Asteroids& l& Asteroid facts, pictures and information
On the first day of January 1801,
discovered an object which he first thought was a new comet.
But after its orbit was better determined it was clear that it was not a comet but more like a small planet.
Piazzi named it Ceres, after the Sicilian goddess of grain.
Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno).
By the end of the 19th century there were
several hundred.
Asteroid History
Several hundred thousand asteroids have been discovered and given provisional designations so far.
more are discovered each
year. There are undoubtedly hundreds of thousands more that are too small
to be seen from the .
There are 26 known asteroids larger than 200 km
in diameter.
Our census of the largest ones is now fairly complete: we probably
know 99% of the asteroids larger than 100 km in diameter.
Of those in the 10 to
100 km range we have cataloged about half.
But we know very few of the smaller
there are probably considerably more than a million asteroids
in the 1 km range.
The total mass of all the asteroids is less than that of the
11 comets and asteroids have been explored by spacecraft so far, as follows: ICE flyby of Comet Giacobini-Zinner. Multiple flyby missions to Comet Halley. Giotto (retarget) to Comet Grigg-Skellerup. Galileo flybys of asteroids Gaspra and Ida (and Ida satellite Dactyl). NEAR-Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros. DS-1 flybys of asteroid Braille and Comet Borrelly. Stardust flyby of asteroid Annefrank and recent sample collection from Comet Wild 2. For future we can expect: Hayabusa (MUSES-C) to asteroid Itokawa, Rosetta to Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres.
were photographed by the
spacecraft on its way to Jupiter.
mission flew by
(left) on 1997 June 27
returning many images.
NEAR (now renamed "NEAR-Shoemaker") entered orbit around 433 Eros (right)
in January 1999 and returned a wealth of images and data.
At the end of its mission
it actually landed on Eros.
The largest asteroid by far is 1 Ceres.
It is 974 km in diameter
and contains
about 25% of the mass of all the asteroids combined. The next largest are
2 Pallas, 4 Vesta and
10 Hygiea which are between 400 and 525 km in diameter. All
other known asteroids are less than 340 km across.
There is some debate as to the
of asteroids, comets and moons.
There are many planetary satellites that are probably better thought of as
captured asteroids. 's tiny moons
's outermost moon,
and perhaps some of the newly discovered moons of,
are all more similar to asteroids than to the larger moons.
(The composite image at the top of this page shows Ida, Gaspra, Deimos and Phobos
approximately to scale.)
Asteroids are classified into a number of types
according to their spectra (and hence their chemical composition) and
C-type, includes more than 75% of known asteroids:
extremely dark (albedo 0.03);
similar to carbonaceous
approximately the same chemical composition as the
Sun minus hydrogen, helium and
S-type, 17%: relatively bright (albedo .10-.22); metallic
nickel-iron mixed with iron- and magnesium-
M-type, most of the rest:
bright (albedo .10-.18); pure
nickel-iron.
There are also a dozen or so other rare types.
Because of biases involved in the observations (e.g. the dark C-types
are harder to see),
the percentages above may not be representative of the true distribution of
asteroids.
(There are actually several classification schemes in use today.)
There is little data about the densities
of asteroids.
But by sensing the Doppler effect on radio waves
returning to Earth from NEAR owing to the (very slight)
gravitational tug between asteroid and spacecraft, Mathilde's mass
could be estimated.
Surprisingly, its density turns out to be not
much greater than that of water, suggesting that it is not a solid
object but rather a compacted pile of debris.
Asteroids are also categorized by their position
in the solar system:
Main Belt: located between Mars and Jupiter roughly
from the S
further divided into subgroups: Hungarias, Floras,
Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are
named after the main asteroid in the group).
Near-Earth Asteroids (NEAs): ones that closely approach the Earth
semimajor axes less than 1.0
distances greater than 0.983 AU;
semimajor axes greater than 1.0 AU and
less than 1.017 AU
perihelion distances between 1.017 and 1.3 AU;
located near Jupiter's
points (60 degrees
ahead and behind Jupiter in its orbit). Several hundred such
ast it is estimated that there may be a
thousand or more altogether. Curiously, there are
many more in the leading Lagrange point (L4) than in the trailing one (L5).
(There may also be a few small asteroids
in the Lagrange points of Venus and Earth
that are also sometimes known as
5261 Eureka is a "Mars Trojan".)
Between the main concentrations of asteroids in the
Main Belt are
relatively empty regions known as the Kirkwood gaps. These are
regions where an object's orbital period would be a simple fraction of that
An object in such an orbit is very
likely to be accelerated by Jupiter into a different orbit.
There also a few "asteroids" (designated as "Centaurs")
in the outer solar system: 2060 Chiron
(aka 95 P/Chiron) orbits
the orbit of 5335 Damocles ranges from near
U 5145 Pholus orbits from Saturn to past
There are probably many more, but such planet-crossing orbits are unstable and
they are likely to be
in the future. The composition of these objects
is probably more like that of comets or
objects than that of ordinary asteroids.
In particular, Chiron is now classified as a comet.
has been studied recently with
particularly interesting asteroid in that it seems to have been differentiated
into layers like the
This implies some internal heat source
in addition to the heat released by long-lived radio-isotopes which alone would be
insufficient to melt such a small object.
There is also a gigantic impact basin
so deep that it exposes the mantle beneath Vesta's outer crust.
Though they are never visible with the unaided eye, many asteroids are visible with
binoculars or a small telescope.
Asteroid table
A few asteroids and comets are listed below for comparison.
(distance is the mean distance to the Sun in tho
masses in kilograms).
Discoverer
---- ---------
----------
1566 Icarus
1862 Apollo
2212 Hephaistos
15 Eunomia
De Gasparis 1851
Goldschmidt 1858
De Gasparis 1849
511 Davida
911 Agamemnon
2060 Chiron
More about asteroids
, a very special asteroid!
(see also the
fact sheet from
images from
lots more info from Zeljko Lipanovic
the Near-Earth Asteroid Rendezvous Mission
more about the , the first known near-Earth asteroid
Keck II images of Vesta
NEAT, Near-Earth Asteroid Tracking Home Page
two pages about 4179 Toutatis
On-Line Asteroid Data
from LANL; a diagram of Vesta's history from B. Zellner (136k postscript)
More on the
and its significance from STScI
, which will pass close to Earth in the year 2028 (from JPL)
Minor Planet Designations, search for asteroid names and numbers
various Minor Planet information from the MPC
Orbital elements from
of Lowell Observatory
Earth's strange companion, Asteroid 3753 Cruithne (1986 TO)
Open Issues
Why are there all those asteroids instead of a planet between
What mechanism(s) are responsible for the differentiation of the asteroids
into metallic and rocky types?
Why are there more Trojan asteroids in Jupiter's L4 point than its L5 point?
Is 4 Vesta really differentiated?
What is its geologic history?
How do asteroids get pushed from their 'normal' orbits into Earth-crossing
What is the probability that a large comet or asteroid will hit the Earth
in any given year?
Nine Planets - Asteroids
1994 - 2015 (C)
A guide to our solar system and beyond.}