inner Solar System . Larger asteroids have also been called planetoids . These terms have historically been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail . As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt .[1]

253 Mathilde, a C-type asteroid measuring about 50 km (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.

Diagram of the Solar System's asteroid belt

2014 JO25 imaged by radar during its 2017 Earth flyby

In this article, the term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter .

Overview

Millions of asteroids exist, many are shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets .[2] The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the

Jupiter trojans ). However, other orbital families exist with significant populations, including the

near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type , M-type , and S-type . These were named after and are generally identified with carbon-rich , metallic, and silicate (stony) compositions, respectively. The sizes of asteroids varies greatly; the largest, Ceres, is almost 1,000 km (600 mi) across and massive enough to qualify as a dwarf planet .

Asteroids are somewhat arbitrarily differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are primarily composed of dust and ice. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice.[3] The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. [4] Finally, meteoroids can be composed of either cometary or asteroidal materials. [5]

Only one asteroid, 4 Vesta, which has a relatively reflective surface , is normally visible to the naked eye, and this is only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time. [6] As of March 2020, the Minor Planet Center had data on 930,000 minor planets in the inner and outer Solar System, of which about 545,000 had enough information to be given numbered designations. [7]

The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. [8][9]

In April 2018, the B612 Foundation reported "It is 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent sure when." [10] Also in 2018, physicist Stephen Hawking , in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. [11][12][13] In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. [14][15][16][17][18] According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched. [19]

Discovery

Sizes of the first ten asteroids to be discovered, compared to the Moon

243 Ida and its moon Dactyl . Dactyl is the first satellite of an asteroid to be discovered.

The first asteroid to be discovered, Ceres, was originally considered to be a new planet. [a] This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", [b] coined in Greek as ἀστεροειδής, or

asteroeidēs , meaning 'star-like, star-shaped', and derived from the Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably. [c]

Discovery timeline: [23]

10 by 1849

1 Ceres, 1801

2 Pallas – 1802

3 Juno – 1804

4 Vesta – 1807

5 Astraea – 1845

in 1846, planet Neptune was discovered [24]

6 Hebe – July 1847

7 Iris – August 1847

8 Flora – October 1847

9 Metis – 25 April 1848

10 Hygiea – 12 April 1849 tenth asteroid discovered

100 asteroids by 1868[23]

1,000 by 1921[23]

10,000 by 1989 [23]

100,000 by 2005[25]

1,000,000 by 2020[7]

Historical methods

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Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. [26] This task required that hand-drawn sky charts be prepared for all stars in the

zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.

First asteroid image ( Ceres and

Vesta) from Mars – viewed by

Curiosity (20 April 2014).

The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi , director of the observatory of Palermo in Sicily . He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be between the planets Mars and Jupiter . Piazzi named it after Ceres, the Roman goddess of agriculture. [26]

Three other asteroids ( 2 Pallas , 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches. [ citation needed ]

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were

J.R. Hind, A. de Gasparis , R. Luther , H.M.S. Goldschmidt, J. Chacornac , J. Ferguson, N.R. Pogson , E.W. Tempel , J.C. Watson , C.H.F. Peters , A. Borrelly , J. Palisa , the Henry brothers and A. Charlois .

In 1891, Max Wolf pioneered the use of

astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia , whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", [27] a phrase variously attributed to E. Suess [28] and E. Weiss. [29] Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was

photographed by a wide-field telescope, or

astrograph . Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope . Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations. [30]

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74 ).

The last step of discovery is to send the locations and time of observations to the Minor Planet Center , where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union .

Computerized methods

2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an

artificial satellite.

Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2017

There is increasing interest in identifying asteroids whose orbits cross Earth 's, and that could, given enough time, collide with Earth (see

Earth-crosser asteroids) . The three most important groups of near-Earth asteroids are the

Apollos , Amors , and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor , 1862 Apollo ,

2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the

Cretaceous–Paleogene extinction , and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites , built to detect nuclear explosions , had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.

All these considerations helped spur the launch of highly efficient surveys that consist of charge-coupled device ( CCD) cameras and computers directly connected to telescopes. As of 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.[31] A list of teams using such systems includes:[32][33]

Lincoln Near-Earth Asteroid Research (LINEAR)

Near-Earth Asteroid Tracking (NEAT)

Spacewatch

Lowell Observatory Near-Earth-Object Search (LONEOS)

Catalina Sky Survey (CSS)

Pan-STARRS

NEOWISE

Asteroid Terrestrial-impact Last Alert System (ATLAS)

Campo Imperatore Near-Earth Object Survey (CINEOS)

Japanese Spaceguard Association

Asiago-DLR Asteroid Survey (ADAS)

As of 29 October 2018, the LINEAR system alone has discovered 147,132 asteroids.[34] Among all the surveys, 19,266 near-Earth asteroids have been discovered[35] including almost 900 more than 1 km (0.6 mi) in diameter. [36]

Terminology

Euler diagram showing the types of bodies in the Solar System. (see Small Solar System body )

A composite image, to the same scale, of the asteroids imaged at high resolution prior to 2012. They are, from largest to smallest:

4 Vesta, 21 Lutetia , 253 Mathilde, 243 Ida and its moon Dactyl , 433 Eros, 951 Gaspra ,

2867 Å teins , 25143 Itokawa .

The largest asteroid in the previous image, Vesta (left), with Ceres (center) and the

Moon (right) shown to scale.

Traditionally, small bodies orbiting the Sun were classified as comets, asteroids, or meteoroids, with anything smaller than one meter across being called a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition including size limits. [37][38] The term "asteroid", from the Greek word for "star-like", never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union .

However, following the discovery of asteroids below ten meters in size, Rubin and Grossman's 2010 paper revised the previous definition of meteoroid to objects between 10 µm and 1 meter in size in order to maintain the distinction between asteroids and meteoroids. [4] The smallest asteroids discovered (based on

absolute magnitude H) are 2008 TS26 with

{{{1}}} and 2011 CQ with {{{1}}} both with an estimated size of about 1 meter. [39]

In 2006, the term "small Solar System body " was also introduced to cover both most minor planets and comets. [40][d] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English especially for larger minor planets such as the

dwarf planets as well as an alternative for asteroids since they are not star-like. [41] The word "planetesimal " has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, [42] but is not in common use. The three largest objects in the asteroid belt, Ceres, Pallas, and Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet , the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.[43]

For almost two centuries, from the discovery of

Ceres in 1801 until the discovery of the first

centaur , Chiron in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as Hidalgo ventured far beyond Jupiter for part of their orbit. Those located between the orbits of Mars and Jupiter were known for many years simply as The Asteroids. [44] When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered asteroids or as a new type of object. Then, when the first

trans-Neptunian object (other than Pluto ),

Albion , was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object , trans-Neptunian object , scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. [45] They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.[46] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, [47] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line. [48]

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. [e] However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. [45] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt ,

Jupiter trojans , and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets – those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred." [49] Currently only the largest object in the asteroid belt, Ceres, at about 975 km (606 mi) across, has been placed in the dwarf planet category.

Artist's impression shows how an asteroid is torn apart by the strong gravity of a white dwarf .[50]

Formation

It is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. [51] Ceres and Vesta grew large enough to melt and differentiate , with heavy metallic elements sinking to the core, leaving rocky minerals in the crust. [52]

In the Nice model , many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres. [53]

Distribution within the Solar System

See also: List of minor-planet groups, List of notable asteroids, and List of minor planets

The asteroid belt (white) and Jupiter's trojan asteroids (green)

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the

Yarkovsky effect. Significant populations include:

Asteroid belt

Main article: Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and

Jupiter , generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, [54] and millions of smaller ones. These asteroids may be remnants of the

protoplanetary disk, and in this region the

accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by

Jupiter .

Trojans

Main article: Trojan (astronomy)

Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two

Lagrangian points of stability, L4 and L5 , which lie 60° ahead of and behind the larger body. The most significant population of trojans are the

Jupiter trojans . Although fewer Jupiter trojans have been discovered (as of 2010), it is thought that they are as numerous as the asteroids in the asteroid belt. Trojans have been found in the orbits of other planets, including Venus ,

Earth , Mars, Uranus, and Neptune .

Near-Earth asteroids

Known Near-Earth objects as of January 2018

[ https://www.youtube.com/watch?v=vfvo-Ujb_qk Video (0:55; 23 July 2018)

Frequency of bolides , small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere

Main article: Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers . As of June 2016, 14,464 near-Earth asteroids are known [31] and the number over one kilometer in diameter is estimated to be 900–1,000.

Characteristics

Size distribution

The asteroids of the Solar System, categorized by size and number

Asteroids vary greatly in size, from almost

1000 km for the largest down to rocks just 1 meter across. [f] The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, [55] and are thought to be surviving

protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.

The dwarf planet Ceres is by far the largest asteroid, with a diameter of 940 km (580 mi). The next largest are 4 Vesta and 2 Pallas, both with diameters of just over 500 km (300 mi). Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis .

The mass of all the objects of the asteroid belt , lying between the orbits of Mars and Jupiter, is estimated to be in the range of (2.8–3.2) × 1021 kg, about 4% of the mass of the Moon. Of this, Ceres comprises 0.938 × 1021 kg, about a third of the total. Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to half, whereas the three most-massive asteroids after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), constitute only another 3%. The number of asteroids increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with size. Although this generally follows a

power law , there are 'bumps' at 5 km and

100 km, where more asteroids than expected from a logarithmic distribution are found. [56]

Approxim

D 0.1 km 0.3 km 0.5 km 1 km

N 25 000 000 4 000 000 2 000 000 750 000 20

Largest asteroids

The four largest asteroids: 1 Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea

See also: Largest asteroids

Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas , are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth largest asteroid,

Hygiea, appears nearly spherical although it may have an undifferentiated interior [citation needed ]

, like the majority of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.

Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a dwarf planet .[40] It has a much higher absolute magnitude than the other asteroids, of around 3.32, [57] and may possess a surface layer of ice. [58] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.[58] No meteorites from Ceres have been found on Earth.

Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water; [59][60] its composition is mainly of basaltic rock with minerals such as olivine. [61] Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites , which constitute 5% of all meteorites on Earth.

Pallas is unusual in that, like Uranus , it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. [62] Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.[63] Pallas is the parent body of the Palladian family of asteroids.

Hygiea is the largest carbonaceous asteroid [64] and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic . [65] It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family, and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope 's SPHERE imager in 2017 and 2018, and announced in late 2019, revealed that Hygiea has a nearly spherical shape, which is at consistent both with it being in hydrostatic equilibrium (and thus a dwarf planet ), or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.[66][67]

Attributes

Name Orbital

radius

( AU) Orbital

period

(years) Inclination

to ecliptic Orbital

eccentricity

Ceres 2.77 4.60 10.6° 0.079 9

(

Vesta 2.36 3.63 7.1° 0.089 5

(

Pallas 2.77 4.62 34.8° 0.231 5

Hygiea 3.14 5.56 3.8° 0.117 4

The relative masses of the twelve largest asteroids known, [68][g] compared to the remaining mass of the asteroid belt. [69]

1 Ceres

4 Vesta

2 Pallas

10 Hygiea 31 Euphrosyne

704 Interamnia

511 Davida

532 Herculina 15 Eunomia

3 Juno

16 Psyche

52 Europa

all others

Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period smaller than 2.2 hours. [70] For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.[71]

Further information: List of fast rotators (minor planets) and List of slow rotators (minor planets)

Composition

Cratered terrain on 4 Vesta

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. [72] 10 Hygiea, however, which appears to have a uniformly primitive composition of

carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries : Rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or, possibly, a planet .[73]

Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (also see panspermia) .[74][75] In August 2011, a report, based on NASA studies with meteorites found on Earth , was published suggesting DNA and RNA components ( adenine,

guanine and related organic molecules ) may have been formed on asteroids and comets in

outer space.[76][77][78]

Asteroid collision – building planets (artist concept).

Composition is calculated from three primary sources: albedo , surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia , though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.[79]

On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility . The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. [80][81][82][83] Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the

Moon . The presence of ice on 24 Themis supports this theory. [82]

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61.[84] On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. [85] The detection was made by using the far-infrared abilities of the Herschel Space Observatory .[86] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." [86]

In May 2016, significant asteroid data arising from the Wide-field Infrared Survey Explorer and

NEOWISE missions have been questioned. [87]

[88][89] Although the early original criticism had not undergone peer review, [90] a more recent peer-reviewed study was subsequently published. [91][18]

In November 2019, scientists reported detecting, for the first time, sugar molecules, including

ribose , in meteorites , suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life , and supporting the notion of an

RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of

panspermia .[92][93]

Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have ice fossils inside it – the first direct evidence of water ice in the composition of asteroids.[94]

[ unreliable source? ]

Surface features

Most asteroids outside the " big four" (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50 km (31 mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300 km (186 mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. [95] Medium-sized asteroids such as Mathilde and

243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid . Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.[96]

Color

Asteroids become darker and redder with age due to space weathering. [97] However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.

A High Court in Owerri, Imo state capital has ordered the final and absolute forfeiture of Royal Palm Springs Hotel and other properties linked to Senator Rochas Okorocha and other members of his family.

Justice Fred Njemanze in his ruling on the case involving properties in pages 226 to 272 of the Imo state Government white paper report by a Judicial Commission of Enquiry on Recovery of Lands and other related matters, said Okorocha’s counsel failed to give concrete and verifiable reasons why a final and absolute forfeiture order should not be given in favour of the Imo state government.

Njemanze said the suit filed against Imo state Government by Okorocha’s counsel was not properly filed and further described it as surplusage (a term in lawn which means a useless statement completely irrelevant to a matter).

Insisting that the white paper gazette by the Imo state Government is a legal binding document, the high court judge added that those who might have made purchase on such properties are at liberty to approach the court to prove their titles. Some of the properties forfeited include:

1. Eastern Palm University, Ogboko 2. Royal Spring Palm Hotels and Apartments 3. IBC staff quarters said to have been illegally acquired for the purpose of Rochas Foundation College 4. Owerri magistrate quarters, Orlu road/cooperative office/Girls Guide allegedly converted to private use housing market square and Kilimanjaro eatery 5. Public building situated at plot B/2 Otamiri South Extension Layout given to the ministry of women affairs for establishing a skills acquisition centre for women, allegedly acquired for the benefit of Nneoma Nkechi Okorocha’s all-in Mall along Aba road. 6. Plot P5, Naze residential layout, initially part of primary school management board but now annexed to All-In Mall along Aba Road, belonging to Mrs Nkechi Okorocha 7. And all the properties contained from pages 226 to 272 of the government white paper on the recommendation of the judicial commission of inquiry into land administration in Imo state from June 2006 to May 2019”.

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5. Scorpion can survive for up to 1 year

Scorpions are the bad guys most of the time, but they sure aren’t on the list of foodies in the animal world. Scorpions naturally eat one-third of their body weight in a single meal.

This alone is known to help them to slow down their metabolism and deal with their nutrition. Now you can guess why it is possible for them to go up to a year without food.

4. Galápagos tortoises

The Galápagos tortoise complex or Galápagos giant tortoise complex ( Chelonoidis nigra and related species) are the largest living species of tortoise. The largest tortoise can live without any food or water and can stay up to 2 to 3 years. Interesting!

3. Crocodiles: 

Time without food: 3 years. Crocodiles are ancient reptiles that regularly go for long periods without food. As crocodiles rely solely on the external temperature to regulate their internal body temperature (aside from panting or moving between sunshine and shade), they don't require food to maintain a metabolism that maintains body temperature. They become more active in hot months and less active in the cooler months. Crocodiles are very efficient at storing energy in their bodies and are capable of eating large meals. This, and their ability reduce remain motionless for long periods means they can go without food for as long as three years.

2. Olms

This carnivore vertebrate, which is commonly found in Balkans and Italy underwater, is a true king! It can survive up to 10 years with no food and only use its stored lipids and glycogen in the liver.

1. Tardigrade — 30 years

A tardigrade goes into cryptobiosis, also known as a lowered metabolism. Their metabolism lowers to 0.01% of their normal rate and their water content can also drop to 1%. This makes them able to go without food for more than 30 years. Do you know any other animals we could add to the list? Please share them with us in the comment section!