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(meteorobs) Fireballs, dark flight and meteorites (was Re: Bolide booms)
Johnie <Iansplanet@aol.com> asked:
>Maybe I will reveal my ignorance, but if a [fireball] losts space velocity
>does it cease to heat the air so that we see the fiery result and/or trail?
It sure does, John! Any object below a certain mass, even one that enters
the atmosphere at the maximum interplanetary velocity relative to the Earth
of 72 km/s, will quickly lose ALL of its celestial velocity... After falling
to a certain altitude in the atmosphere (usually above 30 km altitude, but
for very small particles, as high as 100km), it will slow to the point where
Earth's gravity is the only reason it continues to fall - it has become just
a free-falling rock at that point!
This period of freefall is called the "dark flight" for the particle: for
almost ALL fireballs people report, the particle is small enough (less than
a ton let's say!) that this dark flight starts above 20 km. So now you know
why, when someone says they saw a fireball "fall all the way to the ground",
they almost certainly mean it APPEARED to do so because it dipped below the
local horizon - NOT that it is actually really landed nearby! :)
>Would what is found on the earth be all of the [fireball] or how much would
>be lost while it traveled through the air? Thanks.
That's a tougher question to answer, John. I sure don't know all the physics
involved! The problem is you have to know a lot of things to say for sure how
much will "burn off" (be ablated) from the meteoroid: What is the surface area
of its cross-section from different angles? What is its density? Is it a single
solid "rock", a composite of smaller particles - a "bag of rocks", or something
else? What is the angle of incidence? What mix of materials is it made of? How
well does heat transfer to the inside of the particle?
These questions go on and on: but maybe someone can answer you who is more
familiar with meteorITES - the particles that occasionally actually make it
through the fires of atmospheric entry and manage to strike earth. Maybe there
are some simplifying assumptions I don't know, that can be used to figure out
how much on average will be ablated during entry?
Now of course, MOST meteoroids - even those REALLY bright fireballs we enjoy
reading about during the Perseids or Leonids - are particles that completely
ablate long before they could reach the ground! For cometary debris, such as
meteor showers (including the GEMs?) are made of, is closer to the density
and consistency of DUST BUNNIES than it is to that of solid rocks...
Sorry about the EXTENSIVE answer, folks: I get long-winded when the clouds
keep me inside during major showers. So pray for clear skies! ;>
Later,
Lew
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