[meteorite-list] Observed lunar meteorite impacts hit 100

From: jcomet2 at comcast.net <jcomet2_at_meteoritecentral.com>
Date: Fri, 23 May 2008 20:44:08 +0000
Message-ID: <052320082044.16446.48372C98000465480000403E2205886014CD9B0A03010C06_at_comcast.net>

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.


--
Jerry Armstrong
IMCA #5151
Received on Fri 23 May 2008 04:44:08 PM PDT


Help support this free mailing list:



StumbleUpon
del.icio.us
reddit
Yahoo MyWeb