Station Info :: Asteroids
An asteroid is a small, solid
object in our Solar
System, orbiting the Sun. An asteroid is an example
of a minor planet (or planetoid), which are much smaller than
planets. Most asteroids are believed to be
remnants of the protoplanetary disc which were not incorporated
into planets during the system's formation. Some asteroids
have moons. The vast majority of the asteroids are within
the main asteroid belt, with elliptical orbits between those
of Mars and Jupiter.
The term "asteroid", meaning star-like was coined in 1802 by Sir William Herschel shortly after Olbers discovered
the second one, 2 Pallas, in late March of the same year,
to describe their star-like appearance; the other then-known
planets all show discs, by comparison.
He also applied that term to the small moons of the giant
planets. The first scientific paper to use the word in its
title was published in 1840 by Erman.
The exact definition of an asteroid is unsettled. The term
"Minor planet" carries no strong suggestion about
the composition of the object or its general location in the
solar system, and some argue that not every minor planet should
be called an "asteroid".
One way to classify asteroids is in terms of size. A working
definition is that asteroids are larger than 50 m in diameter,
distinguishing them from meteoroids, which are typically boulder-sized
or smaller. The distinction is made because asteroids are
large enough to survive passage through Earth's atmosphere
and strike Earth largely intact while the smaller meteoroids generally break
up high in Earth's atmosphere.
Thus, it would be safest to use the term "asteroid"
for Solar System objects that are bigger than meteoroids,
smaller than planets, and made out of rock, not ice. See Solar
System for a complete taxonomy of objects in our system, and
minor planet for taxonomy of the sub planetary objects that
include asteroids. The term artificial asteroid is sometimes
used to designate man-made objects which have ended up in
solar orbits, such as the Mariner IV probe.
Asteroids in the Solar System
Hundreds of thousands of asteroids
have been discovered within the solar system. As of October
19, 2005, from a total of 299,733 minor planets with calculated
orbits, 118,161 asteroids had been calculated well enough
to be given official numbers and 12,712 of these had been
officially given trivial names to go along with the numbers
(at least 610 of which have names requiring diacritics). The
lowest-numbered but unnamed minor planet is (3360) 1981 VA;
the highest-numbered named minor planet is 99942 Apophis.
The Minor Planet Circular
(MPC) of October 19, 2005 was a historical one, as it saw
numbered asteroids jump from 99947 to 118161, causing a small
"Y2k" like crisis for various automated data services
—up until then, only five digits were allowed in most
data formats for the asteroid number. (This was solved in
some data fields by having the leftmost digit, the ten-thousand
places, use the alphabet as a digit extension. A=10, B=11…
Z=35, a=36… z=61. The highest number 118161 thus is
cross-referenced as B8161 on some lists.)
Current estimates put the
total number of asteroids in the solar system
at several million. The largest asteroid in the inner solar
system is 1 Ceres, with a diameter of 900-1000 km. Two other
large inner solar system belt asteroids are 2 Pallas and 4
Vesta; both have diameters of ~500 km. Vesta is the only main
belt asteroid that is sometimes visible to the naked eye .The
mass of all the asteroids of the Main Belt is estimated to
be about 2.3x1021 kg, or about 3% of the mass of our moon.
Of this, 1 Ceres comprises 940 to 950x1018 kg, some 40% of
the total. Adding in the next three most massive asteroids,
4 Vesta (12%), 2 Pallas (9%), and 10 Hygiea (4%), bring this
figure up 66%; while the three after that, 511 Davida (1.6%),
704 Interamnia (1.4%), and 3 Juno (1.2%), only add another
4% to the total mass. The number of asteroids then increases
exponentially as their individual masses decrease.
Until the age of space travel,
were merely pinpricks of light in even the largest telescopes
and their shapes and terrain remained a mystery.
The first close-up photographs of asteroid-like
objects were taken in 1971 when the Mariner 9 probe imaged
Phobos and Deimos, the two small moons of Mars, which are
probably, captured asteroids. These images revealed the irregular,
potato-like shapes of most asteroids, as did subsequent images
from the Voyager probes of the small moons of the gas giants.The
first true asteroid to be photographed in close-up was 951
Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl,
all of which were imaged by the Galileo probe en route to
The first dedicated asteroid probe was NEAR
Shoemaker, which photographed 253 Mathilde in 1997, before
entering into orbit around 433 Eros, finally landing on its
surface in 2001. Other asteroids briefly visited by spacecraft
en route to other destinations include 9969 Braille (by Deep
Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).
In September 2005, the Japanese
Hayabusa probe started studying 25143 Itokawa in detail and
will return samples of its surface to earth. Following that,
the next asteroid encounters will involve the European Rosetta
probe (launched in 2004), which will study 2867 steins and
21 Lutetia in 2008 and 2010. NASA is planning to launch the
Dawn Mission in 2006, which will orbit both 1 Ceres and 4
Vesta in 2010-2014.