Radiation astronomy/Fieries

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This is a fireball meteor trail with some burning still visible above the Urals city of Chelyabinsk, Russia, on February 15, 2013. Credit: Reuters/www.chelyabinsk.ru.

On the right is a visual astronomy image of a fireball trail with some burning still visible from a meteor as it passed overhead in Chelyabinsk, Russia, on February 15, 2013.

This image shows "[t]he trail of a falling object ... seen above the Urals city of Chelyabinsk [on] February 15, 2013".[1]

Fireballs[edit]

This meteor image of October 17, 2012, is prior to the meteorite fall on the same day. Credit: Paola-Castillo; and Petrus M. Jenniskens, SETI Institute/NASA ARC.
Fireball is over the Bering Sea viewed from space (18 December 2018). Credit: NASA/GSFC/LaRC/JPL-Caltech, MISR Team.

A fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude −4 or greater).[2] The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude −6.[3]

For 2011 there are 4589 fireballs records at the American Meteor Society.[4]

At right is a cell phone camera image of the green fireball over San Mateo, California, that left meteorite fragments. "The asteroid entered at a speed of 14 km/s, typical but on the slow side of other meteorite falls for which orbits were determined. ... The orbit in space is also rather typical: perihelion distance close to Earth's orbit (q = 0.987 AU) and a low-inclination orbit (about 5 degrees). ... 2012, October 17 - At 7:44:29 pm PDT this evening, a bright fireball was seen in the San Francisco Bay Area."[5]

"At 66 kilometers (41 miles) per second, they appear as fast streaks, faster by a hair than their sisters, the Eta Aquarids of May. And like the Eta Aquarids, the brightest of family tend to leave long-lasting trains. Fireballs are possible three days after maximum."[6]

"The NASA All-sky Fireball Network is a network of cameras set up by the NASA Meteoroid Environment Office (MEO) with the goal of observing meteors brighter than the planet Venus, which are called fireballs."[7]

Fireball Sightings reported to the American Meteor Society [8]
Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Number 724 668 941 1,653 2,172 3,556 3,778 4,233 5,371 5,470 4,301[9]

Bolides[edit]

This diagram maps the data gathered from 1994-2013 on small asteroids impacting Earth's atmosphere. Credit: NASA/Planetary Science.

Def. a fireball reaching magnitude −14 or brighter.[10] is called a bolide.

A bolide is a very bright meteor of an apparent magnitude of −14 or brighter. Credit: Thomas Grau.

"This diagram [center] maps the data gathered from 1994-2013 on small asteroids impacting Earth's atmosphere to create very bright meteors, technically called "bolides" and commonly referred to as "fireballs". Sizes of red dots (daytime impacts) and blue dots (nighttime impacts) are proportional to the optical radiated energy of impacts measured in billions of Joules (GJ) of energy, and show the location of impacts from objects about 1 meter (3 feet) to almost 20 meters (60 feet) in size."[11]

"A map released [...] by NASA's Near Earth Object (NEO) Program reveals that small asteroids frequently enter and disintegrate in the Earth's atmosphere with random distribution around the globe. Released to the scientific community, the map visualizes data gathered by U.S. government sensors from 1994 to 2013. The data indicate that Earth's atmosphere was impacted by small asteroids, resulting in a bolide (or fireball), on 556 separate occasions in a 20-year period. Almost all asteroids of this size disintegrate in the atmosphere and are usually harmless. The notable exception was the Chelyabinsk event which was the largest asteroid to hit Earth in this period."[11]

Superbolides[edit]

Footage of a superbolide, a very bright fireball that exploded over Chelyabinsk Oblast, Russia in 2013. Credit: Aleksandr Ivanov.

Def. a fireball reaching an magnitude −17 or brighter is called a superbolide.

Earth-grazing fireballs[edit]

A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball).

Visuals[edit]

A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere.

There are many definitions of a meteor ranging from any atmospheric phenomenon to a fast-moving streak of light in the night sky caused by the entry of extraterrestrial matter into the earth's atmosphere: A shooting star or falling star.

Meteors become visible between about 75 to 120 kilometers (34 - 70 miles) above the Earth. They disintegrate at altitudes of 50 to 95 kilometers (31-51 miles). Most meteors are observed at night, when darkness allows fainter objects to be recognized. Most meteors glow for about a second.

Kamchatka bolide[edit]

The bolide is captured by the Himawari 8 operated by the Japan Meteorological Agency. Credit: Himawari 8 satellite operated by the Japan Meteorological Agency.

Cause was a 10-14-meter (32-45-foot) asteroid[12]
Impact energy: 173 kiloton
Radiated energy: 130 TJ[13]

The Kamchatka bolide was a meteor that exploded in an air burst off the east coast of the Kamchatka Peninsula in eastern Russia on 18 December 2018.[14] At around midday, local time,[15] an asteroid roughly 10 meters in diameter entered the atmosphere at a speed of 32.0 kilometres per second (72,000 mph), with a TNT equivalent energy of 173 kilotons, more than 10 times the energy of the Little Boy bomb dropped on Hiroshima in 1945.[14] The object entered at a steep angle of 7 degrees, close to the zenith, terminating in an air burst at an altitude of around 16 miles (26 km).[14][16]

2008 TC3[edit]

Estimated path and altitude of the meteor in red, with the possible location for the METEOSAT IR fireball (bolide) as orange crosshairs and the infrasound detection of the explosion in green. Credit: George William Herbert (graphic overlay) / US Government (original map).{{free media}}
An animation of 2008 TC3's excited rotation prior to entering the atmosphere is shown. Credit: Astronomical Institute of the Charles University: Josef Ďurech, Vojtěch Sidorin.{{free media}}
Meteosat 8/EUMETSAT infrared image is of the explosion. Credit: EUMETSAT.{{fairuse}}
This webcam frame was shot. Credit: Webcam at kitepower, Mangroovy Beach, El Gouna, Red Sea governate, Egypt.{{free media}}
2008 TC3 fragment was found on February 28, 2009 by Peter Jenniskens, with help from students and staff of the University of Khartoum. Nubian Desert, Sudan. Credit: NASA / SETI / P. Jenniskens.{{free media}}
Meteosat 8 / EUMETSAT visual image is first light flare from 2008 TC3 with lat/long reference. Credit: EUMETSAT [2].{{fairuse}}
Meteosat 8 / EUMETSAT IR image of main fireball from 2008 TC3. Credit: EUMETSAT [3].{{fairuse}}
Meteosat images combined, showing offset from first light flare to main IR flare. Credit: EUMETSAT [4].{{fairuse}}

2008 TC3 (Catalina Sky Survey temporary designation 8TA9D69) was an 80-tonne (80-long-ton; 90-short-ton), 4.1-meter (13 ft) diameter asteroid[17] that entered Earth's atmosphere on October 7, 2008.[18] It exploded at an estimated 37 kilometers (23 mi) above the Nubian Desert in Sudan. Some 600 meteorites, weighing a total of 10.5 kilograms (23.1 lb), were recovered; many of these belonged to a rare type known as ureilites, which contain, among other minerals, nanodiamonds.[17][19][20]

It was the first time that an asteroid impact had been predicted prior to its entry into the atmosphere as a meteor.[21]

The asteroid was discovered by Richard A. Kowalski at the Catalina Sky Survey (CSS) 1.5-meter telescope at Mount Lemmon, north of Tucson, Arizona, US, on October 6, 06:39 UTC, 19 hours before the impact.[22][23][24]

It was notable as the first such body to be observed and tracked prior to reaching Earth.[21] The process of detecting and tracking a near-Earth object, an effort sometimes referred to as Spaceguard, was put to the test. In total, 586 astrometric and almost as many photometric observations were performed by 27 amateur and professional observers in less than 19 hours and reported to the Minor Planet Center, which in eleven hours issued 25 Minor Planet Electronic Circulars with new orbit solutions as observations poured in. On October 7, 01:49 UTC,[24] the asteroid entered the shadow of the Earth, which made further observations impossible.

Impact predictions were performed by University of Pisa's CLOMON 2 semi-automatic monitoring system[25][26] as well as Jet Propulsion Laboratory's Sentry system. Spectral observations that were performed by astronomers at the 4.2-meter William Herschel Telescope at La Palma, Canary Islands are consistent with either a C-type or M-type asteroid.

The meteor entered Earth's atmosphere above northern Sudan at 02:46 UTC (05:46 local time) on October 7, 2008 with a velocity of 12.8 kilometers per second (29,000 mph) at an azimuth of 281 degrees and an altitude angle of 19 degrees to the local horizon. It exploded tens of kilometers above the ground with the energy of 0.9 to 2.1 kilotons of TNT over a remote area of the Nubian Desert, causing a large fireball or bolide.[27]

The meteor's "light was so intense that it lit up the sky like a full moon and an airliner 1,400 km (870 mi) away reported seeing the bright flash."[28] A webcam captured the flash lighting up El-Gouna beach 725 kilometres north of the explosion (see this webcam frame).[29]

"Une webcam de surveillance, située sur la plage de la Mer Rouge à El Gouna en Egypte, a enregistré indirectement le flash de l'explosion qui s'est produit à environ 725 km plus au sud."[29]

A low-resolution image of the explosion was captured by the weather satellite Meteosat 8.[30] The Meteosat images place the fireball at 21°00′N 32°09′E / 21.00°N 32.15°E / 21.00; 32.15 (2008 TC3 fireball).[31] Infrasound detector arrays in Kenya also detected a sound wave from the direction of the expected impact corresponding to energy of 1.1 to 2.1 kilotons of TNT.[32] Asteroids of this size hit Earth about two or three times a year.[33]

The trajectory showed intersection with Earth's surface at roughly 20°18′N 33°30′E / 20.3°N 33.5°E / 20.3; 33.5 (2008 TC3 projected impact)[34] though the object was expected to break up perhaps 100–200 kilometers (60–120 mi) west as it descended, somewhat east of the Nile River, and about 100 kilometers (60 mi) south of the Egypt–Sudan border.

According to U.S. government sources[35][36] U.S. satellites detected the impact at 02:45:40 UT, with the initial detection at 20°54′N 31°24′E / 20.9°N 31.4°E / 20.9; 31.4 (2008 TC3 initial detection) at 65.4 kilometres (40.6 mi) altitude and final explosion at 20°48′N 32°12′E / 20.8°N 32.2°E / 20.8; 32.2 (2008 TC3 final explosion) at 37 kilometres (23 mi) altitude. These images have not been publicly released.

A search of the impact zone that began on December 6, 2008, turned up 10.5 kilograms (23 lb) of rock in some 600 fragments. These meteorites are collectively named Almahata Sitta,[37] which means "Station Six"[38] in Arabic and is a train station between Wadi Halfa and Khartoum, Sudan. This search was led by Peter Jenniskens from the SETI Institute, California and Muawia Shaddad of the University of Khartoum in Sudan and carried out with the collaboration of students and staff of the University of Khartoum. The initial 15 meteorites were found in the first three days of the search. Numerous witnesses were interviewed, and the hunt was guided with a search grid and specific target area produced by NASA's Jet Propulsion Laboratory in Pasadena, California.[39][40][41][42][43]

Samples of the Almahata Sitta meteorite were sent for analysis to a consortium of researchers led by Jenniskens, the Almahata Sitta consortium, including NASA Ames Research Center in California, the Johnson Space Center in Houston, the Carnegie Institution of Washington, and Fordham University in New York City. The first sample measured was an anomalous ultra-fine-grained porous polymict ureilite achondrite, with large carbonaceous grains. Reflectance spectra of the meteorite, combined with the astronomal observations, identified asteroid 2008 TC3 as an F-type asteroid class. These fragile anomalous dark carbon-rich ureilites are now firmly linked to the group of F-class asteroids.[17] Amino acids have been found on the meteorite.[44] The nanodiamonds found in the meteorite were shown to have grown slowly, implying that the source is another planet in the solar system.[45]

Richard Kowalski, who discovered the object, received a tiny fragment of Almahatta Sitta, a gift from friends and well-wishers on the Minor Planet Mailing List, which Kowalski founded in order to help connect professional and amateur astronomers.[46]

2015 Thailand bolide[edit]

On September 7, 2015, at about 08:40 local time a bolide meteor appeared over Thailand and burned up approximately 100 km (62 mi) above the ground.[47] The meteor briefly flared up producing a green and orange glow before disappearing without a sound of explosion and leaving a white smoke trail. The meteor was recorded by several dashcams during the morning rush hour in Bangkok,[47] and sightings were also reported in Thai towns of Kanchanaburi and Nakhon Ratchasima.[48] The meteor was visible for about four seconds before fading out. As of September 8, 2015 no strewn field has been found. The impact energy was the largest of 2015 at 3.9 TNT equivalent (kiloton).[13] The last impact this large was on 23 August 2014 over the Southern Ocean.[13]

Sound from the meteor was reported in three districts of Kanchanaburi Province: Thong Pha Phum, Sai Yok and Si Sawat.[49] Governor of Kanchanaburi Province Wan-chai Osukhonthip ordered police and Sai Yok National Park rangers to search Wang Krachae and Bong Ti subdistricts in Sai Yok District for meteor debris.[49]

2009 Sulawesi superbolide[edit]

The 2009 Sulawesi superbolide was an atmospheric fireball blast over Indonesia on October 8, 2009, at approximately 03:00 UTC, near the coastal city of Watampone (colloquially named Bone) in South Sulawesi, island of Sulawesi. The meteoritic impactor broke up at an estimated height of 15–20 km. The impact energy of the bolide was estimated in the 10 to 50 kiloton TNT equivalent range with the higher end of this range being more likely. The likely size of the impactor was 5–10 m diameter.[50][51][52][53]

Earth-grazing meteoroid of 13 October 1990[edit]

All-sky photo with the Earth-grazing meteoroid of 13 October 1990 (the faint near-vertical track just to the right of the pole star) taken at Červená hora, Czechoslovakia. Credit: .
Part of the track of the meteoroid above Czechoslovakia and Poland that was captured by European Fireball Network cameras. Credit: .
Orbit of the meteoroid before and after grazing Earth's atmosphere is shown. Credit: .

The event started at 03:27:16±3 UT[note 1] and the observed bright meteor (fireball) moved from south to north. It left a track that was visible for 10 seconds.[54]

Most data about the encounter was acquired using photographic observations by cameras of the European Fireball Network. It was the first event of this type recorded by cameras from two distant locations, at Červená hora and Svratouch (both in the present-day Czech Republic), which enabled the calculation of the meteoroid's orbital characteristics by geometrical methods.[54] Both were equipped with all-sky fisheye objectives.[54]

The Červená hora image was especially valuable. It recorded the fireball's trajectory over approximately 110°, starting 51° above the southern horizon, passing the zenith just 1° westward and disappearing only 19° above the northern horizon (thus crossing about 60% of the sky). Its camera was also equipped with a rotating shutter that interrupted the exposure 12.5 times per second and divided the captured track of the fireball, allowing the determination of its speed. Over the last 4°, the fireball's angular velocity was lower than the resolution of the instrument.[54] The Svratouch image recorded the trajectory only for about 15°, beginning at 30° above the northwest horizon, and the pictured fireball was quite weak. Despite this, the data was sufficient for the calculations.[54]

Gotfred M. Kristensen also detected the fireball in Havdrup, Denmark, using a pen recorder connected to a radio receiver for 78 seconds, at 03:27:24±6 UT.[note 1][55]

The meteoroid grazed Earth's atmosphere quite gently (in comparison to, for example, the 1972 Great Daylight Fireball above the United States and Canada). It became visible north of Uherský Brod, Czechoslovakia, at a height of 103.7 km, approaching the Earth's surface to 98.67 km[note 2] northeast of Wrocław, Poland, and disappearing from sight at a height of 100.4 km north of Poznań, Poland. It would probably still have been visible until it reached a height of 110 km above the southern Baltic Sea.[54]

The meteoroid's absolute magnitude (the apparent magnitude it would have at an altitude of 100 km at the observer's zenith) was approximately −6 and did not vary significantly during the few seconds of observation. It traveled a distance of 409 km in 9.8 seconds during the time it was observed. It moved at a speed of 41.74 km per second, which did not change measurably during the flight.[57] Jiří Borovička and Zdeněk Ceplecha from the Ondřejov Observatory in Czechoslovakia estimated that the deceleration caused by the friction of the atmosphere reached only 1.7 m/s2 at the fireball's perigee (closest approach to Earth), and its velocity was reduced by only 0.012 km per second (less than 0.03%).[54] This corresponds well with computer simulations provided by D. W. Olson, R. L. Doescher and K. M. Watson at the Southwest Texas State University, who concluded that the deceleration was less than 0.5 m/s2 except for a few seconds near perigee.[58]

The meteoroid was a type I fireball,[54] i.e. an ordinary chondrite.[59] When it entered Earth's atmosphere its mass was about 44 kg, which was estimated on the basis of the measured values of its absolute magnitude and velocity. It lost approximately 350 g during the encounter.[54] Computer simulations showed that it started losing mass approximately at the moment it became visible to the cameras of the European Fireball Network, at a height of 100.6 km. It lost mass for 25 seconds, until it reached a height of 215.7 km.[58] Its surface melted and solidified again after leaving,[54] which means its surface became a typical meteoritic fusion crust.[60]

The meteoroid was not dangerous to life on Earth. Even if it had headed towards lower parts of the atmosphere it would have heated so much that it would have exploded high above the ground and only some small particles (meteorites) eventually might have made it to Earth's surface.[61]

Because the fireball was recorded by two cameras of the European Fireball Network, it was possible to calculate the trajectory of its flight through the atmosphere, and afterward also the characteristics of both its pre- and post-encounter orbit in the Solar System.[54] The calculations were published by Czech astronomers Pavel Spurný, Zdeněk Ceplecha, and Jiří Borovička of the Ondřejov Observatory,[60][54][57] who specialize in meteor observations. They demonstrated that the swing by the earth significantly altered the meteoroid's orbit. Its aphelion (the farthest it travels from the Sun) and orbital period were lowered to almost half of their original values.[57] The object was initially in a highly inclined orbit (71°) and ended in an orbit with a slightly higher inclination (74°).

1972 Great Daylight Fireball[edit]

The Great Daylight Fireball (or US19720810) was an Earth-grazing fireball that passed within 57 kilometres (35 mi; 187,000 ft) of Earth's surface at 20:29 UTC on August 10, 1972. It entered Earth's atmosphere at a speed of 15 kilometres per second (9.3 mi/s)[62] in daylight over Utah, United States (14:30 local time) and passed northwards leaving the atmosphere over Alberta, Canada. It was seen by many people and recorded on film and by space-borne sensors.[63]

"It was first detected by satellite at an altitude of about 73 km, tracked as it descended to about 53 km, and then tracked as it climbed back out of the atmosphere".[63]

"object is still in an Earth-crossing orbit around the Sun and passed close to the Earth again in August 1997".[63]

The atmospheric pass modified the object's mass and orbit around the Sun, but it is probably still in an Earth-crossing orbit and passed close to Earth again in August 1997.[63]

Analysis of its appearance and trajectory showed the object was about 3–14 m (10–45 ft) in diameter, depending on whether it was a comet made of ices, or a carbonaceous chondrite (stony) and therefore denser asteroid.[62][64] Other sources identified it as an Apollo asteroid in an Earth-crossing orbit that would make a subsequent close approach to Earth in August 1997.[63] In 1994, Czech astronomer Zdeněk Ceplecha reanalysed the data and suggested the passage would have reduced the asteroid's mass to about a third or half of its original mass (reducing its diameter to 2–10 metres (6.6–32.8 ft).)[64]

The object was tracked by military surveillance systems and sufficient data obtained to determine its orbit both before and after its 100-second passage through Earth's atmosphere. Its velocity was reduced by about 800 metres per second (2,600 ft/s) and the encounter significantly changed its orbital inclination from 15 degrees to 7 degrees.[62]

1947 Sikhote-Alin bolide[edit]

The 10th anniversary stamp reproduces a painting. Credit: Pyotr Ivanovich Medvedev.

On November 20, 1957[65] the Soviet Union issued a stamp for the 10th anniversary of the Sikhote-Alin meteorite shower. It reproduces a painting by Pyotr Ivanovich Medvedev, a Soviet artist who witnessed the fall: he was sitting in his window starting a sketch when the fireball appeared, so he immediately began drawing what he saw.[66]

An iron meteorite fell on the Sikhote-Alin Mountains, in southeastern Russia, in 1947. Though large iron meteorite falls had been witnessed previously and fragments recovered, never before in recorded history had a fall of this magnitude been observed.[67] An estimated 23 tonnes[68] of fragments survived the fiery passage through the atmosphere and reached the Earth.

1930 Curuçá River event[edit]

Most values for the 1930 Curuçá River event put it well below 1 megaton.[69][70][71]

The 1930 Curuçá River event refers to the possible fall of objects on 13 August 1930 over the area of Curuçá River in Brazil.[72][73]

1913 Great Meteor Procession[edit]

Painting is of the 1913 Great Meteor Procession. Credit: Gustav Hahn.

The 1913 Great Meteor Procession occurred on February 9, 1913.[74][75] It was a unique meteoric phenomenon reported from locations across Canada, the northeastern United States, and Bermuda, and from many ships at sea, including eight off Brazil, giving a total recorded ground track of over 7,000 miles (11,000 km).[76][77][78] The meteors were particularly unusual in that there was no apparent radiant, that is to say, no point in the sky from which the meteors appeared to originate. The observations were analysed in detail, later the same year, by the astronomer Clarence Chant, leading him to conclude that as all accounts were positioned along a great circle arc, the source had been a small, short-lived natural satellite of the Earth.[79][80]

The evening of February 9 was cloudy across much of the densely populated northeast United States, meaning that some 30 million potential observers were for the most part unaware of the phenomenon.[81] Nevertheless, more than a hundred individual reports – largely from more remote areas of Canada – were later collected by Clarence Chant, with additional observations unearthed by later researchers.[80] At around 21hr Eastern Standard Time Zone (EST), witnesses were surprised to see a procession of between 40 and 60 bright, slow-moving fireballs moving from horizon to horizon in a practically identical path.[76] Individual fireballs were visible for at least 30 to 40 seconds, and the entire procession took some 5 minutes to cross the sky. An observer at Appin, Ontario, described its appearance at one of the most easterly parts of its track across Canada:

"A huge meteor appeared travelling from northwest by west to southeast, which, as it approached, was seen to be in two parts and looked like two bars of flaming material, one following the other. They were throwing out a constant stream of sparks and after they had passed they shot out balls of fire straight ahead that travelled more rapidly than the main bodies. They seemed to pass over slowly and were in sight about five minutes. Immediately after their disappearance in the southeast a ball of clear fire, that looked like a big star, passed across the sky in their wake. This ball did not have a tail or show sparks of any kind. Instead of being yellow like the meteors, it was clear like a star."[80]

Subsequent observers also noted a large, white, tail-less body bringing up the rear, but the various bodies making up the meteor procession continued to disintegrate and to travel at different rates throughout their course, so that by the time observations were made in Bermuda, the leading bodies were described as "like large arc lights in appearance, slightly violet in colour", followed closely by yellow and red fragments.[82]

At Escanaba, Michigan, the Daily Press stated the "end of the world was apprehended by many" as numerous meteors travelled across the northern horizon.[83] In Batavia, New York, a few observers saw the meteors and many people heard a thundering noise, while other reports were made in Nunda-Dansville, Livingston County, New York (where several residents again thought the world was ending) and Osceola, Pennsylvania.[84]

One observer, an A. W. Brown from Thamesville, Ontario, reported seeing both the initial meteor procession and a second one on the same course at 02:20 the next morning.[85] Chant's original report also referred to a series of three groups of "dark objects" which passed, on the same course as the previous meteors, from west to east over Toronto on the afternoon of February 10, which he suggested were "something of a meteoric nature".[85]

The orbit was later discussed by Pickering and G. J. Burns, who concluded that it was essentially satellitic.[86]

The meteors had most likely represented a body, or group of bodies, which had been temporarily captured into orbit about the Earth before disintegrating.[86]

The meteors, which he referred to as the "Cyrillids", could have in fact represented the last remnant of a circumterrestrial planetary ring, formed from the ejecta of a postulated lunar volcano.[79] This theory was a development of O'Keefe's unusual hypothesis on the origin of tektites.[87]

1860 Great Meteor[edit]

Oil painting is of the 1860 Great Meteor. Credit: Frederic Church.

The 1860 Great Meteor procession occurred on July 20, 1860. It was a unique meteoric phenomenon reported from locations across the United States.[88][89] American landscape painter Frederic Church saw and painted a spectacular string of fireball meteors cross the Catskill evening sky, an extremely rare Earth-grazing meteor procession.[90][91] It is believed that this was the event referred to in the poem Year of Meteors, 1859-60, by Walt Whitman.[92][93] In 2010, 150 years later, it was determined to be an Earth-grazing meteor procession.[94]

1783 Great Meteor[edit]

The 1783 Great Meteor, is seen from the East Angle of the North Terrace, Windsor Castle, original watercolour. Credit: Paul Sandby.

The event occurred between 21:15 and 21:30 on 18 August 1783, a clear, dry night. Analysis of observations has indicated that the meteor entered the Earth's atmosphere over the North Sea, before passing over the east coast of Scotland and England and the English Channel; it finally broke up, after a passage within the atmosphere of around a thousand miles (1610 km), over south-western France or northern Italy.[95]

"Some flashes of lambent light, much like the aurora borealis, were first observed on the northern part of the heavens, which were soon perceived to proceed from a roundish luminous body, whose apparent diameter equaled half that of the moon, and almost stationary in the same point of the heavens [...] This ball at first appeared of a faint bluish light, perhaps from appearing just kindled, or from its appearing through the haziness; but it gradually increased its light, and soon began to move, at first ascending above the horizon in an oblique direction towards the east. Its course in this direction was very short, perhaps of five or six degrees; after which it directed its course towards the east [...] Its light was prodigious. Every object appeared very distinct; the whole face of the country, in that beautiful prospect before the terrace, being instantly illuminated."[96]

Earth crossers[edit]

The close approach of apollo asteroid 2007 VK184 was in May 2014. Credit: Osamu Ajiki (AstroArts) and Ron Baalke (JPL).

EC denotes Earth-crossing.[97]

"50 % of the MB Mars-crossers [MCs] become ECs within 59.9 Myr and [this] contribution ... dominates the production of ECs".[97]

Apollo asteroids[edit]

This a diagram showing the Apollo asteroids, compared to the orbits of the terrestrial planets of the Solar System.
  Mars (M)
  Venus (V)   Mercury (H)
  Sun
  Apollo asteroids
  Earth (E)
Credit: AndrewBuck.
Photograph of the full disc of the asteroid 162173 Ryugu, as it appeared to the Hayabusa2 spacecraft's Optical Navigation Camera – Telescopic (ONC-T) at a distance of 20 kilometres (12 miles) at 03:50 UTC on 26 June 2018. Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.{{fairuse}}
Asteroid Bennu imaged by the OSIRIS-REx probe on arrival 3 December 2018. Credit: NASA/Goddard/University of Arizona.{{free media}}
Photo of 101955 Bennu was taken by the OSIRIS-REx probe on 3 December 2018. Credit: NASA/Goddard/University of Arizona.

Note that sizes and distances of bodies and orbits are not to scale in the image on the right.

As of 2015, the Apollo asteroid group includes a total of 6,923 known objects of which 991 are numbered (JPL SBDB).

Ryugu shown on the left was discovered on 10 May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States.[98]

The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015.[99]

Initial images taken by the Hayabusa-2 spacecraft on approach at a distance of 700 km were released on 14 June 2018 and revealed a diamond shaped body and confirmed its retrograde rotation.[100]

Between 17 and 18 June 2018, Hayabusa 2 went from 330 km to 240 km from Ryugu and captured a series of additional images from the closer approach.[101]

On 21 September 2018, the first two of these rovers, which will hop around the surface of the asteroid, were released from Hayabusa2.[102]

On September 22, 2018, JAXA confirmed the two rovers had successfully touched down on Ryugu's surface which marks the first time a mission has completed a successful landing on a fast-moving asteroid body.[103]

"This series of images [second down on the right] taken by the OSIRIS-REx spacecraft shows Bennu in one full rotation from a distance of around 50 miles (80 km). The spacecraft’s PolyCam camera obtained the 36 2.2-millisecond frames over a period of four hours and 18 minutes."[104]

101955 Bennu (provisional designation 1999 RQ36[105], a C-type carbonaceous asteroid in the Apollo group discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) Project on September 11, 1999, is a potentially hazardous object that is listed on the Sentry monitoring system, Sentry Risk Table, with the second-highest cumulative rating on the Palermo Technical Impact Hazard Scale.[106] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.[107][108]

101955 Bennu has a mean diameter of approximately 492 m (1,614 ft; 0.306 mi) and has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Communications Complex NASA Deep Space Network.[109][110][111]

Asteroid Bennu has a roughly spheroidal shape, resembling a spinning top, with the direction of rotation about its axis retrograde with respect to its orbit and a fairly smooth shape with one prominent 10–20 m boulder on its surface, in the southern hemisphere.[108]

There is a well-defined ridge along the equator of asteroid Bennu that suggests that fine-grained regolith particles have accumulated in this area, possibly because of its low gravity and fast rotation.[108]

Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of 484±10 m, which is in line with other studies. It has a low visible geometric albedo of 0.046±0.005. The thermal inertia was measured and found to vary by ±19% during each rotational period suggesting that the regolith grain size is moderate, ranging from several millimeters up to a centimeter, and evenly distributed. No emission from a potential dust coma has been detected around asteroid Bennu, which puts a limit of 106 g of dust within a radius of 4750 km.[112]

Astrometric observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the semimajor axis to drift on average by 284±1.5 meters/year; analysis of the gravitational and thermal effects give a bulk density of ρ = 1,260±70 kg/m3, which is only slightly denser than water, the predicted macroporosity is 40±10%, suggesting that the interior has a rubble pile structure, with an estimated mass is 7.8±0.9×1010
 kg
.[113]

Photometric observations of Bennu in 2005 yielded a synodic rotation period of 4.2905±0.0065 h, a B-type asteroid classification, which is a sub-category of C-type asteroid or carbonaceous asteroids. Polarimetric observations show that Bennu belongs to the rare F-type asteroid or F subclass of carbonaceous asteroids, which is usually associated with cometary features.[114] Measurements over a range of phase angles show a phase function slope of 0.040 magnitudes per degree, which is similar to other near-Earth asteroids with low albedo.[115]

Asteroid Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation into more complex minerals.[108] Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km, where simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the 495 Eulalia (Eulalia family).[116]

Subsequently, the orbit drifted as a result of the Yarkovsky effect and mean motion resonances with the giant planets, such as Jupiter and Saturn modified the asteroid, possibly changing its spin, shape, and surface features.[117]

A possible cometary origin for Bennu, based on similarities of its spectroscopic properties with known comets, with the estimated fraction of comets in the population of Near Earth asteroids is 8±5 %.[114]

Eta Aquariids[edit]

Animation is of 1P/Halley orbit - 1986 apparition.   1P/Halley   Earth   Sun. Credit: Phoenix7777.

The current orbit of Halley's Comet does not pass close enough to the Earth to be a source of meteoric activity.[118]

The shower is best viewed from the equator to 30 degrees south latitude.[118]

The meteoroids are from very old ejection from the parent 1P/Halley and are trapped probably in resonances to Jupiter's orbit (similar to the Orionids observed between 2007 and 2010).[119]

The peak ZHR reached 135 ± 16.[120] Updated information on the expected time and rates of the shower is provided through the annual IMO Meteor Shower Calendar.[119]

"At 66 kilometers (41 miles) per second, they appear as fast streaks, faster by a hair than their sisters, the Eta Aquarids of May. And like the Eta Aquarids, the brightest of family tend to leave long-lasting trains. Fireballs are possible three days after maximum."[6]

Geminids[edit]

A Geminid meteor in 2007, seen from San Francisco. Credit: Brocken Inaglory.
Geminid meteors clearly show the position of the radiant. Credit: Berkó Ernő.
A bolide from the Geminids meteor shower (Special Astrophysical Observatory of the Russian Academy of Science (SAO RAS), vmag  −3) in December 2010. Credit: Участник:S. Korotkiy.

The Geminids are a prolific meteor shower caused by the object 3200 Phaethon,[121] which is thought to be a Palladian asteroid[122] with a "rock comet" orbit.[123]

Lyrids[edit]

Radiant point of the April Lyrid meteor shower is shown, active each year around April 22. Credit: NASA/Don Pettit.
On the night of April 21, the 2012 Lyrid meteor shower peaked in the skies over Earth. While NASA allsky cameras were looking up at the night skies, astronaut Don Pettit aboard the International Space Station trained his video camera on Earth below. Credit: NASA/Don Pettit.{{free media}}

The April Lyrids (LYR, IAU shower number 6)[124] is a meteor shower lasting from April 16 to April 26[125]

The source of the meteor shower is particles of dust shed by the long-period Comet C/1861 G1 Thatcher.[126]

The Lyrids have been observed and reported since 687 BC; no other modern shower has been recorded as far back in time.[127]

The shower usually peaks on around April 22 and the morning of April 23. Counts typically range from 5 to 20 meteors per hour, averaging around 10.[125]

April Lyrid meteors are usually around magnitude +2. However, some meteors can be brighter, known as "Lyrid fireballs", cast shadows for a split second and leave behind smokey debris trails that last minutes.[128]

Occasionally, the shower intensifies when the planets steer the one-revolution dust trail of the comet into Earth's path, an event that happens about once every 60 years.[126]

The one-revolution dust trail is dust that has completed one orbit: the stream of dust released in the return of the comet prior to the current 1862 return. This mechanism replaces earlier ideas that the outbursts were due to a cloud of dust moving in a 60-year orbit.[129]

In 1982, amateur astronomers counted 90 April Lyrids per hour at the peak and similar rates were seen in 1922. A stronger storm of up to 700 per hour occurred in 1803,[130] observed by a journalist in Richmond, Virginia:

"Shooting stars. This electrical [sic] phenomenon was observed on Wednesday morning last at Richmond and its vicinity, in a manner that alarmed many, and astonished every person that beheld it. From one until three in the morning, those starry meteors seemed to fall from every point in the heavens, in such numbers as to resemble a shower of sky rockets ...[128]"[130]

The oldest known outburst, the shower on March 23.7,[131] 687 BC (proleptic Julian calendar) was recorded in Zuo Zhuan, which describes the shower as "On the 4th month in the summer in the year of Sexagenary cycle (xīn-mǎo) (of year 7 of King Zhuang of the State of Lu), at night, (the sky is so bright that some) fixed stars become invisible (because of the meteor shower); at midnight, stars fell like rain."[132] In the Australian Aboriginal astronomy of the Boorong tribe, the Lyrids represent the scratchings of the Mallee fowl (represented by Vega), coinciding with its nest-building season.[133]

Perseids[edit]

Perseid meteor shower is from September 6 and 7, 1880-81. Credit: unknown.{{free media}}
A Perseid shower occurs in 2007. Credit: Brocken Inaglory.
Animation of 109P/Swift–Tuttle orbit from 1875 to 2100.
   Sun ·   Earth ·    Jupiter  ·   Saturn ·   Uranus ·   109P/Swift–Tuttle. Credit: Phoenix7777.{{free media}}
Radiant point is from August 8, 2006. Credit: Olga Berrios.{{free media}}

In 1835, Adolphe Quetelet identified the shower as emanating from the constellation Perseus.[134][135]

Right ascension = 03h 04m[136] and Declination = +58°[136]

The Perseid meteor shower, usually the richest meteor shower of the year, peaks in August. Over the course of an hour, a person watching a clear sky from a dark location might see as many as 50-100 meteors. Parent body is Comet Swift–Tuttle.[136] The first record is from 36 CE.[134][135]

The radiant point image on the right is from September 6 and 7, 1880-81.[137]

Velocity = 58 km/s[138] and Zenithal hourly rate = 100[136].

The stream of debris is called the Perseid cloud and stretches along the orbit of the comet Swift–Tuttle. The cloud consists of particles ejected by the comet as it travels on its 133-year orbit.[139] Most of the particles have been part of the cloud for around a thousand years. However, there is also a relatively young filament of dust in the stream that was pulled off the comet in 1865, which can give an early mini-peak the day before the maximum shower.[140] The dimensions of the cloud in the vicinity of the Earth are estimated to be approximately 0.1 AU across and 0.8 AU along the latter's orbit, spread out by annual interactions with the Earth's gravity.[141]

The shower is visible from mid-July each year, with the peak in activity between 9 and 14 August, depending on the particular location of the stream. During the peak, the rate of meteors reaches 60 or more per hour. They can be seen all across the sky; however, because of the shower's radiant in the constellation of Perseus, the Perseids are primarily visible in the Northern Hemisphere.[142] As with many meteor showers the visible rate is greatest in the pre-dawn hours, since more meteoroids are scooped up by the side of the Earth moving forward into the stream, corresponding to local times between midnight and noon, as can be seen in the accompanying diagram.[143] While many meteors arrive between dawn and noon, they are usually not visible due to daylight. Some can also be seen before midnight, often grazing the Earth's atmosphere to produce long bright trails and sometimes fireballs. Most Perseids burn up in the atmosphere while at heights above 80 kilometres (50 mi).[144]

Northern Taurids[edit]

11/6/2015 - Image shows the Taurid Meteor Shower - Joshua Tree , CA. Credit: Channone Arif.{{free media}}

Parent body = 2004 TG10[145][136]

Radiant point = RA 03h 52m Dec = +22°.[146]

Occurs during October 20 – December 10, with a peak at 12 November.[146]

Velocity = 29 km/s.[146]

Zenithal hourly rate is 5.[146]

The Northern Taurids originated from the asteroid 2004 TG10.[147]

The Taurids are also made up of weightier material, pebbles instead of dust grains.[148]

Typically, Taurids appear at a rate of about 5 per hour, moving slowly across the sky at about 28 km/s (17 mi/s), or 100,800 km/h (65,000 mph).[148] If larger than a pebble, these meteors may become bolides as bright as the moon and leave behind smoke trails.[148]

The Beta Taurids could be the cause of the Tunguska event of June 30, 1908.[149]

In 1962 and 1963, the Mars 1 probe recorded one micrometeorite strike every two minutes at altitudes ranging from 6,000 to 40,000 km (3,700 to 24,900 mi) from Earth's surface due to the Taurids meteor shower, and also recorded similar densities at distances from 20 to 40 million kilometres (12,000,000 to 25,000,000 mi) from Earth.[150][151]

The Taurid stream has a cycle of activity that peaks roughly every 2,500 to 3,000 years,[149] when the core of the stream passes nearer to Earth and produces more intense showers. In fact, because of the separate "branches" (night-time in one part of the year and daytime in another; and Northern/Southern in each case) there are two (possibly overlapping) peaks separated by a few centuries, every 3000 years. The next peak is expected around 3000 AD.[149]

Over Poland in 1995, all-sky cameras imaged an absolute magnitude –17 Taurid bolide that was estimated to be 900 kg and perhaps a meter in diameter.[152]

In 1993, it was predicted that there would be a swarm of activity in 2005.[148] Around Halloween in 2005, many fireballs were witnessed that affected people's night vision.[148] Astronomers have taken to calling these the "Halloween fireballs."[148] The Tunguska event may have been caused by a Beta Taurid.[153]

A brief flash of light from a lunar impact event was recorded on November 7, 2005, while testing a new 250 mm (10 in) telescope and video camera built to monitor the Moon for meteor strikes.[154] This may be the first photographic record of such a strike.[155]

Southern Taurids[edit]

During the Southern Taurid meteor shower in 2013, fireball sightings were spotted over southern California, Arizona, Nevada, and Utah.[156]

The Southern Taurids originated from Comet Encke, while the Northern Taurids originated from the asteroid 2004 TG10.[157]

Encke and the Taurids are believed to be remnants of a much larger comet, which has disintegrated over the past 20,000 to 30,000 years.[158]

Wildfires[edit]

The European Space Agency’s Copernicus Sentinel-2 satellite snapped this image July 27 of wildfires near the Mackenzie River in Canada’s Northwest Territories. Credit: Pierre Markuse.{{fairuse}}
The European Space Agency’s Copernicus Sentinel-3 satellite snapped this image August 1 of burn scars from a recent fire and smoke from an active fire on the west Greenland coast. Credit: Pierre Markuse.{{fairuse}}
Inpe said it had detected more than 74,000 fires between January and August - the highest number since records began in 2013. Credit: Reporter, Reuters.{{fairuse}}
On November 5, 2019, the Operational Land Imager (OLI) on Landsat 8 acquired this natural-color image of fire and smoke in southern Western Australia. Credit: Lauren Dauphin.{{fairuse}}

"Record-breaking temperatures and strong winds are fueling an unprecedented number of wildfires across the region this summer. In Siberia alone, hundreds of wildfires captured by satellite images July 28 spanned about 3 million hectares of land. Across Alaska, as many as 400 wildfires were burning as of mid-July."[159]

"The scale and intensity of the June 2019 wildfires are unparalleled in the 16 years that the European Union’s Copernicus Atmosphere Monitoring Service, or CAMS, has been tracking global wildfire data."[159]

July’s numbers "have been of similar proportions. I’ve been surprised at the duration of the fires in the Arctic Circle, in particular."[160]

"Wildfires most often occur in the Arctic in July and August, sparked by lightning strikes. But this year, unusually hot and dry conditions in the Northern Hemisphere in June exacerbated the problem and drove the fire season’s start earlier."[161]

"Unusually high temperatures and low precipitation in the region were almost certainly fueling the July wildfires as well. In early August, CAMS will release its monthly bulletin summarizing the July data and I wouldn’t be surprised if the July fires correspond to [those climate] anomalies."[160]

"In Alaska, a heat record toppled July 4, with temperatures reaching as high as 32.2° Celsius (90° Fahrenheit). Average June temperatures in parts of Siberia were almost 10 degrees higher than the average temperatures from 1981 to 2010. That same month, more than 100 intense wildfires were burning within the Arctic Circle."[159]

"On August 1, the Copernicus Sentinel-3 satellite took this image [on the left] of western Greenland. To the left of the island’s ice sheet, an active wildfire burns."[159]

"Layers of black charcoal in sediments in the Canadian Arctic suggest that wildfires frequently raged across the region during the Pliocene Epoch, when global atmospheric CO₂ levels were between 350 and 450 parts per million — similar to today [...]. In June, CO₂ levels averaged 413.92 ppm, according to data collected at the Mauna Loa Observatory in Hawaii."[159]

"Brazil's Amazon rainforest has seen a record number of fires this year [...] The National Institute for Space Research (Inpe) said its satellite data showed an 84% increase on the same period in 2018."[162]

"The largest rainforest in the world, the Amazon is a vital carbon store that slows down the pace of global warming."[162]

"Inpe said it had detected more than 74,000 fires between January and August - the highest number since records began in 2013. It said it had observed more than 9,500 forest fires since Thursday, mostly in the Amazon region."[162]

"In comparison, there are slightly more than 40,000 in the same period of 2018, it said. However, the worst recent year was 2016, with more than 68,000 fires in that period."[162]

"The satellite images showed Brazil's most northern state, Roraima, covered in dark smoke, while neighbouring Amazonas declared an emergency over the fires."[162]

"There is nothing abnormal about the climate this year or the rainfall in the Amazon region, which is just a little below average."[163]

"The dry season creates the favourable conditions for the use and spread of fire, but starting a fire is the work of humans, either deliberately or by accident."[163]

On the lower left is a Landsat 8 image of fires in southwestern Australia. "On November 5, 2019, the Operational Land Imager (OLI) on Landsat 8 acquired this natural-color image of fire and smoke in southern Western Australia. The fire burned in the Goldfields region, about 230 kilometers northeast of Norseman."[164]

"The fire follows what has been a dry winter and spring, leaving vegetation prone to burning and the fire danger high. Experts say the dryness is a result of the Indian Ocean Dipole, which can affect rainfall and temperatures across Australia."[164]

Volcanic eruptions[edit]

Eruption of Stromboli (Isole Eolie/Italia), ca. 100m (300ft) vertically. Exposure of several seconds. The dashed trajectories are the result of lava pieces with a bright hot side and a cool dark side rotating in mid-air. Credit: Wolfgang Beyer.{{free media}}

Strombolian eruptions consist of ejection of incandescent cinders, lapilli, and lava bombs, to altitudes of tens to a few hundreds of metres. The eruptions are small to medium in volume, with sporadic violence.

The gas coalesces into bubbles, called gas slugs, that grow large enough to rise through the magma column, bursting near the top due to the decrease in pressure and throwing magma into the air. Each episode thus releases volcanic gases, sometimes as frequently as a few minutes apart. Gas slugs can form as deep as 3 kilometers, making them difficult to predict.[165][166]

Lava flows[edit]

An 'a'ā lava flow from Mauna Loa during its 1984 eruption. Credit: R.W. Decker, USGS.
Glowing `a`a flow front advances over pahoehoe on the coastal plain of Kilauea Volcano, Hawai`i. Credit: USGS.
Advancing Pahoehoe toe, Kilauea Hawaii 2003, results when Kohola breakouts. Credit: Hawaii Volcano Observatory (DAS).
This Pahoehoe is from Kīlauea in Hawaii. Credit: Tari Noelani Mattox, USGS.

An effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. There are two major groupings of eruptions: effusive and explosive.[167] Effusive eruption differs from explosive eruption, wherein magma is violently fragmented and rapidly expelled from a volcano. Effusive eruptions are most common in basaltic magmas, but they also occur in intermediate and felsic magmas. These eruptions form lava flows and lava domes, each of which vary in shape, length, and width.[168]

Def. a "form of lava flow [associated with Hawaiian-type volcanoes, consisting][169] of basaltic rock, usually dark-colored with a jagged and loose, clinkery surface"[170] is called an aa (fifth image down on the right).

"`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified `a`a flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an `a`a flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an `a`a flow."[171]

Def. a "form of lava flow of basaltic rock, usually dark-colored with a smooth or ropey surface"[172] is called pahoehoe.

"Ropy pahoehoe [shown in the third image down on the right, taken 11 June 1995] is the most common surface texture of pahoehoe flows. The numerous folds and wrinkles ("ropes") that are characteristic of ropy pahoehoe form when the thin, partially solidified crust of a flow is slowed or halted (for example, if the crust encounters an obstruction or slower-moving crust). Because lava beneath the crust continues to move forward, it tends to drag the crust along. The crust then behaves like an accordian that is squeezed together--the crust is flexible enough to develop wrinkles or a series of small ridges and troughs as it is compressed and driven forward."[173]

Lava fountains[edit]

Lava fountain erupting from the Pu`u `O`o cinder and spatter cone on Kilauea Volcano, Hawai`i, 1983. Credit: USGS Hawaiian Volcano Observatory.{{free media}}

On the right: "A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is called a lava fountain. Lava fountains typically range from about 10 to 100 m in height, but occasionally reach more than 500 m. Lava fountains erupt from isolated vents, along fissures, within active lava lakes, and from a lava tube when water gains access to the tube in a confined space [...]."[174]

"Kīlauea's current eruption is a natural laboratory for volcanologists Tephra falling from a lava fountain on September 6, 1983, helped build the Pu‘u ‘Ō‘ō cone, which eventually reached a maximum height of 255 m (835 ft) in 1986."[174]

Lava balls[edit]

This is an accretionary lava ball. Credit: J. D. Griggs, USGS HVO.

The image at top right is an "[a]ccretionary lava ball [coming] to rest on the grass after rolling off the top of an ‘a‘a flow in Royal Gardens subdivision. Accretionary lava balls form as viscous lava is molded around a core of already solidified lava."[175]

See also[edit]

References[edit]

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