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]

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]

"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]

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]

"Spectrally blue (B-type) asteroids are rare, with the second discovered asteroid, Pallas, being the largest and most famous example."[8]

"[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."[8]

"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."[9]

Carbonaceous asteroids[edit]

C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids,[10] 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]

"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)."[11]

E asteroids[edit]

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]

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]

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

H asteroids[edit]

"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."[12]

I asteroids[edit]

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."[13]

J asteroids[edit]

"The remaining six are interpreted as diogenitic in composition and were designated as J asteroids (mnemonic for the Johnstown diogenite)."[14]

K asteroids[edit]

"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]

"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."[15]

M asteroids[edit]

"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".[16]

N asteroids[edit]

"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."[17]

O asteroids[edit]

"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."[18]

P asteroids[edit]

"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."[19]

Q asteroids[edit]

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."[20]

R asteroids[edit]

"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]

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."[21]

"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.

T asteroids[edit]

The "T-type asteroids appear to contain at least two mineralogically distinct types."[22]

U asteroids[edit]

"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]

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.[23][24]

"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."[25]

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."[26]

"A HCM numerical analysis (by Zappala 1995) determined a large group of 'core' family members, whose proper orbital elements lie in the approximate ranges"[26]

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"[26]

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

"The Zappala 1995 analysis[27] 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."[26]

W asteroids[edit]

"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."[28]

"CB chondrites have only very minor amounts of phyllosilicates and may come from W-type asteroids (“wet-M” asteroids)."[29]

X asteroids[edit]

"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."[30]

Y asteroids[edit]

""

Z asteroids[edit]

"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."[31]

Earth crossers[edit]

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

EC denotes Earth-crossing.[5]

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

Apollo asteroids[edit]

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

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

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

Ryugu shown on the left was discovered on 10 May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States.[32]

The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015.[33]

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.[34]

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.[35]

On 21 September 2018, the first two of these rovers, which will hop around the surface of the asteroid, were released from Hayabusa2.[36]

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.[37]

"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."[38]

101955 Bennu (provisional designation 1999 RQ36[39], 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.[40] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.[41][42]

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.[43][44][45]

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.[42]

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.[42]

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.[46]

Astrometric observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the semimajor axis to drift on average by 284±1.5 meters/year; analysis of the gravitational and thermal effects give a bulk density of ρ = 1,260±70 kg/m3, which is only slightly denser than water, the predicted macroporosity is 40±10%, suggesting that the interior has a rubble pile structure, with an estimated mass is 7.8±0.9×1010
 kg
.[47]

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.[48] 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.[49]

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.[42] 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).[50]

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.[51]

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 %.[48]

Mars crossers[edit]

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

Amor asteroids[edit]

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).
  Sun
  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]

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]

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"[52] 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[53] 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)."[54]

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,[55] and millions of smaller ones.[56]

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."[57]

"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."[9]

Baptistina family[edit]

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.[58][59]

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.[60]

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.[61]


Eos family[edit]

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.

Koronis family[edit]

These are some of the Koronis family asteroids. Credit: NASA.

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

Maria family[edit]

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

Nysian asteroids[edit]

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]

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"[62] are called the Trojan points.

Def. "an asteroid occupying the Trojan points of the Sun-Jupiter system"[63] is called a Trojan asteroid.

Centaurs[edit]

Def. an "icy planetoid that orbits the Sun between Draft:Jupiter and Draft:Neptune"[64] 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."[65] "[T]he intrinsic number of such objects is roughly an order of magnitude greater than that for a<60 AU".[65]

Radars[edit]

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."[66]

See also[edit]

References[edit]

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  5. 5.0 5.1 5.2 5.3 5.4 5.5 Patrick Michel, Fabbio Migliorini, Alessandro Morbidelli, Vincenzo Zappalà (June 2000). "The Population of Mars-Crossers: Classification and Dynamical Evolution". Icarus 145 (2): 332-47. doi:10.1006/icar.2000.6358. http://www.obs-nice.fr/morby/papers/6358a.pdf. Retrieved 2011-10-06. 
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  7. 7.0 7.1 7.2 V. Reddy, P. S. Hardersen, M. J. Gaffey, and P. A. Abell (2005). Mineralogic and Temperature-Induced Spectral Investigations of A-type Asteroids 246 Asporina and 446 Aeternitas, In: Lunar and Planetary Science (PDF). XXXVI. USRA. p. 2. Retrieved 2015-09-04.CS1 maint: Multiple names: authors list (link)
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