northeast to southwest more-or-less constantly all year round, causing sand dunes to migrate in the same direction, and Ripert stated in 1932 that he thought it possible that the meteorite would already have been covered by the dune. Taking Ripert's estimate that the meteorite was 40 m high, it would require a dune more than 40 m high to cover it.

Sand dunes in a desert do not typically stack up against one-another; instead, they are usually discrete structures, with flat spaces between them. Warren et al. identified dunes higher than 30 m high in their remote sensing dataset, in order to give an error of margin, creating a map showing dunes which meet this criterion within the two dune fields. Since dunes are unlikely to have moved more than 100  m since 1916, the meteorite, if buried, must be within 100 m of the western edge of the dune covering it. 

Since a height of 30-40 m is reached within 300-400 m of the western flank of the dunes, it would in theory be possibly for a walk along the western flank of the dunes with a magnetometer (a passive instrument that measures changes in the Earth's magnetic field), and be confident of passing within 500 m of the meteorite, a distance at which it ought to be highly detectable.

Warren et al. also not that a magnetic survey of the area has been carried out by aircraft on behalf of the Mauritanian Ministry of Petroleum Energy and Mines by the Fugro geological surveying company, using funds provided by the World Bank, and this data has subsequently been made available to teams of scientists working on other projects. With this in mind, Warren et al. wrote to the Ministry requesting access to the data, but have yet to receive an answer.

Between 13 and 17 December 2022 Warren et al. carried out a magnetometer survey of the eastern part of the Les Boucles Dune Field on foot, covering the western edges of six large dunes, based upon which they are confident that the presence of a large iron meteorite beneath these dunes can be ruled out. Based upon the time this took, they estimate that a survey of all the potential dunes would require an expedition lasting three weeks. 

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A meteoric iron arrowhead from the late Bronze Age of Switzerland.

The Iron Age is considered to have begun when people started smelting iron from iron oxide ores. However, some iron artefacts predate this, having been produced from a source which did not require smelting: meteoric iron. In the Old World, Bronze Age meteoric iron artefacts are known from Turkey, Greece, Syria, Iraq, Lebanon, Egypt, Iran, Russia, China, and Poland. To date, the entire complement of Bronze Age meteoric iron artefacts from Europe comprises two rings and an amulet from Greece, a pair of bracelets from Czestochowa-Rakowa in Poland, and an iron axe from Wietrzno, also in Poland. Attempts have recently been made to locate other meteoric iron objects in archaeological collections, using X-ray fluorescence analysis, concentrating on areas where meteoric iron is thought likely to have been available. One such potential source is the Twannberg iron meteorite strewn field in the Jura Mountains of Switzerland, which has led archaeologists to re-examine many Bronze Age artefacrs in Swiss museum collections.

In a paper published in the Journal of Archaeological Science on 25 July 2023, Beda Hofmann of the Naturhistorisches Museum Bern and the Institute of Geological Sciences at the University of Bern, Sabine Bolliger Schreyer of the Bernisches Historisches Museum, Sayani Biswas and Lars Gerchow of the Paul Scherrer Institute, Daniel Wiebe, Marc Schumann, Sebastian Lindemann, and Diego Ramírez Garíca of the Physics Insitute at the University of Freiburg, Pierre Lanari, also of the Naturhistorisches Museum Bern, Frank Gfeller, also of the Naturhistorisches Museum Bern and the Institute of Geological Sciences at the University of Bern, Carlos Vigo, Darbachan Das, and Fabian Hotz, also of the Paul Scherrer Institute, Katharina von Schoeler of the Institute for Particle Physics and Astrophysics at ETH Zürich, Kuzihiko Ninomiya of the Institute of Radiation Sciences at Osaka University, Megumi Niikura of the RIKEN Nishina Center for Accelerator Based Science, Narongrit Ritjoho of the School of Physics at Suranaree University of Technology, and Alex Amato, again of the Paul Scherrer Institute, describe the discovery of a Bronze Age meteoric iron arrowhead in the collection of the Bernisches Historisches Museum.

The arrowhead (specimen number A/7396), was recovered from the Mörigen Pile Dwelling, a Bronze Age stilt house settlement attributed to the Urnfield Culture, on Lake Biel in Bern Canton, which is about 4-8 km south of the Twannberg iron meteorite strewn field. The Mörigen site was discovered by local fishermen in 1843, and was the subject of various amateur excavations until 1873, when the Bern government banned such activities, and arranged for a formal exploration of the site under the leadership of archaeologist Eduard von Jenner and geologist Edmund von Fellenburg. Exactly when arrowhead A/7396 was found is unclear, but it is thought to have been recovered during Jenner and Fellenburg's excavations in 1873 and 1874. It was first observed that the arrowhead was iron rather than bronze by Monika Bernatzky-Goetze in 1987, during a wider examination of arrowheads from Mörigen, though she made no further investigation of it at that time. It has a mass of 2.904 g, and measures 39.3 mm long, 25 mm wide, and 2.6 mm wide, and has a triangular blade with a 13 mm tang. 

(a) Overview of the Mörigen arrowhead (A/7396). Note adhering bright sediment material. Remnants of an older label on the left of the sample number. Total length is 39.3 mm. ( b) Side view of the Mörigen arrowhead. Layered texture is well visible. Point is to the right. Thomas Schüpbach in Hofmann et al. (2023).

Hofmann et al. carried out a metallurgical comparison of arrowhead A/7396, comparing it to two fragments of the Twannberg Meteorite, TW1 (NMBE 36467) and TW934 (NMBE 43747), but found that it was metallurgically quite distinct from these, and therefore derived from a different meteorite. The arrowhead was also examined by light microscopy, X-ray micro-computer tomography, muon induced X-ray emission spectography, scanning electron microscopy, gamma spectroscopy, and Ramen spectroscopy.

The arrowhead is comprised of rust covered iron with a very laminated texture, in places patches of sediment can still be seen attached to the surface, and very small amounts of unrusted iron are visible within a crack on the surface. The surface of the arrowhead has grinding or scratch marks in several places, which are beneath the attached organic material and sediment particles where these are found on the same part of the arrowhead.

X-ray tomography revealed that the rust layer, although covering most of the sirface, is very thin (less than 0.1 mm). The crack observed visually can be seen to extend across almost the whole width of the arrowhead in X-ray tomography, and is largely filled with fine-grained silt sediment. -ray tomography also showed the arrowhead to be of uneven thickness, being 1.2 mm thick on one side, while the other is only 0.6 mm thick. The metal has a pronounced layering parallel to the frontal plane of the arrowhead, something which would not be expected in an iron meteorite, and which is therefore presumed to be an artefact of the way in which the arrowhead was made.

X-ray tomographic sections of the Mörigen arrowhead. (a) Shows four sagittal sections, (b) shows 10 transversal sections. Brightest (densest) areas correspond to metallic iron, brightness of iron metal is variable due to flatness of the object. The layered structure and fractures filled with iron (hydr)oxides/sediment material resulting from oxidative volume expansion are well visible. Hofmann et al. (2023).

The Mörigen arrowhead is very flat, and has probably had its thickness increased somewhat by oxidation. This is not a natural shape for meteors or meteor fragments, suggesting that the metal has been flattened as well as being sharpened. Such working of the metal is a plausible origin for the laminations visible in the X-ray tomograph images of the arrowhead, which is probably a deformed Widmanstätten pattern (Widmanstätten patterns are interleaving of kamacite and taenite bands found in nickel-iron meteorites, where they are believed to be formed by very slow cooling of the metal, probably over millions of years). Similar patterns have been observed in artefacts from Greenland, which are known to have been made by cold working of material from the Cape York meteorite. These Greenland artefacts also have a very flat form, and a layered microstructure made from flattening of large kamacite and taenite grains. Hot working is also a possibility, though heating to above about 700° would probably result in the loss of the banding due to recrystallization. The grinding marks seen on the surface of the arrowhead in places may be a result of this working process. Thus, although the arrowhead is of a similar shape to the bronze arrowheads also found at Mörigen, it appears to have reached this shape via quite a different working process. 

An undeformed Widmanstätten pattern in a section of a meteorite from the Gibeon Cluster in Namibia. Kevin Walsh/Wikimedia Commons.

The oxidised surface of the arrowhead is a less than ideal target for X-ray fluorescence spectroscopy, and is likely to be responsible for the variation in nickel concentrations across the surface of the object; up to 22%, which is improbable on an unoxidized surface, and probably results from element partition during the corrosion process. Muon induced X-ray emission spectography, which can penetrate the surface of objects, found that the nickel content increased and stabilised with depth in both the arrowhead and meteorite fragment TW934 (which also has an oxidised surface) but not meteorite fragment TW1, which does not. Iron, nickel, cobalt, gallium, and germanium, all typical components of iron-nickel meteorites, were all detected by X-ray fluorescence spectroscopy, as were arsenic and copper, which are much more unusual. High levels of lead were found on the parts of the arrowhead with white numbering, implying that a lead-oxide based paint was used.

Scanning electron microscopy revealed the presence of bith taenite and kamacite, which are nickel-rich and nickel-poor phases found in iron-nickel meteorites. Some organic material was present on the surface, and were sediment particles, showing calcium, carbon, oxygen, and silicon, which would fit with a mixture of calcium carbonate and quartz. The pigment of the label was found to contain bith leand and tin.

Scanning electron microscopy images of typical surface areas of the arrowhead. (A) Thin lamina of taenite (Ta) surrounded by oxidation products (Feox) and nearby kamacite (Ka), Backscattered electron image; (B) Iron oxidation products (Feox) covered by organic material, probably Birch tar (Org, dark) and a latest layer of adhering sediment (Sed), Backscattered electron image image. (C), (D) Scratched surface (Scr) below organic material (wood tar; Org) and sediment (Sed). Scanning electron images. Hofmann et al. (2023).

Gamma spectrometry of the arrowhead was able to detect the presence of the isotopes aluminium²⁶, potassium⁴⁰, uranium²³⁸, thorium²²⁸, cobalt⁶⁰, and cesium¹³⁷. Ramen spectroscopy of the organic material produced a signal typical of a tar-like material, which was probably birchwood tar used to attach the arrowhead to the arrow. 

The presence of aluminium²⁶ strongly supports the meteoric origin of the metal suggested by the presence of nickel, cobalt, gallium, and germanium, and the ratios of iron to nickel and nickel to cobalt. The presence of the Widmanstätten patterns and taenite rule out an origin from the Twannberg Meteorite, fragments of which have only ever been found to contain kamacite. The concentrations of nickel and germanium in the metal are consistent with the parent meteor having mostly likely been an IAB type iron meteorite, such as the Cañon Diablo Meteorite from Arizona or the Campo de Cielo meteorites from Argentina. The composition of the metal could also correspond to an IC group meteorite, although these are much rarer, with only 13 known examples, none of them from Europe. 

Aluminium²⁶ is a cosmogenic isotope, found close to the surface of iron-nickel Solar System bodies, where it is formed by cosmic rays bombarding magnesium²⁶, the element to which it also decays, with a half-life of 717 000 years. This short half-life means that aluminium²⁶ and magnesium²⁶ reach an equilibrium point, witht the proportion of aluminium²⁶ decreasing deeper within the body. The proportion in the metal of the Mörigen arrowhead implies that it was at a depth of about 40 cm when it was in the parent body, implying a meteorite with an original diameter of about 80 cm. Such a meteorite would have had a minimum mass of about two tonnes.

The metal of the arrowhead is likely to have undergone some modification since it arrived on Earth. The most obvious modification is the layer of rust (iron oxide) which has formed on its surface, but the presence of copper and arsenic, elements not usually found in nickel-iron meteorites, is probably a result of Human actions, possibly originating when the metal was worked with tools used to work on bronze, but also quite possibly a result of being stored with bronze items.

The chemical and isotopic composition of the Mörigen arrowhead suggests that it derived from an IAB type meteorite with a minimum mass of about two tonnes. Three large IAB meteorites with compositions compatible with the Mörigen arrowhead are known from Europe; the Bohumilitz Metoerite from the Czech Republic, the Retuerte de Bullaque Meteorite from Spain and the Kaalijarv Meteorite from Estonia. Of these, the Kaalijarv is known to have been particularly large, producing a series of craters, the largest of which, the Kaalijärv Crater on the island of Saarema in Estonia, is 110 m in diameter. This object is thought to have had an original mass of several hundred tonnes, most of which was destroyed during the impact, leaving only small fragments of shrapnel. A piece of shrapnel from the Kaalijarv Meteorite would be a plausible source for the metal of the Mörigen arrowhead, although it is possible that the metal was broken off a larger mass, with other iron artefacts (now lost to us) being made from the remaining material. About 10 kg of material has been recovered from the Kaalijarv Meteorite to date, with dating based upon the stratigraphic location of these fragments suggesting the meteorite fell between 1870 and 1440 BC. This Bronze Age date, combined with the parent body having been sufficiently large to produce a fragment with the aluminium²⁶ signature seen in the Mörigen arrowhead, and the fact that it fell in an area known to have been inhabited during the Bronze Age, and therefore would have been observed, makes the Kaalijarv Meteorite the most likely source for the material used to make the arrowhead. 

Kaalijärv Crater on the island of Saarema in Estonia. Kaspars Priede/Wikimedia Commons.

However, this does not rule out other meteorites, such as Bohumilitz or Retuerta de Bullaque, or even an unknown impactor, as sources of the material. The Morasko IAB strewn field in Poland, which has been dated to about 3000 BC and which produced craters up to 90 m in diameter, can be ruled out, as all recovered fragments of this meteorite have much higher levels of germanium (about 500 parts per million) than seen in the Mörigen arrowhead. The Wietrzno Axe and Czestochowa-Rakowa Bracelets are close in time to the Mörigen arrowhead, but have much higher nickel contents, suggesting that they were made with material from a different meteorite.

A search for meteoric iron artefacts near to the Twannberg strewn field produced only a single item, and this was clearly derived from a different meteorite. This suggests that Bronze Age peoples were not aware of the Twannberg Impact, and had no means of detecting and utilizing metal from buried fragments of this object. The artefact uncovered, an iron arrowhead from the Mörigen Pile Dwelling in Bern Canton appears to have been derived from the Kaalijarv Meteorite, which fell in Estonia in about 1500 BC, implying that meteoric iron was a commodity traded across Europe before 800 BC (the approximate age of the Mörigen settlement), with the arrowhead, or the metal from which it was made, apparently having been transported about 1600 km. 

Since it is highly unlikely that only a single artefact would have been made from a source such as the Kaalijarv Meteorite once people were aware of it, there is a distinct possibility that other objects made from iron derived from this source are present in archaeological collections elsewhere in Europe, and possibly beyond. While it is possible that larger objects were made from this source, the highly fragmented nature of the material makes it more likely that most artefacts were small, and out current understanding of the ability of Bronze Age people to work iron, also suggests any objects will be very flat, giving a clear set of parameters for searching archaeological collections for more objects.

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Could the 1908 Tunguska Event have been caused by an extra-terrestrial body passing through the Earth's atmosphere without impacting?

On 30 June 1908 a explosion near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Siberia, flattened around 80 million trees over an area of about 2150 km², and is thought to have killed at least three people. The cause of this is unclear; the most possible current explanation is that a stony meteorite with a diameter of about 100 m exploded in an airburst 5-10 km above the ground, although no debris from such an event has ever been found, which is surprising. Since the Chelyabinsk meteor impact of February 2013, when a meteor estimated to have been 20 m in size passed over the city of Chelyabinsk, creating a shockwave that caused considerable damage on the ground, before breaking up to the west of the city, it has been clear that a bolide passint through the Earth's atmosphere could cause considerable damage on the ground, even without impacting or detonating in an airburst. 

In a paper published in the Monthly Nottices of the Royal Astronomical Society on 4 February 2020, Daniil Khrennikov of the Siberian Federal University, Andrei Titov of the Moscow Institute of Physics and Technology, Alexander Ershov, also of the Siberian Federal University, and of the Institute of Computational Modeling, Vladimir Pariev of the PN Lebedev Physical Institute, and Sergei Karpov, again of the Siberian Federal University, and also the LV Kirensky Institute of Physics and the Siberian State University of Science and Technology, examine the possibility of through passage of asteroid bodies across the Earth’s atmosphere, and the likelyhood that this might have caused the damage associated with the 1908 Tunguska Event.

Fallen trees in the aftermath of the 1908 Tunguska Event. Leonid Kulik.

The problem of the motion in the Earth’s atmosphere of a large bolide, capable of falling on to the surface of the planet in the form of meteorites, is now of great interest. An equally urgent concern is the study of the conditions for the passage of such bodies through the upper atmosphere, even without collision with the Earth’s surface, since the shockwaves produced by this passage have a colossal destructive effect.

Large bolides (1–10 km in size and larger) that carry the potential danger of collision with the Earth are detected by ordinary astronomical observations. The bodies of intermediate dimensions began to be registered relatively recently. Observations of such bodies and the interpretation of observational data make it possible to determine the probability of their collision with the Earth, their properties, and the characteristic features of passage through the atmosphere, as well as the consequences of fall. The clarification of these questions will enable us to assess more accurately the degree of asteroid hazard.

 

One of the fundamental problems of meteor physics is the determination of the pre-atmospheric mass of bolides, since the intensity of the meteor phenomenon is determined by the kinetic energy of the body when entering the atmosphere of the planet. It is known that the velocity of the bodies belonging to the Solar  System at the entrance to the Earth’s atmosphere should be inside a relatively narrow range, between 11.2 and 72.8 km per second, so that the variance of the contribution of the velocitysquared factor to the kinetic energy does not exceed 50 times. At the same time, the mass of a meteor body can vary in a much wider range: from fractions of a gram (micrometeor) to tens of millions of tons or more (the Tunguska space body), that is, by 13–15 orders of magnitude.

The goal of Khrennikov et al.'s study was to evaluate the effect on the trajectory of the bolide of its passage through dense layers of the atmosphere, taking into account the acting forces, the initial velocity, and the mass and its variation during the flight, to determine the conditions for possible passage of a large bolide through the atmosphere with a minimum loss of mass without collision with the Earth’s surface. The obtained results are compared with observational data on the Tunguska space body with an estimated altitude of maximum energy release of about 10–15 km to receive evidence in favour of a new explanation of the Tunguska phenomenon, which attributes the absence of meteoritic material on the Earth’s surface near the epicentre to the through passage of the bolide across the atmosphere with a small loss of velocity.

Firstly, Khrennikov et al. imagine a model explaining the entry of a bolide into the Earth’s atmosphere with respect to a chosen X–Y coordinate system coinciding with the centre of the Earth and corotating with the rotation of the Earth. The altitude of the entry of the bolide into the atmosphere is measured from the starting value 160 km, at which the temperature of the bolide begins to increase. The angle of entry into the atmosphere relative to the local horizontal line at the altitude 160 km.is one of the most important parameters of the problem. 

Schematic diagram of the motion of a bolide in the Earth’s atmosphere and the angle of entry into the atmosphere (β) at a given point relative to the X–Y coordinate system. Rₑ is the radius of the Earth. Thickness of the atmosphere is exaggerated. The trajectory of the bolide and its length within the atmosphere are indicated by the line with the arrow. Khrennikov et al.(2020).

Khrennikov et al. describe the ballistics of the bolide by a system of equations, including the equation of motion under the action of applied forces: the force of the aerodynamic drag and the gravitational force.

The contribution of the lifting force to the ballistic motion of the bolide is also neglected as Khrennikov et al.assume that its shape is close to spherical. The Coriolis and centrifugal forces in the rotating reference frame are negligible for fast-moving bolides compared to the aerodynamical forces from stratospheric winds, which they also neglect here because bolides move much faster than the wind speed.

In accordance with existing ideas, Khrennikov et al. assume the main contribution to the force of aerodynamic drag is made by the difference in pressure between the frontal and rear parts of the bolide's surface (low-pressure cavity forms near the rear surface).

Mass-loss of bolides occurs due to heating to a temperature much higher than the melting point. In Khrennikov et al.'s model the main contributor to this heating is the radiant heat transfer between the the bolide and the boundary layer of the shock wave, whose temperature reaches several thousand degrees close to the surface of the bolide. One of the most difficult problems in calculating the radiant heat transfer is the determination of the radiant heat transfer coefficient. Its magnitude is affected by the velocity of motion in the atmosphere, flight altitude, air density, temperature of the boundary layer and the nature of the processes in the boundary layer (dissociation and ionization of air molecules), the degree of blackness of the radiating and absorbing surfaces, etc.

Khrennikov et al.'s model does not involve the process of bolide fragmentation, since the initial dimensions of the bolide are taken to be quite significant (from 50 to 200 m) as well as moderate velocities, when most of the bolide remains intact, despite extreme external influences. First of all, maximum resistance to fragmentation is characteristic of iron bolidess, which is associated with the high homogeneity of their internal structure. In contrast to the iron bolidess, the internal structure of stone and ice bolidess is heterogeneous with an abundance of numerous microcracks. The results of the study of the conditions for the fragmentation of iron bolidess will be presented in the future.

Khrennikov et al. denote the mass-loss by the term ‘ablation’, which includes two processes: The first process is the low-temperature blowing off a liquid film from the bolide's surface (at a temperature about 1000°C) with the formation of small droplets. These droplets are typical for a slow fall of small bolides or their fragments at the final stage of the flight in the atmosphere. The second process is the high-temperature sublimation of material occurring when the surface temperature exceeds several thousand degrees. In this case, a mass-loss occurs in the form of vapours of single atoms and their ions. Under the conditions in consideration, Khrennikov et al.'s model includes the sublimation as a dominant process responsible for the mass-loss at high velocities (over 12 km per second).

As a typical example of Khrennikov et al's calculations, in th case of the trajectory of a spherical iron bolide with a radius of 50 m entering into the atmosphere at 20 km per second when passing through it at the entry angle 11.2° and a minimum altitude of 11 km, the perturbation of the trajectory of the SB deviates it from the initial direction by an angle of 11.25° when neglecting the aerodynamic drag effect and 16.9° when the aerodynamic drag effect is taken into account. These results demonstrate the significant effect of aerodynamic drag on the bolide trajectory.

 

Changes in the trajectory of SB during a through passage via the atmosphere. The bolide parameters are radius 50 m, the velocity of entry into the atmosphere is 20 kmper second, and the minimum altitude is 11 km. Khrennikov et al. (2020).

At present, there are over 100 hypotheses about the nature of the Tunguska phenomenon, among which three to four versions  are predominant theories. They include the fall on to the Earth of a small asteroid measuring several dozen metres, consisting of typical asteroid materials, either metal or stone, as well as ice, which is characteristic of cometary nuclei. The most probable material of the Tunguska bolide mentioned in literature is ice. According to the available observational data, there are several variants of the direction and the trajectory length of the Tunguska bolide, from 450 to 600 km, in particular, with a propagation direction from ‘south–north’ to ‘east–west’. The value of the angle of entry into the atmosphere mentioned in literature is 30°–40°. The radius of the Tunguska bolide was estimated based on the amplitude of the shock wave recorded by the seismic stations and amounted to about 25 m. The minimum trajectory altitude of the Tunguska bolide approximately corresponded to the point of maximum energy release.

The results of comparative calculations of the velocity variations of iron, stone, and ice bolides with radii 100 and 50 m along the trajectory of through passage across the atmosphere for an initial velocity of 20 km per second, suggests that stone bolides lose their velocity faster than iron bolides, and ice bolides do not survive passage through the atmosphere.

Khrennikov et al. calculated the trajectories of bolides with radii of 50 m and iron and stone compositions. Both enter the atmosphere at 160 km, reach a minium altitude of 11 km, then exit the atmosphere at 160 km agian. However, there is a considerable lengthening of trajectory of the stone bolide compared to the iron body. The iron bolide passes through the atmosphere with a minimum loss of velocity and minimum deflection due to a high initialmass, whereas the stone bolide subsequently re-enters the atmosphere due to a significant decrease in velocity. Although quite improbable, such an bolide could manifest itself as a pair of explosive phenomena in the atmosphere separated by thousands of kilometres in distance and tens of minutes in time.

Khrennikov et al. calculated the trajectories of an ice bolide with a radius of 100 m at different entry angles and changes inmass. They found that such a bolide suffered a dramatic loss of mass at angles over 11°. At angle 10°, the initial mass is preserved due to the high altitude, with the bolide remaining over 50 km above the Earth's surface. 

Next Khrennikov et al. calculated the reduction of the masses of ice bolides with radii of 100, 50, and 25 m on the trajectory of collision with the surface of the Earth. The residual fractions of the mass at an initial velocity of 15 km per second were 49 per cent, 21.3 per cent, and 4.8 per cent, respectively, for radii of 100, 50, and 25 m. The length of the trajectory until the moment of the collision with the surface of the Earth is about 325 km for the initial velocity of 15 km per second. At an entry velocity of 25 km per second, for radii of 100 and 50 m, ice bolides fall with a preservation of 6 per cent and 0.000 04 per cent of the initial mass respectively. For radii of 25 m and entry velocity of 25 km per second, an ice bolide loses all its mass completely within a trajectory length of about 329 km.

Of course, the fall of a bolide with preservation of a significant part of the initial mass results in the formation of a crater with a diameter larger than 1 km. However, there are no craters near the epicentre of the Tunguska Event or in the surrounding area. The actual length of the trajectory based on the results of visual observations was estimated to be about 450–700 km, which is over 1.5 times longer than the calculated value for the ice bolide. Therefore, the hypothesis of the ice origin of the Tunguska bolide, which enters the atmosphere at an angle of 30°–40°, is hardly justified accorind to Khrennikov et al.'s model.

Moreover, the decrease in the mass of the ice bolide with an initial radius of 100 m along the trajectory at small angles of entry into the atmosphere, while preserving a significant fraction of mass is possible only at a minimum altitude above 40 km, which contradicts with the estimated minimum altitude of about 10–15 km in the Tunguska event.

Khrennikov et al.'s calculations showed that the trajectory length of the ice bolide when it passes through the atmosphere at a minimum altitude of 15.5 km and small entry angles (less than 15°) until the moment of its complete loss of mass even at a radius of 100 m is two times shorter compared to the case of the iron bolide. Thus, the through passage of the ice bolide at small entry angles with a minimum trajectory altitude 10–15 km is impossible.

For the ice bolide with a radius of 25 m, the length of the trajectory to the moment of the total loss of mass is reduced by four to five times. In addition, it was shown that a considerable part of the initial mass is preserved by iron and stone bolides with radii of 100, 50, and 25 m at an entry angle of 30°. But their fall would be accompanied by the formation of craters with a diameter larger than 1 km and a depth over 200 m.

Khrennikov et al. did not deal with the problem of the formation of a shock wave, although when comparing the Tunguska phenomenon with the Chelyabinsk meteorite with a size of about 10 m and an altitude of maximum energy release of about 30 km, they have no reason to doubt that the body that is 10–20 times larger with an altitude of maximum energy release of 10–15 km at a velocity of 20 km per second will create a shock wave with a huge amplitude and destructive force, capable of causing tree-fall over an area exceeding 1600 km². Experimental modelling of the knock-down effect of a shock wave from the source with cylindrical geometry was performed in a 1966 study. The cylindrical source of the shock wave was modelled by a long detonating cord inclined at a certain angle to a plane planted with small sticks, which imitated trees in the Siberian forest. It was shown that the shape of the area of fallen sticks was similar to the shape of real treefall territory. However, that study did not model the dependence of the strength of the cylindrical shock wave on the height of its source above the ground. Instead, they added a point explosive at the lower end of their cord to model a presumed spherical component of the shock wave. Because rates of the mass and energy losses of the bolide that caused the Tunguska event depend strongly on its altitude above the ground a sharp increase in energy release close to the minimum altitude reached by the through passing bolide can be interpreted as an explosion creating a spherical component of the shock wave. Clearly, making a detailed prediction for the patterns of tree-fall in the framework of our hypothesis of a through-passed bolide as a cause for the Tunguska event will be an important subject of future research.

In solving the main problems in this work, Khrennikov et al. confined themselves to the need to make an upper estimate for calculating the residual mass of space body using the parameters maximising the massloss. They did not consider the problem of the mass-loss of the space body due to its fragmentation. This will be the subject of future research and the results will be published elsewhere.

Based on the obtained results, Khrennikov et al. make the following statements: (i) The conditions for the possible through passage of a large space body composed of various materials across the Earth’s atmosphere with a minimal loss of mass and without collision with the surface of the planet are established. It was shown that this corresponds to the entry angles of space body into the atmosphere of at least 11.5°. (ii) It was shown that the Tunguska space body could hardly consist of ice, since the length of the trajectory of such a body in the atmosphere before the complete loss of its mass would be less than the length of its trajectory estimated on the basis of observational data.  (iii) The value of the angle of entry into the atmosphere of 30°–40° mentioned in the literature for the Tunguska space body looks unrealistic, since it corresponds to the trajectory of a fall of a body with a large residual mass and trajectory length, which is 1.5–2 times shorter than the estimated trajectory length based on the observational data. Such a fall would be accompanied by the formation of a large crater, absent near the epicentre and around. (iv) The most realistic version explaining the Tunguska phenomenon is the through passage of the iron asteroid body as the most resistible to fragmentation across the Earth’s atmosphere at a minimum altitude of 10–15 km with the length of the trajectory in the atmosphere of about 3000 km and a subsequent exit of this asteroid body into the outer space to the near-solar orbit. This version is supported by the fact that there are no remnants of this body and craters on the surface of the Earth. Within this version,  Khrennikov et al. can explain optical effects associated with a strong dustiness of high layers of the atmosphere over Europe, which caused a bright glow of the night sky.

If Khrennikov et al. admit the version of the complete loss of mass of the bolide after the passage of the epicentre or close to it, then the evidence of its reality would be the presence of droplets of meteoric iron of millimetre sizes on the Earth’s surface along the trajectory of the bolide. It follows that the smaller the bolide size and its mass are, the faster it loses a velocity (the amplitude of the shock wave near the epicentre also becomes smaller). Finally, when the velocity of a diminishing bolide reduces to such an extent that its surface temperature approaches 1000°C, the sublimation ceases and the dominant mechanism of mass-loss consists of blowing off a liquid film from the surface of the body. In this case, the bolide becomes the source of a huge amount of droplets, which will be sprayed by the bolide. However, such microformations have not been found despite intensive searches around the epicentre and far beyond. The absence of iron droplets around the epicentre is explained by the high velocity of the bolide during through passage across the Earth’ s atmosphere, always over 11.2 km per second when the surface temperature exceeds several thousands of degree Celcius. The dominant mechanism of mass-loss at these temperatures is the sublimation of material in the form of single atoms, which can be found on the Earth’s surface as iron oxides, which do not differ from the same widespread iron oxides of terrestrial origin.

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