Most of the asteroids are much small objects compared with the planets. Typically most are about one kilometre in diameter. Several are hundreds of kilometres across, with Ceres being the largest at just under 1 000 km. Of all these planetary objects, we can see only Vesta near opposition with the naked eye.
Originally termed planetoids, they are now preferably called as minor planets or asteroids. First of these objects was Ceres discovered by the Sicilian observer Giuseppe Piazzi on 1st January 1801. The second was Pallas in March 1802, followed in succession by, Juno in 1804 and Vesta in 1807. Yet nearly forty years were to pass when the fifth minor planet, Astraea, was discovered in 1845. After this, there was a litany of new discoveries. By the late 1890’s, some 322 were known.
After this fever-pithched telescopic observational period, astronomers began the discovery of most new asteroids photographically. Many were found accidentally during several important whole sky-surveys of the heavens. By 1991 more than 4 500 were catalogued, with 4 200 having identified and accurate enough orbital parameters to predicting future positions. In recent years the numbers have exploded. By October 2006, some 13 500 having proper or given names, with about 125 000 having been numbered and an estimated 360 000 - increasing by between 4000-7000 each month! Mostly, we have little information on the majority of minor planets ’ physical characteristics, such as their general size and shape.
Many asteroids orbit the Sun roughly between the orbits of Mars and Jupiter with distances of between 2.0 and 2.5 Astronomical Units. (300 - 375 million km.) Due to perhaps millions of fragments orbiting the Sun at similar distances, Science once popularly described for any years the theory of the destruction of a Moon or Mars-like planet. Among astronomers this theory has generally been in slow decline. We now believe the origin of these objects is some incomplete remnant from the beginning of the Solar System, which never formed into a planet.
Other asteroids have highly elliptical orbits that sometimes crosses the Earth’s path. Several hundred of these are known described as Apollo-type asteroids. Examples include, the first discovered Apollo, followed by others like; Eros, Icarus, Hermes and Geographos. Asteroids also exist inside the Earth’s orbit and are called called Atens. Those that crossing either Mars orbit, or those with terrestrial-like orbits, are often called Amors.
Other asteroid groups also exist, such as the groups that precede and follow the orbit of Jupiter. These are believed to be captured minor planets by Jupiter’s powerful gravitational field. The Achilles group lie 60o ahead, while the Patroclus group follows 60o behind. The technical term for these places is called the Lagrangian Points; L4 and L5. They have name both groups of asteroids after the major combatants in the classical story of Trojan War as told in the Iliad and Odyssey written by Homer. For reasons, which we do not really know, there are about 40% more asteroids in the Patroclus group.
The closest asteroid to the Sun is Phaethon. At perihelion its closest approach is 0.15 A.U. or merely 10 million kilometres! Futherest to the Sun is Hidalgo at 5.5 A.U.
Sometimes close approaches of asteroids to the Earth do occur. For example, in 1937 Hermes, have a diameter about 15 km, came within 700 000 km of the Earth. Another in January, 1991 the tiny asteroid designated 1991BA streaked past the Earth. Although only at 17th magnitude, the object passed within 130 000 km, only half the distance from the Moon ! The object was calculated to be 5 to 10 metres in diameter, and so far the closest approach to date (2003).
Another was discovered in March 1989 passed within just ten times the Moon’s distance, whose estimated to be about 150 metres across. All these Earth-crossing asteroids hold great interest (and fear), as one of these bodies could crash into the planet, creating mass destruction. Some scientists believe that the dinosaur extinction some 65 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 large sized object would throw enormous amounts of dust into the atmosphere, which would block out the Sun for month, similar to the burning of the oil-wells in Kuwait, during the first Iraqi war. Under such conditions, temperatures on our planet could fall on average by 60o. Most 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 the dust settled.
Such collisions with asteroid sized objects are fortunately rare - happening once perhaps every two-hundred million years or so.
The majority of the asteriods have very dark surfaces, perhaps dark as lumps of coal. Others have highly reflectivities or albedos. (The albedo being the measure of the amount of received light against the reflected light. Ie. Earth has an albedo of 0.39.)
All asteroids are classified into a number of different types that are decided by observing the spectra displayed by them and the albedo. Such important signatures tell us much about the composition of the asteroids themselves, and in some instances correspond to similar meteorites. 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. Ie, K-class. Most today are placed in a variety of different classification systems, which is confusing to the average observerbut 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 Halley’s 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 asteriods.
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 the 600-metre long potato-shaped asteroid, 25143 Itokawa (1998 SF36).
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 planeṯ position is never near this precision. Asteriodal occultations can only be exactly predicted by photographing the position several days before the event. The possible error in position is still large, with the path being several thousands of kilometers 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 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! In 1977, observations of the 225-kilometre 532 Herculina werr found to have two separate pairs of occultations, indicating that it was a ‘double-asteroid’, merely 0.9 arc seconds away! Calculations revealed that this was actually possible. These two asteriods are tens of kilometres across and believed to be gently touching each other, or more correctly sitting together. Others have also been discovered. Some observers believe that multiple or double asteriods are rather common. Several minor planets also suggest axial rotations because of the fluctuations in magnitude. Many have periods between 1 and 40 days. The dog-boned shaped asteroid 216 Kleopatra (217±94 km.) and the K-type 104-km. diameter 221 Eos vary by as much as 1.5 magnitudes.
The naming of Minor Planets began by using the traditional method of using the plethora of Greek and Roman Gods. The first several hundred asteroids discovered were all given female names. For example, Ceres as named after the 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 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 the messenger of the various Muses, Graces, Horae, nymphs, etc.
After the 1860’s this system became impracticable due to the vast number of discoveries. Today, several hundred are discovered annually, all requiring names. The I.A.U. decides the naming of newly discovered asteriods. Many have been named after famous people; Ie. Tycho Brahe (1677), Newton and Hypatia (238); States within countries ie. Ohio (439), Chemicals ie. Polyxo (308), or variations on common names Ie. Kassandra (114), Eleonora (354) and Pauly (537).
Each asteroid is listed in order of discovery with the temporary designation, Ie. 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. Ie. Ariadne (43) or Adonis (2101).
(1) C E R E S |
(2) P A L L A S |
(3) J U N O |
(4) V E S T A |
(5) A S T R A E A |
(6) H E B E |
(7) I R I S |
(8) F L O R A |
(9) M E T I S |
(10) H Y G I E A |
P L U T O |
E R I S |
(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 |
***************************************** No. Name Period Dia. Opp. Disc (yr.) (km.) Mag. Year ***************************************** 1 Ceres 4.6 848.4 06.8 1801 2 Pallas 4.61 498.1 08.3 1802 3 Juno 4.362 233.9 10.1 1804 4 Vesta 3.627 468.3 05.9 1807 5 Astraea 4.136 119.1 09.8 1845 6 Hebe 3.777 185.2 08.6 1847 7 Iris 2.686 199.8 09.0 1847 8 Flora 3.266 135.9 09.2 1847 9 Metis 3.686 151 09.6 1848 10 Hygeia 5.589 407.1 10.0 1849 11 Partherope 3.841 150 09.5 1850 12 Victoria 5.587 126 09.6 1850 13 Egeria 4.133 224 09.9 1851 14 Irene 4.167 158 09.1 1851 15 Enunomia 4.297 272 09.1 1851 16 Psyche 5.006 250 10.1 1852 17 Thetis 3.89 109 10.0 1852 18 Melpomena 3.479 150 09.5 1852 19 Fortuna 3.815 215 09.7 1852 20 Massilia 3.74 230 09.3 1852 21 Lutetia 3.081 115 10.5 1852 22 Calliope 4.962 177 10.0 1852 23 Thalia 4.293 111 11.5 1852 24 Themis 5.57 177 11.0 1853 25 Phocaea 3.723 249 10.6 1853 26 Proserpine 4.329 80 09.7 1853 27 Euterpe 3.596 108 09.7 1853 28 Bellora 4.631 126 10.1 1853 29 Amphitrate 4.087 195 09.1 1854 30 Urania 3.655 91 10.0 1854 *****************************************
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