[meteorite-list] Observed lunar meteorite impacts hit 100

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Fri, 23 May 2008 14:05:09 -0700 (MST)
Message-ID: <58442.71.226.60.25.1211576709.squirrel_at_timber.lpl.arizona.edu>

Hello Jerry:

Good summary!

There are actually other compositional (spectral) classes of asteroids,
but you hit on the main ones. While most C-asteroids are "wet," some
appear to be dry (at least on their surfaces). Also, there are "wet"
M-asteroids, which are clearly NOT metallic, but it is not yet clear what
they really are.

And yes, a "dead" comet may look spectrally like an asteroid!

Larry

On Fri, May 23, 2008 1:44 pm, jcomet2 at comcast.net wrote:
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>
> 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
>
>
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Received on Fri 23 May 2008 05:05:09 PM PDT