User:Marshallsumter/Radiation astronomy1/Meteoroids

This is a scanned photograph on the right of the bolide EN131090, originally captured on a glass photographic plate. The Earth-grazing meteoroid flew above Czechoslovakia and Poland on 13 October 1990 and left to space again. It was taken by an all-sky camera equipped with a fish-eye objective Zeiss Distagon 3.5/30mm located at the hydrometeorological station at Červená hora, Czechoslovakia (now in the Czech Republic). The bolide travels from the south to the north and its track is interrupted by a shutter rotating 12.5 times per second, which allows determining its speed.

"As of 2011 the International Astronomical Union officially defines a meteoroid as a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".

Meteors
Some wanderers are meteors.

A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble.

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. Most meteors are actually pieces of rock that have broken off a comet and continue to orbit the Sun. The Earth travels through the comet debris in its orbit. As the small pieces enter the Earth's atmosphere, friction causes them to burn up.

Def. "[a]ll other objects [not a planet or dwarf planet], except satellites, orbiting the Sun" are called collectively Small Solar-System Bodies.

"Coronal mass ejections (CMEs) are large‐scale expulsions of plasma and magnetic field from the solar corona to the interplanetary space. During a large CME event, ∼1016 g of coronal material with energies of ∼1032 ergs are ejected from the Sun [Hundhausen, 1997; Vourlidas et al., 2002]. While accelerating away from the Sun, CMEs present speeds between few tens up to ∼2500 km/s. CMEs with speeds exceeding the magnetosonic speed can drive fast shocks ahead of them. CME‐driven fast shocks are able to accelerate charged particles up to very high energies (∼GeV/nucleon) [Wang and Wang, 2006]."

Current "knowledge of the orbital structure of the outer solar system, [is] mostly slanted towards that information which has been learned from the Canada-France-Ecliptic Plane Survey (CFEPS: www.cfeps.net). Based on our current datasets (inside and outside CFEPS) outer solar system modeling is now entering the erra of precission cosmogony."

"Since the discovery of the first members of the Kuiper belt (Jewitt and Luu, 1993) the growth in knowledge of the outer solar system has been marked (perhaps driven) by the discovery of individual objects whose dynamics pointed at previously unknown reserviours; for example: 1993 RO and the plutinos, 1996 TL66 and the ‘scattering disk’, 2003 CR103 and the detectatch disk, 90377 Sedna and the Inner Oort Cloud."

The "‘main Kuiper belt’ is populated by dynamically ‘hot’ and ‘cold’ subcomponents (Brown 2001), the dyncamically ‘cold’ component is further sub-divided into a ‘stirred’ and ‘kernel’ component (Petit et al., 2011). The plane of the Ecliptic does not match the ecliptic or invariable planes of the solar sytem (Elliot et al., 2005). Collisional families exists, Haumea (Brown et al., 2007)."

Theoretical meteoroids
Def. a "fast-moving streak of light in the night sky caused by the entry of extraterrestrial matter into the earth's atmosphere" is called a meteor.

Def. "a relatively small (sand- to boulder-sized) fragment of debris in a solar system" is called a meteoroid.

Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 µm and 10 m across. Following the discovery and naming of asteroids below 10 m in size (e.g., 2008 TC3), Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter. The near-Earth object (NEO) definition includes larger objects, up to 50 m in diameter, in this category.

Most of our short-period meteor showers are not from the normal water vapor drag of active comets, but the product of infrequent disintegrations, when large chunks break off a mostly dormant comet.

Alpha Capricornids
Alpha Capricornids is a meteor shower that takes place as early as 15 July and continues until around 10 August.

The parent body is asteroid 2002 EX12 [169P/NEAT], which in the return of 2005 was found weakly active near perihelion.

"Minor planet 2002 EX12 ... is identified as the parent body of the alpha Capricornid shower, based on a good agreement in the calculated and observed direction and speed of the approaching meteoroids for ejecta 4500-5000 years ago....The bulk of this matter still passes inside Earth's orbit, but will cross Earth's orbit 300 years from now. As a result, the alpha Capricornids are expected to become a major annual shower in 2220-2420 A.D., stronger than any current annual shower"

The meteor shower was created about 3,500 to 5,000 years ago, when about half of the parent body disintegrated and fell into dust.

The Alpha Capricornids are expected to become a major annual storm in 2220–2420 A.D., one that will be "stronger than any current annual shower."

Alpha Monocerotids
Most years, those trails would miss the Earth altogether, but in some years the Earth is showered by meteors. This effect was first demonstrated from observations of the 1995 alpha Monocerotids,

The swarm is visible every year from 15 to 25 November; its peak occurs on 21 or 22 November.

The speed of its meteors is 65 km/s.

Normally it has a low Zenithal Hourly Rate (ZHR), but occasionally it produces remarkable meteor storms that last less than an hour: such outbursts were observed in 1925, 1935, 1985, and 1995.

The 1995 return was predicted based on the hypothesis that these outbursts were caused by the dust trail of a long period comet occasionally wandering in Earth's path due to planetary perturbations, during observations in southern Spain, assisted by a team of observers of the Dutch Meteor Society, and confirmed the brevity of Alpha Monocerotids outbursts as less than one hour, where the parent body, probably a long-period comet, is unknown.

Andromedids
The Andromedids meteor shower is associated with Biela's Comet, the showers occurring as Earth passes through old streams left by the comet's tail. The comet was observed to have broken up by 1846; further drift of the pieces by 1852 suggested the moment of breakup was in either 1842 or early 1843, when the comet was near Jupiter. The breakup led to particularly spectacular showers in subsequent cycles (particularly in 1872 and 1885).

Radiant of the Andromedids in December 2013 is near γ Cassiopeiae (near the middle of the W). Right ascension = 01:36:0 and Declination = +37°

Occurs during September 25 – December 6, date of peak is November 9, Velocity = 19 km/s and its Zenithal hourly rate = 3.

The first known sighting of the Andromedids was December 6, 1741, over St Petersburg, Russia.

The 1872 shower consisted mainly of faint (5th to 6th magnitude) meteors with "broad and smoke-like" trains and a predominantly orange or reddish colouration. The same shower produced at least 58,600 visible meteors between 5.50 and 10.30 pm, observed in England and that the meteors were much slower than the Leonids], with noises "like very distant gun-shots" several times to the north-west. In Burma, the 1885 shower was perceived as a fateful omen and was indeed followed swiftly by the collapse of the Konbaung dynasty and the conquest by Britain.

The November 27, 1885, shower was the occasion of the first known photograph of a meteor, taken by Austro-Hungarian astronomer, Ladislaus Weinek, who caught a 7 mm-long trail on a plate at his Prague observing station.

Since the 19th century the Andromedids have faded so substantially that they are no longer generally visible to the naked eye, though some activity is still observable each year in mid-November given suitable detection equipment. In recent years, peak activity had been less than three meteors per hour, around November 9 to 14. Andromedid activity of November comes from the newest streams, while that of early December comes from the oldest.

On December 4, 2011, six Canadian radar stations detected 50 meteors in an hour. The activity was likely from the 1649 stream. On December 8, 2013, Meteor specialist Peter Brown reported that the Canadian Meteor Orbit Radar had recorded an outburst from the Andromedid meteors in the past 24 hours. Scientists postulate a somewhat weaker return in 2018, but a yield of up to 200 meteors an hour in 2023. Canadian Meteor Orbit Radar (CMOR) data also detected a spike of 30 meteors per hour on November 27, 2008.

During the 2012 shower an inconspicuous maximum occurred on November 9.

Arietids
The Arietids are a strong meteor shower that lasts from May 22 to July 2 each year, and peaks on June 7. The Arietids, along with the Zeta Perseids, are the most intense daylight meteor showers of the year. The source of the shower is unknown, but scientists suspect that they come from the asteroid 1566 Icarus, although the orbit also corresponds similarly to 96P/Machholz.

First discovered at Jodrell Bank Observatory in England during the summer of 1947, the showers are caused when the Earth passes through a dense portion of two interplanetary meteoroid streams, producing an average of 60 shooting stars each hour, that originate in the sky from the constellation Aries and the constellation Perseus. However, because both constellations are so close to the Sun when these showers reach their peak, the showers are difficult to view with the naked eye. Some of the early meteors are visible in the very early hours of the morning, usually an hour before dawn. The meteors strike Earth's atmosphere at speeds around 39 km/s.

By June 22 the radiant has migrated to the constellation Taurus (3h 51m +27) which is the same constellation that the Beta Taurids peak on June 28.

Aurigids
Aurigids is a meteor shower occurring primarily within September.

The comet Kiess (C/1911 N1) is the source of the material that causes the meteors, with an orbital period as approximately 1800 to 2000 years, and showers observed in the years 1935, '86,  '94 and 2007.

The Alpha were discovered by C. Hoffmeister and A. Teichgraeber, during the night of 31 August 1935.

Beta Taurids
The Beta Taurids are normally active from June 5 to July 18. They emanate from an average radiant of right ascension 5h18m, declination +21.2 and exhibit maximum activity around June 28–29 (Solar Longitude=98.3 deg). The sun has a solar longitude (λ⊙) of 90 degress on June 21 (Summer solstice) and as there are 365 days/year moves roughly 1 degree/day. The meteor shower radiant of RA=79.4 degrees converts to 5h 18m as each hour is 15 degrees. The Zenithal Hourly Rate typically reaches about 25 km/s as seen on radar. Non-radio observers are faced with a very difficult prospect, because the Beta Taurid radiant is just 10–15 degrees west of the Sun on June 28.

Asteroids associated with the β–Taurids include 2201 Oljato, 5143 Heracles, 6063 Jason, (8201) 1994 AH2 and 1991 BA.

2019 will be the closest post-perihelion encounter with Earth since 1975. The Taurid swarm is expected to pass 0.06 AU below the ecliptic between June 23 – July 17.

During 2019 astronomers hope to search for hypothesized asteroids ~100 meters in diameter from the Taurid swarm between July 5–11, and July 21 – August 10. There is circumstantial evidence that the daytime June 30 Tunguska event came from the same direction in the sky as the Beta Taurids. The next June close approach to the Taurid swarm is expected in 2036.

Draconids
The October Draconids, in the past also unofficially known as the Giacobinids, are a meteor shower whose parent body is the periodic comet 21P/Giacobini-Zinner. They are named after the constellation Draco, where they seemingly come from. Almost all meteors which fall towards Earth ablate long before reaching its surface. The Draconids are best viewed after sunset in an area with a clear dark sky. RA 17.467:0:0 and Declination = +54°. Velocity = 20 km/s.

The 1933  and 1946 Draconids had Zenithal Hourly Rates of thousands of meteors visible per hour, among the most impressive meteor storms of the 20th century. Rare outbursts in activity can occur when the Earth travels through a denser part of the cometary debris stream; for example, in 1998, rates suddenly spiked and spiked again (less spectacularly) in 2005. A Draconid meteor outburst occurred as expected on 2011 October 8, though a waxing gibbous Moon reduced the number of meteors observed visually.

"Observers in the UK and Northern Europe are ideally placed to see the peak of the Draconids. Unfortunately the peak occurs in the day time for North America. There will also be a bright Moon which may drown out many but the brightest meteors, but if predictions are correct, you will still see many. You may see Draconid meteors on the 7th an the 9th also, so it is worth going out and checking the skies."

During the 2012 shower radar observations detected up to 1000 meteors per hour. The 2012 outburst may have been caused by the narrow trail of dust and debris left behind by the parent comet in 1959.

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

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

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).

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

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

June Bootids
"The June Bootid meteor shower is active each year from June 26th until July 2nd. It peaks on June 27th. Normally the shower is very weak, but occasional outbursts produce a hundred or more meteors per hour."

"The shower's radiant lies in the constellation Bootes (right ascension 14h 56m, declination 48°)."

"The source of the June Bootids is periodic comet 7P/Pons-Winnecke."

"June Bootid meteoroids hit Earth's atmosphere with a velocity of 18 km/s (40,000 mph).They are considered slow-moving meteors."

"On June 27th, 1998, northern sky watchers were surprised when meteors suddenly began to stream out of the constellation Bootes. Observers saw as many as 100 meteors per hour during the 7-hour-long outburst. It wasn't the first time: similar outbursts from Bootes had been recorded in 1916, 1921 and 1927. Astronomers call these unpredictable meteors the June Bootids."

Kappa Cygnids
Kappa Cygnids, abbreviated KCG, is a minor meteor shower that takes place in August along with the larger Perseids meteor shower.

The Kappa Cygnids in 2009 were Active between August 3-August 25 August, with Peak of shower at August 17, and ZHR = 3 km/s.

Leonids
"The Leonid meteor shower peaked early Saturday (Nov. 17 [2012]), and some night sky watchers caught a great view. The Leonids are a yearly meteor display of shooting stars that appear to radiate out of the constellation Leo. They are created when Earth crosses the path of debris from the comet Tempel-Tuttle, which swings through the inner solar system every 33 years."

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

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

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

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.

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.

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.

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.

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, 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 ... "

The oldest known outburst, the shower on March 23.7, 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." 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.

Northern Taurids
Parent body = 2004 TG10

Radiant point = RA 03:52:0 Dec = +22°.

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

Velocity = 29 km/s.

Zenithal hourly rate is 5.

The Northern Taurids originated from the asteroid 2004 TG10.

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

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). If larger than a pebble, these meteors may become bolides as bright as the moon and leave behind smoke trails.

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

In 1962 and 1963, the Mars 1 probe recorded one micrometeorite strike every two minutes at altitudes ranging from 6000 to 40000 km from Earth's surface due to the Taurids meteor shower, and also recorded similar densities at distances from 20 to 40 e6km from Earth.

The Taurid stream has a cycle of activity that peaks roughly every 2,500 to 3,000 years, 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.

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.

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

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. This may be the first photographic record of such a strike.

Orionids
"The Orionid meteor shower [leftover bits of Halley's Comet] is scheduled to reach its maximum before sunrise on Sunday morning (Oct. 21 [2012]). This will be an excellent year to look for the Orionids, since the moon will set around 11 p.m. local time on Saturday night (Oct. 20) and will not be a hindrance at all ... The orbit of Halley's Comet closely approaches the Earth's orbit at two places. One point is in the early part of May producing a meteor display known as the Eta Aquarids. The other point comes in the middle to latter part of October, producing the Orionids."

Perseid meteor showers
In 1835, Adolphe Quetelet identified the shower as emanating from the constellation Perseus.

Right ascension = 03:04:0 and Declination = +58°

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. The first record is from 36 CE.

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

Velocity = 58 km/s and Zenithal hourly rate = 100.

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. 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. 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.

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. 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. 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 km.

Phoenicids
The Phoenicids get their name from the location of their radiant, which is in the constellation Phoenix, active from 29 November to 9 December, with a peak occurring around 5/6 December each year, and are best seen from the Southern Hemisphere.

The Phoenicids appear to be associated with a stream of material from the disintegrating comet D/1819 W1 (Blanpain).

A very minor meteor shower with a radiant in Phoenix also occurs in July; this shower is referred to as the July Phoenicids.

Pi Puppids
"The Pi Puppids are a meteor shower associated with the comet 26P/Grigg-Skjellerup."

"The Pi Puppids get their name because their radiant appears to lie in the constellation Puppis, at around Right ascension 112 degrees and Declination -45 degrees."

Quadrantids
The Quadrantids (QUA) are a January meteor shower, with the zenithal hourly rate (ZHR) of this shower as high as that of two other reliably rich meteor showers, the Perseids in August and the Geminids in December.

The meteor rates exceed one-half of their highest value for only about eight hours (compared to two days for the August Perseids), which means that the stream of particles that produces this shower is narrow, and apparently deriving within the last 500 years from some orbiting body. The parent body of the Quadrantids was tentatively identified in 2003 as the minor planet (196256) 2003 EH1, which in turn may be related to the comet C/1490 Y1 that was observed by Chinese, Japanese and Korean astronomers some 500 years ago.

Southern Delta Aquariids
Meteors radiating from near the star Delta Aquarii (declension "-i") are called the Delta Aquariids.

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

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

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.

Ursids
The Ursids were probably discovered by William F. Denning who observed them for several years around the start of the 20th century. While there were sporadic observations after, the first coordinated studies of shower didn't begin until Dr. A. Bečvář observed an outburst of 169 per hour in 1945. Further observations in the 1970s and ongoing to current have established a relationship with comet 8P/Tuttle.

Parent body = 8P/Tuttle.

Right ascension = 14:28:0, Declination = +78°

Constellation = Ursa Minor (near Kochab)

Occurs during December 17 – December 26.

Date of peak = December 22.

Velocity = 33 km/s.

Zenithal hourly rate = 10.

Asteroids
Def. a "naturally occurring solid object, [which is] smaller than a planet and is not a comet, that orbits a star" is called an asteroid.

Usage notes

"The term "asteroid" has never been precisely defined. It was coined for objects which looked like stars in a telescope but moved like planets. These were known from the asteroid belt between Mars and Jupiter, and were later found co-orbiting with Jupiter (Trojan asteroids) and within the orbit of Mars. They were naturally distinguished from comets, which did not look at all starlike. Starting in the 1970s, small non-cometary bodies were found outside the orbit of Jupiter, and usage became divided as to whether to call these "asteroids" as well. Some astronomers restrict the term "asteroid" to rocky or rocky-icy bodies with orbits up to Jupiter. They may retain the term planetoid for all small bodies, and thus tend to use it for icy or rocky-icy bodies beyond Jupiter, or may use dedicated words such as centaurs, Kuiper belt objects, transneptunian objects, etc. for the latter. Other astronomers use "asteroid" for all non-cometary bodies smaller than a planet, even large ones such as Sedna and (occasionally) Pluto. However, the distinction between asteroid and comet is an artificial one; many outer "asteroids" would become comets if they ventured nearer the Sun. The official terminology since 2006 has been small Solar System body for any body that orbits the Sun directly and whose shape is not dominated by gravity."

From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see Meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. ... Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second). Meteoroids moving through the earth's orbital space average about 20 km/s.

Rocky objects
The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe.

The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.

In meteoroid ablation spheres from deep-sea sediments, "[t]he silicate spheres are the most dominant group."

Micrometeoroids
Def. "an extraterrestrial particle less than a millimeter in size" is called a micrometeoroid.

Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

Y asteroids
The possible importance of the Yarkovsky effect is the movement of meteoroids about the Solar System.

The diurnal effect is the dominant component for bodies with diameter greater than about 100 m.

On very long timescales over which the spin axis of the body may be repeatedly changed due to collisions (and hence also the direction of the diurnal effect changes), the seasonal effect will also tend to dominate.

The effect was first measured in 1991–2003 on the asteroid 6489 Golevka which drifted 15 km from its predicted position over twelve years (the orbit was established with great precision by a series of radar observations in 1991, 1995 and 1999 from the Arecibo Observatory radio telescope).

The "population of asteroids in comet-like orbits using available asteroid size and albedo catalogs of data taken with the Infrared Astronomical Satellite [I], AKARI [A], and the Wide-field Infrared Survey Explorer [W] on the basis of their orbital properties (i.e., the Tisserand parameter with respect to Jupiter, TJ, and the aphelion distance, Q, [is] 123 asteroids in comet-like orbits [with] Q < 4.5 AU and TJ < 3, [including] a considerable number (i.e., 25 by our criteria) of asteroids in comet-like orbits have high albedo, pv > 0.1. [As] such high-albedo objects mostly consist of small (D < 3 km) bodies distributed in near-Earth space (with perihelion distance of q < 1.3 AU) [may be] susceptible to the Yarkovsky effect and drifted into comet-like orbits via chaotic resonances with planets."

"There are 138,285 asteroids whose albedos and sizes are given in the I–A–W catalog. [...] nearly all high-albedo [asteroids in comet-like orbits] ACOs consist of small asteroids at q < 1.3 AU. This trend cannot be explained by the observational bias. Because the result is obtained based on the mid-infrared data, which, unlike optical observations, are less sensitive to albedo values, it provides reliable sets of asteroid albedo information. If there are big ACOs with high albedo beyond q = 1.3 AU, they would be detected easily. Although further dynamical study is essential to evaluate the population quantitatively, we propose that such ACOs with high albedos were injected from the domain of TJ > 3 via the Yarkovsky effect, because small objects with higher surface temperature are susceptible to the thermal drag force and gradually change their orbital elements to be observed as ACOs in our list."

"Although there are uncertainties in the dynamical simulation such as the value of the Yarkovsky force and the rocket force (for active comets), we conservatively consider that these three objects (2000 SU236, 2008 UM7, and 2009 SC298) are ACOs and PDCs. ["potential dormant comet" (PDC) is one having a low albedo (pv < 0.1) among ACOs. The second term is a paronomasia associating the spectra of potential dormant comets with spectra similar to P-type, D-type, or C-type asteroids (Licandro et al. 2008; DeMeo & Binzel 2008).]"

"Let us consider how the Yarkovsky effect moves an asteroid into a comet-like orbit. As shown [...], high-albedo ACOs concentrate in a range of 2 < a < 3.5 AU, similar to main-belt asteroids and [Jupiter-family comets] JFCs. The Tisserand parameter is a function of a, e, and i, [the semimajor axis, eccentricity, and inclination, respectively] while the Yarkovsky effect changes a. Due to the similarity in a between high-albedo ACOs and main-belt asteroids, we conjecture that subsequent dynamical effects may change e and i. Widely known as a standard model for orbital evolution of near-Earth asteroids, the Yarkovsky effect could move small main-belt asteroids' orbits until they are close to resonances with planets, and subsequently, these resonances can push them into terrestrial planet crossing orbits (see, e.g., Morbidelli et al. 2002). Numerical simulations demonstrated that chaotic resonances cause a significant increase in the e and i of test particles in the resonance regions (Gladman et al. 1997). Bottke et al. (2002) suggested that some objects on TJ < 3 (or even TJ < 2) can result from chaotic resonances. [...] Although there are a couple of ACOs close to resonances, their semimajor axes are not related to these major resonances. Therefore, it may be reasonable to think that encounters with terrestrial planets as well as chaotic resonances with massive planets can drift main-belt asteroids into comet-like orbits."

"In particular, we stress again the significance of high-albedo ACOs. As we discussed through our ground-based observation with the Subaru Telescope, high-albedo ACOs, which may have composition similar to silicaceous asteroids, definitively exist in the I–A–W database. Considering the very low TJ as well as the small size and perihelion distance, we suggest that such high-albedo ACOs have been injected via nongravitational forces, most likely the Yarkovsky effect."

Apollo asteroids
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.

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

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.

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.

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

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.

"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."

101955 Bennu (provisional designation 1999 RQ36, 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. It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199. 101955 Bennu has a mean diameter of approximately 492 m and has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Communications Complex NASA Deep Space Network.

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.

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.

Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of $484 m$, which is in line with other studies. It has a low visible geometric albedo of $0.046$. 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.

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 $284$ meters/year; analysis of the gravitational and thermal effects give a bulk density of ρ = $1,260$ kg/m3, which is only slightly denser than water, the predicted macroporosity is $40$%, suggesting that the interior has a rubble pile structure, with an estimated mass is $7.8 kg$.

Photometric observations of Bennu in 2005 yielded a synodic rotation period of $4.291 h$, 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. 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.

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. 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).

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.

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 $8 %$.

Earth crossers
EC denotes Earth-crossing.

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

"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 [yellow] 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."

"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."

2008 TC3
2008 TC3 (Catalina Sky Survey temporary designation 8TA9D69) was an 80 t, 4.1 m diameter asteroid that entered Earth's atmosphere on October 7, 2008. It exploded at an estimated 37 km above the Nubian Desert in Sudan. Some 600 meteorites, weighing a total of 10.5 kg, were recovered; many of these belonged to a rare type known as ureilites, which contain, among other minerals, nanodiamonds.

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

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.

It was notable as the first such body to be observed and tracked prior to reaching Earth. 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, 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 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 km/s 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.

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

"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."

A low-resolution image of the explosion was captured by the weather satellite Meteosat 8. The Meteosat images place the fireball at 21°N, 32.15°W. 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. Asteroids of this size hit Earth about two or three times a year.

The trajectory showed intersection with Earth's surface at roughly 20.3°N, 33.5°W though the object was expected to break up perhaps 100 - 200 km west as it descended, somewhat east of the Nile River, and about 100 km south of the Egypt–Sudan border.

According to U.S. government sources U.S. satellites detected the impact at 02:45:40 UT, with the initial detection at 20.9°N, 31.4°W at 65.4 km altitude and final explosion at 20.8°N, 32.2°W at 37 km altitude. These images have not been publicly released.

A search of the impact zone that began on December 6, 2008, turned up 10.5 kg of rock in some 600 fragments. These meteorites are collectively named Almahata Sitta, which means "Station Six" 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.

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. Amino acids have been found on the meteorite. The nanodiamonds found in the meteorite were shown to have grown slowly, implying that the source is another planet in the solar system.

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.

Meteorites
The visible path of a meteoroid that enters the Earth's atmosphere (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite.