Originally termed planetoids, they are now preferably called as minor planets or asteroids. Asteroids perhaps is more appropriate, and none show discernible disks in the telescope, and whose term suggest being star-like Most asteroids are very much smaller objects compared with the planets. Typically most are irregularly shaped bodies about one kilometre or less in across. Several are hundreds of kilometres in size, with the largest Ceres being just under one thousand kilometres. Of all these planetary objects, we can see only Vesta near its favourable oppositions with the naked-eye.

In Part 2 of this article section lists the first hundred asteroids that were discovered between the years of 1801 to 1868.

Asteroid Discoveries

First of these smaller discovered planetary objects was Ceres by Sicilian observer Giuseppe Piazzi on 1st January 1801. Second was Pallas in March 1802, followed by Juno in 1804 then Vesta in 1807. Yet nearly forty years passed when the fifth minor planet, Astraea, was found in 1845. After 1846, there was the continuous litany of new discoveries. Three were then found in 1847, being Hebe, Iris, Flora, Metis in 1848 and Hygiea in 1849.

In the 1850s, fifty new ones were added — eight in 1852 alone! By 1860, sixty bodies were list, then one hundred with 100 Hekate in 1868. (See next page Table) By the 1891, the listed ended with 322 Phaeo, and at the end of the 19th Century the last was 452 Hamiltonia. After this period of fever-pitched telescopic observation, astronomers instead discovered many new asteroids photographically. Some were accidentally found during several newer important whole sky-surveys.

In the following fifty years, the first one found in 1950, was 1612 Hirose – adding 1160 new bodies. By 1991 more than 4,500 were catalogued, with 4,200 having identified and accurate enough orbital parameters to predicting future positions. In 2000 A.D., the number of bodies reached 38,679 asteroids. By October 2006, some 13,500 having proper or given names, with about 125 000 having been numbered of an estimated 360,000 — increasing by between 4,000 to 7,000 per month! In quite recent years with improved instrumentation and far more sensitive CCD cameras, the numbers of objects have exploded. In 2012, the numbered asteroids will surpass the 300,000 mark!!

Mostly for the majority of them, we have little information on their own physical characteristics, such as their general size and shape — though in recent years this has improved significantly our knowledge.

[Note : Each of the first ten have detailed Southern Astronomical Delights linked articles with useful ephemerides.]

Naming the Asteroids

Naming of all minor planets began by using the adopted traditional method of the plethora of Greek and Roman gods. The first several hundred discovered asteroids were all given female names. For example, Ceres as named after Sicilian goddess of grain and agriculture. Juno and Vesta were named after Ceres two sisters, who were also sisters to Jupiter, with Juno being his wife as well. Pallas name is derived as the alternative name for the goddess Athena, daughter of Jupiter, and therefore the niece of Ceres. Hebe is the cup bearer of the gods, while Iris is tthe Muses, Graces, Horae, nymphs, etc. messenger

After the 1860s such naming systems became quite impracticable due to the vast numbers of new discoveries. Today, several hundred are discovered annually, all requiring designated names. Officially, the I.A.U. decides the naming of newly discovered asteroids, which are announced new lists each year.

Each asteroid is listed in order of discovery with the temporary designation. I.e. 1982 FA (F = June; A = First one discovered in that month.) If an orbit is ascertained, then numbers are assigned, given in the order of their discovery of the orbital elements. I.e. 43 Ariadne or 2101 Adonis.

Many of them have been named after;

People; 1677 Tycho Brahe, 8000 Isaac Newton [1986 RL5], 1134 Kepler [1929 SA], 238 Hypatia, 4000 Hipparchus [1989 AV], 12155 Hyginus

Continents ; 2404 Antarctica (1980 TE), 1193 Africa/ 1931 HB, 8088 Australia [1990 SL27]

Oceans ; 224 Pacifica

American States. 439 Ohio, 793 Arizona [1907 AD], 48575 Hawaii [1994 NN],
1602 Indiana

Chemicals ; 308 Polyxo

Common Name Variants ; 114 Kassandra, 354 Eleonora, 537 Pauly.

Observing Asteroids

Many asteroids orbit the Sun roughly between the orbits of Mars and Jupiter, and averaging distances of between 2.0 and 2.5 Astronomical Units or 300 to 375 million kilometres. Like all the superior planet of Mars, most asteroids are often best viewed nearest opposition when they are at their brightest. Yet unlike the planets, they can vary significantly in brightness. This is mostly caused by the large possible comparable distance between the observer and asteroid, which using the distance range above 1.0 AU at opposition and 3 AU at conjunction. [For an outer planet like Uranus, has the much smaller half-magnitude difference, as the distance between opposition and conjunction is the smaller range of 18.1 AU compared to 20.1 AU.

On top of this, the majority have highly eccentric orbits, and this causes the brightnesses to have larger variations. It also makes favourable and unfavourable oppositions, leading to periods of several years to about a decade when asteroids are at their best.

Between oppositions can be considerably lengthened, as the period between successive alignments of Earth and the more distant body are very much longer than a year. This time difference is called the synodic period, and for superior planetary bodies, must be greater than one whole years. [We find that the closer the minor planets orbit is to the Earths orbit then the longer this synodic period becomes.]

Table : Common Asteroid Synodic Periods

 Asteroid    Orbital  Synodic  
No.   Name   Period   Period 
             (year)    (day)
01  Ceres     4.599    466.74
02  Pallas    4.619    466.18
03  Juno      4.362    473.90
04  Vesta     3.627    504.29
05  Astraea   4.136    481.73
06  Hebe      3.777    496.78
07  Iris      2.686    581.89
08  Flora     3.266    526.44
09  Metis     3.686    501.24
10  Hygiea    5.589    444.85
30  Urania    3.655    502.83
    Eris    559.550    365.90
    Pluto   247.921    366.73

Locating Asteroids

There are perhaps millions of fragments orbiting the Sun at similar distances. Science once popularly described for many years the popular theory of the destruction of some large Moon or Mars-sized planet, but most astronomers holding this once prominent theory has generally been on slow decline. We now believe the origin of these objects is instead some incomplete remnant from the very beginning of the Solar System, which never properly formed into a small planet.

Some asteroids have highly elliptical orbits that sometimes cross the path of the Earths orbit. Several hundred of these are known and described as Apollos or Apollo-type asteroids. Examples include, the first discovered, Apollo, followed by others like; Eros, Icarus, Hermes and Geographos. Asteroids exist inside the Earths orbit being placed in the sub-group known as Atens. Those crossing either Mars orbit, or similar terrestrial-like planet orbits, are often alternatively called Amors.

Other similar asteroid groups also exist, like those that precede and follow the orbit of Jupiter. These are believed to be naturally captured minor planets by Jupiters own powerful gravitational field.

The Achilles group lie 60º ahead, while the Patroclus group follows some 60º behind. Technically these places are called the Lagrangian Points; specifically being L4 and L5. They have named both asteroidal groups after the major combatants in the classical story of Trojan War as told in the Iliad and Odyssey as written by Homer. For reasons, which we do not really know, there are about 40% more asteroids in the trailing Patroclus group.

Closest known asteroid in approach the Sun is 3200 Phaethon / 1983 TB, being 5.1 km across whose solar distance varies between 2.403 AU down to 0.140 AU. At perihelion its closest approach is 0.15 A.U. or merely 10 million kilometres! Furthest from the Sun of the inner asteroids is 944 Hidalgo / 1920 HZ averages some 5.5 A.U, but whose eccentric orbit (e=0.660) finds the range anywhere between 1.95 AU to 9.54 AU during the 13.77 year period — being longer than even Jupiter. The orbit itself is highly inclined to the ecliptic at 42.6º!

Nearby Asteroidal Approaches to Earth

Sometimes close approaches of asteroids to the Earth do occur. For example, on 30th October 1937 the Apollo-type 300 metre equally massed binary asteroid 69230 Hermes / 1937 UB, came within 700 000 km or 2.0 times the lunar distance from Earth. During WWII in 1942, Hermes made an even closer approach of 1.7 the Moons distance but was not observed. Next approach will be in There is no chance of a direct impact of the Earth in the foreseeable future.

On 22nd March 1989 another even closer approach of 600 000 km (1.8 lunar distances) was made by estimated 300 metre diameter 4581 Asclepius Yet another passed during January 1991, being the tiny asteroid designated 1991 BA streaked past the Earth. Although only reaching a very faint 17th magnitude, the object passed within 130 000 km, only half the distance from the Moon! This object was calculated to be 5 to 10 metres in diameter, and so far the closest approach to date (2003).

All these Earth-crossing asteroids hold great interest (and some fear), as one of these bodies could crash into the planet, creating mass destruction. Some scientists believe that the dinosaur extinction some sixty-five million years ago was caused by an asteroid or comet, hitting the Earth and causing the so-called nuclear winter. The effect of any one kilometre or larger sized object would throw enormous amounts of dust into the atmosphere. This dust would block out the Sun for month or so — similar to the sight of burning oil-wells in Kuwait during the first Iraqi war. Under such conditions, temperatures on our planet could fall by an average by 60ºC. Most living creatures and people would probably just die from malnutrition due to all plants being unable to grow. This effect of nuclear winter could be expected to last six to twelve months — at least till most of the dust had settled.

Such collisions with asteroid sized objects are fortunately rare — happening once perhaps every two-hundred million years or so.

General Composition

The majority of the asteriods have very dark surfaces, perhaps dark as lumps of coal. Others show high reflectivity or albedo — the latter being the measured proportion of the amount of received light against the reflected light. I.e. Earth has an albedo of 0.39.

All asteroids are generally classified into a number of different types that are decided by observing the spectra displayed by them and the albedo value. Such important signatures tell us much about the asteroidal compositions themselves, and in some instances correspond to similar meteorites. Some of the major types include;

C-type: This class is typical of more than three-quarters of all known asteroids they are extremely dark, having typical albedos around 0.03. They are very similar to the carbonaceous chondrite meteorites, whose chemical constitution are mainly lighter than typical rock and contain many complex hydrocarbons. Most seem devoid of water ices like the comets.

S-type: These asteroids comprise 17% to 18% of all these three main types. They are relatively bright objects, whose albedos are brighter than C-types — typically between 0.10 and 0.22. Most are more metallic objects often with either mixtures of nickel-iron, iron or magnesium-silicates.

M-type. These asteroid types comprise of the remaining 7% to 8%, and are typically bright with albedos between 0.10 and 0.18. There composition are almost pure nickel-iron, and can have mean densities similar to the Earth.

Other than these main classes, there are also about another dozen rare types. I.e. K-class. Most today are placed in a variety of different classification systems, which is confusing to the average observer but important for specialists in the field. Several problems also exist when placing objects within some category, as some minor planets can display multiple characteristics. Really, the types are related to their densities, but knowledge of this for the vast majority have not yet accurately determined. Some asteroids are astonishingly light — about the density of water — clearly suggesting they are in many cases just a conglomerate of rocks gathered together gravitationally.

The S-Type can be measured spectroscopically to determine their compositions, with most seemingly being silicate type rocks mixed with dark organic molecules. since the fly-by mission of Halleys Comet in 1986, the very similar spectroscopic analysis obtained between comet and asteroid surfaces, have many planetary astronomers now believing there is real connection between comets and asteroids.

Of the bright C-type reflected asteroids, most are thought to be composed of ices such as water, ammonia or carbon dioxide scattered across their surfaces. In October 1991, the Galileo spacecraft had a fly-by of 951 Gaspra discovering some details about of its nature, followed in August 1993 by 243 Ida, and its moon Dactyl. On August and September 2005, the Japanese Hayabusa spacecraft was placed into orbit around 600-metre long and potato-shaped, 25143 Itokawa (1998 SF36).

Overall Appearances

Several asteroids diameters have been determined by rare stellar occultations — when time when faint stars disappears behind their tiny disks. Stellar positions are usually well known, but predicting any minor planet position is never approaching near this precision. Asteroidal occultations can only be exactly predicted by photographing their position several days before the event. The possible error in position is still large, with the path being several thousands of kilometres wide, even though the true path can only be as large as the asteroid. If several observers fall inside the path the rough shape of the asteroid can be determined. To see one of these events is often more like simply good luck than any real planning.

Asteroid shapes that have so far been determined do indicate only a very few of them are even close to spherical. Even Ceres, now formally classed as a dwarf planet is slightly oblate. Most are very odd cigar or irregular shapes. For example, 624 Hektor, is twice as long as it is wide!

Binary Asteroids

Over the years many asteroids have been found to have companions that are gravitational bound together. Due to the low gravity between them, they are are very close together.

In 1977, observations of the 225-kilometre 532 Herculina was found to have two separate pairs of occultations, indicating a double-asteroid merely 0.9 arc seconds away! Calculations revealed that this was actually possible. These two asteriods were found to be tens of kilometres across and are believed to be gently touching or nudging each other, or more correctly actually sitting together. Others subsequently have also been discovered, so that planetary observers now believe that multiple or double asteriods are rather commonplace, and are no longer deemed as unusual.

Several in recent years have been found to have no one but two companions, such as 87 Sylvia, which has been called after the mythology of the ancient Roman founders Romulus and Remus (or Sylvia I and II). Sylvia was the mother of these two brothers, who were taken from her when they were very young, and in legend were suckled to maturity by wolves. Sylvia was discovered by Norman Pogson on morning of 17th May 1866 in India. When found the magnitude of the body was 11.7v magnitude in northern Scorpius, and slowly fading, whose brightness was at the limits of the charts that were available to Pogson at the time. Its size is about 385×265×230 kilometres (c.±0.5), whose mean distance from the Sun is 3.49 AU or 522 million kilometres. The orbit itself is fairly circular, with an eccentricity of 0.080, whose period is roughly 6.52 years or 2381.68 days. As such, favourable oppositions are fairly rare when compared to the the four major asteroids. This dark asteroid is fairly faint because of it low reflective albedo, and some believe that it is not a solid object, but is merely just a gravitational collection of rubble of low density material. The highly elongated body has been found to have the rotational period of 5.18 hours, based on the light-curve, whose axis is tilted at 29.1º to the ecliptic.

Romulus was discovered on 16th February 2001 and seems quite sizeable compared to Sylvia itself, being about eighteen kilometres across, along with an orbital period of 87.59 hours or 3.6496 days, moving at the mean distance of 1356±0.5 kilometres. His brother Remus, was found on the 9th August 2004, being roughly half the diameter of Romulus or 7 kilometres across and also half the distance at 706±5 km. An orbital period of 33.09 hours or 1.3788 days has been calculated, but presently, little is known about the long term stability of the orbits of these moons.

Another established triple asteroid is 45 Eugenia, whose second little companion was found in 2004. Eugenia was discovered in Paris by Herman Goldschmidt in 27th June 1857, exactly one month after his discovery of 44 Nysa. The name is not from classical mythology, but is named after the last Empress Consort of France, Her Imperial Majesty, Empress Eugénie de Montijo (1826-1920) who was the wife of Napoleon III. Size is 305×220×145 kilometres, and like Sylvia, is thought to be composed of very dark material probably made of collection of rubble-like material. Mean distance of Eugenia is 406.897 million km. or 2.720 AU, the orbital period 4.49 years or 1638.46 days. During 1998, Mauna Kea observers found a small moon in orbit, which was later named Petit-Prince, who was born to Empress Eugénie on the 16th March 1856. With an orbital period of about five days, the estimated size is about 15 kilometres across, and is much like many other double asteroids. The second moon, called S/2004 (45) 1, was found by Franck March being announced in the IAUC 8817 Circular on 7th March 2007, even though the observations were made in February 2004 by the 8.2-metre telescope at Cerro Paranal owned by the European Southern Observatory (ESO). Presently the estimated diameter is merely 6 km. as it orbits closer to Eugenia to the asteroid than Petit-Prince.

Several other minor planets also display significant axial rotations, found more usually by observing the slow fluctuations in magnitude. Many have periods ranging between one and forty days, like the dog-boned shaped asteroid 216 Kleopatra (217±94 km.) and the K-type 104 kilometre diameter — 221 Eos vary by as much as 1.5 magnitudes.

Binary Asteroids

Examples of other binary asteroids include;

22 Kalliope, 41 Daphne, 45 Eugenia, 87 Sylvia, 90 Antiope, 107 Camilla, 121 Hermione, 130 Elecktra, 243 Ida, 283 Emma, 379 Huenna and 69230 Hermes.

Descriptions of the Main Asteroids

Trans-Neptunian Objects

(20000) V A R U N A
(50000) Q U A O A R
(90377) S E D N A
(90482) O R C U S
(136108) 2003 EL 61
(136472) 2005 FY 9
(28978) I X I O N
(2060) C H I R O N

NEXT : List of the First Hundred Asteroids


Last Update : 1st March 2014

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