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.
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
planet’s orbit is to the
Earth’s orbit then the longer this
synodic period becomes.]
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 Earth’s 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 Earth’s
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 Jupiter’s 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 Moon’s 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 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 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.