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Definitions of Asteroids, Comets, NEO's, etc.

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#1 Glassthrower

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Posted 25 May 2008 - 05:08 PM

I ran across this well-written "mini-article" that explains the various solar system bodies that can produce meteorites, or are associated with meteorites. The author drew upon personal expertise and reference materials found on the web.

I thought this article would receive an appreciative audience here in the Space Rocks forum, so I emailed the author and asked him for permission to reprint his work here. His name is Jerry Armstrong, and he is a member of IMCA. He gave his permission for this to be reprinted here. :)

I have added nothing to this article. All I did was clean up the formatting and add some UBB code for use here on CN.

There is a large distinction between the classical comets and the
classical asteroids. Comet Hale-Bopp or Huykatake could never be confused
for an asteroid. On the other hand there are comets that are almost spent
out such as P/Arend-Regeaux which at often times appears stellar like
through even the largest telescopes. Then there is P/Schwassmann-Wachmann 1
which travels in a near circular orbit and is quite faint and stellar appearing until it
goes into an out burst approximately once per month. I have monitored this comet
for years and it can brighten very rapidly several hundred fold and produce a coma.
It may or may not be tied into the rotation due to the out bursts are not predictable
and can vary by 20 to 60 days.

For those of you who have been confused by all the classifications of comets and asteroids,
here is a very simple, and I stress simple classification. It does not get into groups or families.
I'm sure there are many mistakes and I take full responsibilty for the content.

Near Earth Objects – Objects that approach the earth’s orbit.

Atens – NEO’s with average orbital radii smaller than earth’s

Apollos – NEO’s with average orbital radii greater than earth but cross earth’s orbit

Amors – NEO’s with orbital radii between Mars and Earth’s and perihelia just outside Earth’s orbit.

Main belt – Asteroids in roughly circular orbits between Mars and
Jupiter, most have inclinations less than 30 degrees and eccentricities less
than 0.4

C-type – Carbonaceous asteroids with spectra similar to carbonaceous
chondrites and comprising 75% of known Main Belt asteroids, found in the
outer reaches of the asteroid belt.

S-type – Silicate rich asteroids with some metal but no carbon.
Comprise about 17% of visible asteroids.

M-type – Metal rich asteroids comprising about 10% of known asteroids.
Possibly remnant core of differentiated body composed of nickel-iron.
One problem is that some silicate compounds can mimic metal spectra. It
is not yet clear whether all M-types are compositionally similar, or
whether it is a label for several varieties which do not fit neatly into
the main C and S classes.

V-type – Basaltic type asteroids, very rare but evidently more than one
with different histories exist for this type. Vesta is the namesake of
this type.

Trojan asteroids – Asteroids in roughly same orbit as a planet and
found approximately 60 degrees ahead or behind the planet.

Mars Trojans - has only two.

Trojans - Jupiter has several hundred to several thousand, first ones
to be discovered.

Neptune Trojans - is the only other gas giant to date with known
Trojan asteroids. Spectra suggests that they are composed of water ice with
a layer of dust and probably more akin to comets.

Centaurs – Objects with orbits between the gas giants Jupiter and
Neptune. Three of them have exhibited cometary behavior.

Damocloids – Halley type objects some have retrograde orbits, all have
high eccentricity orbits, a few have since been shone to be comets.


Trans-Neptunian Objects :

Kuiper belt objects – extend from Neptune’s orbit to 55 AU from the
Sun. All are icy bodies composed primarily of ammonia, methane and water

Cubewanos – objects with roughly circular orbits and low inclination
and not in resonance with Neptune.

Plutinos – objects with eccentric orbits, often crossing Neptune’s
orbit and inclinations of 10 to 25 degrees. Also in a 2 to 3 resonating
orbit with Neptune. Pluto crosses Neptune’s orbit but due to inclination
and resonation it cannot collide with the planet.

Scattered Disc Objects – Poorly understood objects beyond Neptune in
highly eccentric and highly inclined orbits that computer simulations
revealed their orbits to be unstable.

Ort Cloud – Vast region of space roughly 1000 times further than Kuiper
Belt.

Inner Ort Cloud – Also known as the Hills cloud, roughly 2000 to 20,000
AU and doughnut shaped. Probably the main source of comets.

Outer Ort Cloud – Spherical shaped region with inner edge about 20,000
AU extending out to 50,000 and possibly 100,000 AU or more than one
light year. No known Ort Cloud Objects have been discovered beyond the Kuiper Belt.
The only known Ort Cloud Objects are the long period comets that visit
the inner solar system. All have high eccentricities and can even be
in retrograde orbits.

There are some comets that are confined to the asteroid belt. There
have even been cases when a known asteroid has turned into a comet. And we
have one case where a known asteroid shares an orbit with a known
meteor shower and is indeed postulated to be a dead comet. A few comets are
in fact beginning to slowly turn off (less and less outgassing on each
trip around sun).

Basically it boils down to all asteroids are in prograde orbits and are
rocky type objects. Comets on the other hand can be prograde or
retrograde, and in all sorts of orbits. They are composed of ice and dust
intermixed with rock. Compositions both in asteroids and comets probably
vary greatly. There may be rare cases when it is evident that something
may have hit an asteroid and only a dust type tail is produced, would
this be for classifying this object as a comet?

It is clear that as one descends from stars to brown dwarfs to gas
giants, to rocky planets, to planetoids, to asteroids, to meteoroids and
from asteroids to comets, the distinctions begin to blur. Although there
will always be classical objects in their own classes, there will also
be some overlap of objects that just do not quite fit in any category.
This is what makes astronomy and collecting meteorites interesting
indeed.




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