Radiation astronomy/Asteroids

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Asteroids in the solar system are categorized by size and number. Credit: Marco Colombo, DensityDesign Research Lab.{{free media}}

The center image is a log-log plot of frequency vs. size for asteroids in the solar system.

Theoretical asteroids[edit | edit source]

Def. a "naturally occurring solid object, [which is] smaller than a planet[1] and is not a comet,[2] that orbits a star"[3] 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."[1]

Entities[edit | edit source]

"Asteroids in particularly large classes tend to be broken into subgroups with the first letter denoting the dominant group and the succeeding letters denoting less prominent spectral affinities or subgroups."[4]

"From the dominant group, the asteroids evolve to intersect the Earth's orbit on a median time scale of about 60 Myr."[5]

A asteroids[edit | edit source]

Spectra "of several related asteroid classes (types A, R, and V) were also analyzed for comparison to various S-subtypes."[6]

"Observing 246 Asporina and 289 Nenetta, [6] were the first to identify A-type asteroids as nearly pure olivine assemblages based on their spectral characteristics. These asteroids display a single broad absorption feature centered at 1.06 μm (Band I) without any significant pyroxene feature at ~2.0 μm (Band II)."[7]

Only "a handful of A-type objects were discovered during the taxonomic surveys, assuming that all A-type asteroids are olivine-rich. The study of A-type asteroids may help solve the “missing mantle problem” in the asteroid belt."[7]

Blue asteroids[edit | edit source]

This is an ultraviolet image of Pallas showing its flattened shape taken by the Hubble Space Telescope. Credit: NASA.

Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered (after Ceres), and one of the largest in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt,[8] and its diameter of 544 kilometres (338 mi) is slightly larger than that of 4 Vesta. It is however 10–30% less massive than Vesta,[9] placing it third among the asteroids.

"Spectrally blue (B-type) asteroids are rare, with the second discovered asteroid, Pallas, being the largest and most famous example."[10]

"[T]he negative optical spectral slope of some B-type asteroids is due to the presence of a broad absorption band centered near 1.0 μm. The 1 μm band can be matched in position and shape using magnetite (Fe3O4), which is an important indicator of past aqueous alteration in the parent body. ... Observations of B-type asteroid (335) Roberta in the 3 μm region reveal an absorption feature centered at 2.9 μm, which is consistent with the absorption due to phyllosilicates (another hydration product) observed in CI chondrites. ... at least some B-type asteroids are likely to have incorporated significant amounts of water ice and to have experienced intensive aqueous alteration."[10]

"The [Sloan Digital Sky Survey] SDSS “blue” asteroids are related to the C-type (carbonaceous) asteroids, but not all of them are C-type. They are a mixture of C-, E-, M-, and P-types."[11]

Carbonaceous asteroids[edit | edit source]

C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids,[12] and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except D-types and others common only at the extreme outer edge of the asteroid belt. Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are largely featureless but slightly reddish. The so-called "water" absorption feature around 3 μm, which can be an indication of water content in minerals is also present.

D asteroids[edit | edit source]

"Two comets observed at low activity (visible nuclei) also have properties more consistent with D asteroids than any other class (very low reported geometric albedos of 0.02 and red colors)."[13]

E asteroids[edit | edit source]

The "enstatite achondrite-like E-asteroids almost certainly derive from the melting and differentiation of highly reduced parent materials analogous to enstatite chondrites (e.g., Keil 1989)."[6]

F asteroids[edit | edit source]

F-type asteroids have spectra generally similar to those of the B-type asteroids, but lack the "water" absorption feature around 3 μm indicative of hydrated minerals, and differ in the low wavelength part of the ultraviolet spectrum below 0.4 μm.

G asteroids[edit | edit source]

"G-type asteroids are a relatively uncommon type of carbonaceous asteroid. The most notable asteroid in this class is 1 Ceres. Generally similar to the C-type objects, but containing a strong ultraviolet absorption feature below 0.5 μm.

H asteroids[edit | edit source]

"Group H asteroids are apparently olivine rich. In addition, this group exhibits the lowest mean value for the bend parameter. The asteroid 7 Iris is a typical member of this group."[14]

I asteroids[edit | edit source]

Hardersen (2003) "started by dividing asteroids into two classes: Class I asteroids, which allow diagnostic determinations of their mineralogy, and Class II asteroids, where the determination is nondiagnostic and open to interpretation."[15]

J asteroids[edit | edit source]

"The remaining six are interpreted as diogenitic in composition and were designated as J asteroids (mnemonic for the Johnstown diogenite)."[16]

K asteroids[edit | edit source]

"Bell (1988) distinguished a new class (K-type) from previous members of the S-class and interpreted these objects as composed of material analogous to the undifferentiated CO3 ro CV3 carbonaceous chondrites."[6]

L asteroids[edit | edit source]

"Color dependent photometry calibration, or the HK solar color, might be in error by 0.1 magnitude and cause the L asteroids not to coincide with the carbonaceous chondrites, but we do not think that this is a very likely possibility."[17]

M asteroids[edit | edit source]

"Exposure of compositionally distinct internal layers from within parent planetesimals may be an important source of the diversity within the S-population and is presumably the source of the M- and A-type."[6]

"M-class [...] contains both metallic objects [...] 216 Kleopatra and [...] 16 Psyche".[18]

N asteroids[edit | edit source]

"Emission features attributable to fine-grained silicates are seen in the spectra of two of the M-types and at least four of the C-types. This is the first positive evidence that N-type asteroids have a silicate component on their surface similar to that of other asteroids."[19]

O asteroids[edit | edit source]

"O-type asteroids 3628 Boznemcová and 7472 Kumakiri have absorption bands similar to pyroxenes but with band minima that are not typically found for terrestrial pyroxenes and known pyroxene-dominated meteorite assemblages."[20]

P asteroids[edit | edit source]

"Whereas most of the P-type asteroids spectrally match the Tagish Lake meteorite in the wavelength range longer than 0.9 μm, they are different in the shorter wavelength range."[21]

Q asteroids[edit | edit source]

lf "planetary encounters are important, the distribution of Q-type asteroids in the planet-crossing space should show a correlation with the perihelion distance, exactly as we have found here."[22]

R asteroids[edit | edit source]

"A-type asteroids are a relatively rare taxonomic class with no more than 17 known objects [1,2,3]. They were first identified as a separate group of R-type asteroids based on broadband spectrophotometry by [4], and were later classified based on ECAS data by Tholen (1984) [1]."[7]

Siliceous asteroids[edit | edit source]

This graph depicts carrier wavelength versus band ratios to divide or characterize the siliceous asteroids. Credit: Michael J. Gaffey, Jeffrey F. Bell, R. Hamilton Brown, Thomas H. Burbine, Jennifer L. Piatek, Kevin L. Reed, and Damon A. Chaky.

"The presence of trapped solar gas in stony meteorites places their origin in the regoliths of asteroidal-type bodies. The most plausible sources are the C (carbonaceous) and S (siliceous) asteroids, in spite of the differences between the spectra of S asteroids and ordinary chondrites."[23]

"The S-asteroid class includes a number of distinct compositional subtypes [designated S(I)-S(VII)] which exhibit surface silicate assemblages ranging from pure olivine (dunites) through olivine-pyroxene mixtures to pure pyroxene or pyroxene-feldspar mixtures (basalts)."[6] These are shown in the graph on the right.

3 Juno is a siliceous asteroid type S(IV).[6]

8 Flora may be ambiguously a type S(II-III) siliceous asteroid.[6]

T asteroids[edit | edit source]

The "T-type asteroids appear to contain at least two mineralogically distinct types."[24]

U asteroids[edit | edit source]

"The SU type represents an ambiguous classification with parameters of both the S- and the [unusual] U-types but with the former more likely--Tholen 1984, 1989, Tholen and Barucci 1989."[6]

V asteroids[edit | edit source]

As NASA's Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA.
Location and structure of the Vesta family are depicted. Credit: Deuar.

In the full image of Vesta at right, the rocky-object appears to have suffered from meteor damage.

Vesta, minor-planet designation 4 Vesta, is one of the largest asteroids in the Solar System. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites, a rich source of information about the asteroid.[25][26]

"V-type asteroids are bodies whose surfaces are constituted of basalt. In the Main Asteroid Belt, most of these asteroids are assumed to come from the basaltic crust of Asteroid (4) Vesta."[27]

The Vestian asteroids consist "of 4 Vesta, the second-most-massive of all asteroids (mean diameter of 530 km), and many small asteroids below 10 km diameter. The brightest of these, 1929 Kollaa and 2045 Peking, have an absolute magnitude of 12.2, which would give them a radius of about 7.5 km assuming the same high albedo as 4 Vesta."[28]

"A HCM numerical analysis (by Zappala 1995) determined a large group of 'core' family members, whose proper orbital elements lie in the approximate ranges"[28]

ap ep ip
min 2.26 AU 0.075 5.6°
max 2.48 AU 0.122 7.9°

"This gives the approximate boundaries of the family. At the present epoch, the range of  osculating orbital elements of these core members is"[28]

a e i
min 2.26 AU 0.035 5.0°
max 2.48 AU 0.162 8.3°

"The Zappala 1995 analysis[29] found 235 core members. A search of a recent proper-element database (AstDys) for 96944 minor planets in 2005 yielded 6051 objects (about 6% of the total) lying within the Vesta-family region as per the first table above."[28]

W asteroids[edit | edit source]

"Rivkin et al. also noted that W-type asteroids in their survey are lacking an FeO absorption band (0.4–0.5 μm), an expected feature if the hydrated minerals formed in the presence of significant free metal."[30]

"CB chondrites have only very minor amounts of phyllosilicates and may come from W-type asteroids (“wet-M” asteroids)."[31]

X asteroids[edit | edit source]

"Since the composition of X-type asteroids remains unclear and their relation to mineralogical type is unknown, [87] Sylvia's bulk porosity is not well constrained."[32]

Y asteroids[edit | edit source]

Yarkovsky effect:
1. Radiation from asteroid's surface
2. Prograde rotating asteroid
2.1 Location with "Afternoon"
3. Asteroid's orbit
4. Radiation from Sun. Credit: Graevemoore.

The possible importance of the Yarkovsky effect is the movement of meteoroids about the Solar System.[33]

The diurnal effect is the dominant component for bodies with diameter greater than about 100 m.[34]

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.[34]

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).[35]

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."[36]

"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."[36]

"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).]"[36]

"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."[36]

"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."[36]

Z asteroids[edit | edit source]

"There is a general increase in 'redness' with heliocentric distance in the outer belt asteroids going from the moderately red P asteroids, through the redder D asteroids, to the much redder Z asteroids."[37]

Earth crossers[edit | edit source]

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

EC denotes Earth-crossing.[5]

"50 % of the MB Mars-crossers [MCs] become ECs within 59.9 Myr and [this] contribution ... dominates the production of ECs".[5]

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

"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."[38]

"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."[38]

2019 OK[edit | edit source]

An asteroid dubbed a ‘city-killer’ narrowly missed colliding with the Earth on Thursday. Credit: Michael Brown, NASA.{{fairuse}}

"Asteroid 2019 OK - around 100 metres in diameter and racing at 24 kilometres a second - flew past Earth at around 11.22am on Thursday morning."[39]

"[If it hit Earth] it makes the bang of a very large nuclear weapon – a very large one."[40]

"It would have hit with over 30 times the energy of the atomic blast at Hiroshima. It's a city-killer asteroid. But because it's so small, it's incredibly hard to see until right at the last minute. It's threading tightly between the lunar orbit. Definitely too close for comfort."[41]

2008 TC3[edit | edit source]

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.
Meteosat 8/EUMETSAT infrared image is of the explosion. Credit: .
This webcam frame was shot. Credit: Webcam at kitepower, Mangroovy Beach, El Gouna, Red Sea governate, Egypt.
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: .
Meteosat 8 / EUMETSAT visual image is first light flare from 2008 TC3 with lat/long reference. Credit: .
Meteosat 8 / EUMETSAT IR image of main fireball from 2008 TC3. Credit: .
Meteosat images combined, showing offset from first light flare to main IR flare. Credit:

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[42] that entered Earth's atmosphere on October 7, 2008.[43] 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.[42][44][45]

It was the first time that an asteroid impact had been predicted prior to its entry into the atmosphere as a meteor.[46]

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.[47][48][49]

It was notable as the first such body to be observed and tracked prior to reaching Earth.[46] 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,[49] 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[50][51] 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.[52]

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."[53] A webcam captured the flash lighting up El-Gouna beach 725 kilometres north of the explosion (see this webcam frame).[54]

"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."[54]

A low-resolution image of the explosion was captured by the weather satellite Meteosat 8.[55] 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).[56] 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.[57] Asteroids of this size hit Earth about two or three times a year.[58]

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)[59] 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[60][61] 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,[62] which means "Station Six"[63] 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.[64][65][66][67][68]

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.[42] Amino acids have been found on the meteorite.[69] The nanodiamonds found in the meteorite were shown to have grown slowly, implying that the source is another planet in the solar system.[70]

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.[71]

Apollo asteroids[edit | edit source]

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)
  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.[72]

The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015.[73]

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.[74]

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.[75]

On 21 September 2018, the first two of these rovers, which will hop around the surface of the asteroid, were released from Hayabusa2.[76]

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.[77]

"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."[78]

101955 Bennu (provisional designation 1999 RQ36[79], 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.[80] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.[81][82]

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.[83][84][85]

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.[82]

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.[82]

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.[86]

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 = 1260±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

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.[88] 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.[89]

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.[82] 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).[90]

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.[91]

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 %.[88]

Mars crossers[edit | edit source]

"50 % of the MB Mars-crossers [MCs] become ECs within 59.9 Myr".[5]

Amor asteroids[edit | edit source]

This is a diagram showing the general location of the Amor asteroids and Mars trojans, compared to the orbits of the terrestrial planets Mercury (H), Venus (V), Earth (E) and Mars (M).
  Amor asteroids
  Mars trojans
  Earth (E)
  Mars (M)
  Venus (V)
  Mercury (H)

Note that sizes and distances of bodies and orbits are not to scale. Credit: AndrewBuck.

Mars Trojans[edit | edit source]

From the diagram in the section Amor asteroids, the Mars Trojans occupy comparable positions relative to Mars that the Trojan asteroids do to Jupiter.

Asteroid belts[edit | edit source]

This is a composite image, to scale, of the asteroids which have been imaged at high resolution. As of 2011 they are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa. Credit: NASA/JPL-Caltech/JAXA/ESA.
The asteroid belt is shown in (white) and the Trojan asteroids (green). Credit: .
This is an approximately natural color picture of the asteroid 243 Ida on August 28, 1993. Credit: NASA/JPL.

Def. a "region of the orbital plane of the solar system located between the orbits of Mars and Jupiter which is occupied by numerous minor planets and the dwarf planet Ceres"[92] is called an asteroid belt.

"The MB group is the most numerous group of MCs. ... 50 % of the MB Mars-crossers [MCs] become ECs within 59.9 Myr and [this] contribution ... dominates the production of ECs"[5]. MB denotes the main belt of asteroids.[5]

The interplanetary medium includes interplanetary dust, cosmic rays and hot plasma from the solar wind. The temperature of the interplanetary medium varies. For dust particles within the asteroid belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU[93] The density of the interplanetary medium is very low, about 5 particles per cubic centimeter in the vicinity of the Earth; it decreases with increasing distance from the sun, in inverse proportion to the square of the distance. It is variable, and may be affected by magnetic fields and events such as coronal mass ejections. It may rise to as high as 100 particles/cm³.

"The hydromagnetic approach led to the discovery of two important observational regularities in the solar system: (1) the band structure [such as in the rings of Saturn and in the asteroid belt], and (2) the cosmogonic shadow effect (the two-thirds fall down effect)."[94]

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter ... This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km in diameter,[95] and millions of smaller ones.[96]

At second right is an approximately natural color image of the asteroid 243 Ida. "There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas."[97]

"The [Sloan Digital Sky Survey] SDSS “blue” asteroids are related to the C-type (carbonaceous) asteroids, but not all of them are C-type. They are a mixture of C-, E-, M-, and P-types."[11]

Baptistina family[edit | edit source]

298 Baptistina (center) is one of the largest presumed remnants of the Baptistina family. Credit: .

The Baptistina family (Family identification number, FIN: 403) is an asteroid family of more than 2500 members that was probably produced by the breakup of an asteroid 170 km (110 mi) across 80 million years ago following an impact with a smaller body. The two largest presumed remnants of the parent asteroid are main-belt asteroids 298 Baptistina and 1696 Nurmela. The Batistina family is part of the larger Flora clan.[98][99]

The Baptistina family consists of darkly colored asteroids and meteoroids in similar orbits. Many mountain-sized fragments from the collision would have leaked into the inner solar system through orbital resonances with Mars and Jupiter, causing a prolonged series of asteroid impacts. Previously, this collision was believed to have occurred about 160 million years ago, and many impacts between 100 and 50 million years ago were attributed to it. However, in 2011, data from WISE revised the date of the proposed collision which broke up the parent asteroid to about 80 million years ago.[100]

Following the impact of the Chelyabinsk meteor in 2013, a paper published in the journal Icarus showed that shock produced during impact of a large asteroid can darken otherwise bright silicate material. Spectral analysis of the darkly-colored portions of the non-carbonaceous Chelyabinsk meteorite closely matched the color of members of the Baptistina family, showing that a low albedo does not necessarily indicate the composition of the family.[101]

Eos family[edit | edit source]

Members include 221 Eos, 339 Dorothea, 450 Brigitta, 513 Centesima, 562 Salome, 579, 633 Zelima, 639 Latona, 651 Antikleia, 653 Berenike, 661 Cloelia, 669 Kypria, 742 Edisona, 807 Ceraskia, 876 Scott, 890 Waltraut, 1075, 1199, and 1364.

Flora family[edit | edit source]

Location and structure of the Flora family within the asteroid belt is shown. Credit: Deuar.{{free media}}

In the top diagrams, the proper orbital elements are plotted (inclination ip vs. semi-major axis ap on the left, and inclination ip vs. eccentricity ep on the right) for numbered asteroids in the main belt.

In the bottom enlargements, one has:

  • core family members (shown in red) are those identified by the 1995 Zappala (V. Zappala et al, Icarus Vol. 116, p. 291 (1995)) HCM method analysis. The data set is here.
  • outer family members (shown in purple) were identified by the same analysis, but are somewhat less certain to be members (were given a quality tag of "1" in the data-set)
  • interlopers (shown in green) were identified either:
    • by the spectroscopic analysis of Florczak (M. Florczak A Visible Spectroscopic Survey of the Flora Clan, Icarus Vol. 133, p. 233 (1998)),
    • and also by inspection of the PDS asteroid taxonomy data set for non S-type members of the above Zappala 1995 data-set.
  • numbered asteroids in general (shown as small blue dots), from the AstDys database.

The location of asteroids that are very large and/or explicitly named on the plot is shown by a light cross within the circle.

Asteroid diameters were obtained from two sources:

  • (thick circles): From the IRAS survey, when available (data set here).
  • (thin circles): Otherwise, by assuming an albedo of 0.243 (same as 8 Flora), and estimating from absolute magnitudes from the AstOrb database here.

The diagrams were created by Piotr Deuar using proper element data for 96944 minor planets, which was obtained from the AstDys site. Data was dated March 2005, calculation was by Z. Knezevic and A. Milani.

Koronis family[edit | edit source]

These are some of the Koronis family asteroids. Credit: NASA.
This is an approximately natural color picture of the asteroid 243 Ida on August 28, 1993. Credit: NASA/JPL.

Members include 158 Koronis, 167 Urda, 208 Lacrimosa, 243 Ida,[102] 311, 321, 534, 720, and 1223.

At second right is an approximately natural color image of the asteroid 243 Ida. The dot to the right is its moon Dactyl. "There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas."[97]

Ida was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory.[103] It was his 45th asteroid discovery.[104] Ida was named by Moriz von Kuffner, a Viennese brewer and amateur astronomer.[105][106] In Greek mythology, Ida was a nymph of Crete who raised the god Zeus.[107] Ida was recognized as a member of the Koronis family by Kiyotsugu Hirayama, who proposed in 1918 that the group comprised the remnants of a destroyed precursor body.[108]

Ida's reflection spectrum was measured on 16 September 1980 by astronomers David J. Tholen and Edward F. Tedesco as part of the eight-color asteroid survey (ECAS).[109]

"The Eos and Koronis families ... are entirely of type S, which is rare at their heliocentric distances ..."[109]

Its spectrum matched those of the asteroids in the S-type classification.[109] Many observations of Ida were made in early 1993 by the United States Naval Observatory Flagstaff Station and the Oak Ridge Observatory. These improved the measurement of Ida's orbit around the Sun and reduced the uncertainty of its position during the Galileo flyby from 78 to 60 km (48 to 37 mi).[110]

Maria family[edit | edit source]

Members include 170 Maria, 472, 660, 695, and 714.

Nysian asteroids[edit | edit source]

This is a screen shot of a computer plot for the Nysian family of asteroids. Credit: EasySky.

The Nysian asteroids occur primarily between the orbits of Mars and Jupiter but some are between Earth and Mars.

Trojan asteroids[edit | edit source]

Diagram of Lagrange points is in a system where the primary is much more massive than the secondary. Credit: Cmglee.

Def. "the L4 and L5 Lagrange points of the Sun-Jupiter orbital configuration"[111] are called the Trojan points.

Def. "an asteroid occupying the Trojan points of the Sun-Jupiter system"[112] is called a Trojan asteroid.

The Trojan asteroids orbit 60 degrees ahead of and behind Jupiter as it circles the Sun."[113]

Asteroid (514107) 2015 BZ509 may be a former interstellar object, captured some 4.5 billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun.[114]

Centaurs[edit | edit source]

Def. an "icy planetoid that orbits the Sun between Jupiter and Neptune"[115] is called a Centaur.

"The recent investigation of the orbital distribution of Centaurs (Emel’yanenko et al., 2005) showed that there are two dynamically distinct classes of Centaurs, a dominant group with semimajor axes a > 60 AU and a minority group with a < 60 AU."[116] "[T]he intrinsic number of such objects is roughly an order of magnitude greater than that for a<60 AU".[116]

Radars[edit | edit source]

This image is of asteroid 2012 LZ1 by the Arecibo Observatory in Puerto Rico using the Arecibo Planetary Radar. Credit: Arecibo Observatory.

Radar astronomy is used to detect and study astronomical objects that reflect radio rays.

"The advantages of radar in planetary astronomy result from (1) the observer's control of all the attributes of the coherent signal used to illuminate the target, especially the wave form's time/frequency modulation and polarization; (2) the ability of radar to resolve objects spatially via measurements of the distribution of echo power in time delay and Doppler frequency; (3) the pronounced degree to which delay-Doppler measurements constrain orbits and spin vectors; and (4) centimeter-to-meter wavelengths, which easily penetrate optically opaque planetary clouds and cometary comae, permit investigation of near-surface macrostructure and bulk density, and are sensitive to high concentrations of metal or, in certain situations, ice."[117]

514107 Kaʻepaokaʻawela[edit | edit source]

Retrograde orbit of 2015 BZ509 has 100 day motion markers. Credit: Tomruen.{{free media}}
Orbital diagram is shown. Credit: Nwbeeson.
Orbit (side-view) is compared to Jupiter. Credit: Nwbeeson.
Kaʻepaokaʻawela shows apparent retrograde motion in the sky while it is on the far side of the sun, rather than at opposition with the sun. Credit: Tomruen.

514107 Kaʻepaokaʻawela, provisional designation 2015 BZ509, also nicknamed Bee-Zed,[118] is a small asteroid, approximately 3 kilometers (2 miles) in diameter,[119] in a resonant, co-orbital motion with Jupiter.[120] Its orbit is retrograde, which is opposite to the direction of most other bodies in the Solar System.[121] It was discovered on 26 November 2014, by astronomers of the Pan-STARRS survey at Haleakala Observatory on the island of Maui, United States.[122] The unusual object is the first example of an asteroid in a 1:−1 resonance with any of the planets.[123]

Kaʻepaokaʻawela orbits the Sun at a distance of 3.2–7.1 AU once every 11 years and 8 months (4,256 days; semi-major axis of 5.14 AU), has an eccentricity of 0.38 and an inclination of 163° with respect to the ecliptic.[124]

The orbit of this asteroid is shown in blue when it is above the plane of the orbit of Jupiter, and in magenta when it is below the plane of the orbit of Jupiter.[120]

The plane of the orbit of Jupiter is shown in black, but in this frame of reference Jupiter (the red dot) stays at the right end of the black line. The orbit of this asteroid is shown in blue when it is above (north of) the plane of the orbit of Jupiter, and it is shown in magenta when it is below (south of) the plane of the orbit of Jupiter.[120]

Kaʻepaokaʻawela has been in its retrograde resonance with Jupiter since the origin of the Solar System instead of it being an object that is only briefly in this orbit that was observed by chance using the Copernican principle.[125][126] Since its retrograde orbit is in the opposite direction as objects that formed in the early Solar System they posit that Kaʻepaokaʻawela has an interstellar origin.[127] If confirmed, this origin would have implications on current theories such as the detailed timing and mechanics of planet formation, and the delivery of water and organic molecules to Earth.[125] Others suggest that Kaʻepaokaʻawela originated in the Oort cloud or that it acquired a retrograde orbit due to interactions with Planet Nine, and that it is a short term resident of its current resonance.[125]

See also[edit | edit source]

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