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  #1  
Old May 1st 04, 02:30 AM
|-|erc
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"Prigator" wrote in
geopelia:

A football-sized meteorite falling to earth would be spectacular and
dangerous.


The Earth is rotating and moving in an orbit around the sun, the sun is moving
in an orbit with the galaxy, and our galaxy is moving in some direction. The
meteor may have come from some other system somewhere, moving at its own
velocity through space. It is not "falling" - it is intersecting. It may
collide with Earth's atmosphere at Mach 30. Earth's gravity has little to do
with it.


Football sized meteors hit Earth all day. Stare up at the night sky for 15 minutes and
you have a 50% chance of seeing a shooting star. For a football sized mass to
reach the ground it would have to start out several feet across. (Guesstimate!)



If astronauts on the moon saw one falling there, what would it
look like? (No fiery trail of course and no sound.)
Would there be a lot of heat on impact from kinetic energy?
How close could they be to it to survive? Would a pressure wave knock them
over without air?


They might see it coming if the sun is on it. You see why the moon has so many
craters? Craters within craters. It has been hit by big rocks and little
rocks for a few billion years with no atmosphere to shield the surface. The
astronauts would be in danger of flying shrapnel with a near miss.


Right! Hence the frequency of shooting stars visibly burning up when reaching Earth.
They come in clusters so its common to see 1 a minute!

Herc



  #2  
Old May 1st 04, 02:57 AM
TMG
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|-|erc wrote:



Football sized meteors hit Earth all day. Stare up at the night sky for 15 minutes and
you have a 50% chance of seeing a shooting star. For a football sized mass to
reach the ground it would have to start out several feet across. (Guesstimate!)


Nitpicking: If they actually hit the ground, they're Meteorites - far
more rare. The vast majority of what you're seeing in the night sky,
while spectacular, never reaches the ground. Maybe you meant "hit the
Earth's atmosphere"

http://www.solarviews.com/eng/meteor.htm

Most meteors are in the cm^3 range - most actually in the "grain of
sand" range. One of the most thrilling shows is the summer (northern
hemisphere) Perseid meteor shower.

http://www.space.com/spacewatch/pers..._010731-1.html

- regards

  #3  
Old May 1st 04, 04:04 AM
Rob Duncan
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"TMG"

One of the most thrilling shows is the summer (northern
hemisphere) Perseid meteor shower.

- regards


Is it just me or has it been sucking lately? I seem to remember it being
much livelier when I was a kid. Then again I grew up in a farming community
with few city lights.


Rob


  #4  
Old May 1st 04, 04:14 AM
TMG
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Rob Duncan wrote:

"TMG"


One of the most thrilling shows is the summer (northern
hemisphere) Perseid meteor shower.

- regards



Is it just me or has it been sucking lately? I seem to remember it being
much livelier when I was a kid. Then again I grew up in a farming community
with few city lights.


Rob



They vary year to year - and "yes" a bit disappointing recently. It's
also possible you're remembering the Leonids in the fall, which are more
impressive - I've mixed up the two when "remembering as a kid". I
suspect the biggest variable is the light pollution you didn't have as a
youth.
  #5  
Old May 1st 04, 09:57 AM
Rob Duncan
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"TMG" wrote in message
...
Rob Duncan wrote:

"TMG"


One of the most thrilling shows is the summer (northern
hemisphere) Perseid meteor shower.

- regards



Is it just me or has it been sucking lately? I seem to remember it

being
much livelier when I was a kid. Then again I grew up in a farming

community
with few city lights.


Rob



They vary year to year - and "yes" a bit disappointing recently. It's
also possible you're remembering the Leonids in the fall, which are more
impressive - I've mixed up the two when "remembering as a kid". I
suspect the biggest variable is the light pollution you didn't have as a
youth.


Yah, I think youre right. Plus, I probably didnt even know the name of them
as a child. I just remember our family laying out blankets next to the pool
to lie on as we watched the "Star Show". Boy, I remember being thrilled as
a child seeing all those meteors. And identifying satelites as they crossed
our path of vision. Here, Portland, you have to drive 60 miles just to get
away from the light polution.


Rob


  #6  
Old May 1st 04, 01:15 PM
Gistak
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On 4/30/04 9:30 PM, in article
, "|-|erc"
wrote:

For a football sized mass to
reach the ground it would have to start out several feet across.
(Guesstimate!)


How would something that was several feet across be football sized?

Damn, I sure don't want to play on YOUR team!

P

  #7  
Old May 1st 04, 11:01 PM
Kilolani
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"Gistak" wrote in message
...
On 4/30/04 9:30 PM, in article
, "|-|erc"


wrote:

For a football sized mass to
reach the ground it would have to start out several feet across.
(Guesstimate!)


How would something that was several feet across be football sized?

Damn, I sure don't want to play on YOUR team!


I believe his implication is that it would have to be several feet across
when it entered the atmosphere in order to survive and hit the Earth as a
"football sized" mass. Unfortunately, he is incorrect in his basic premise.
Football sized and larger meteorites are not striking the Earth on a daily
basis. The size of most meteors is somewhere between a grain of sand and a
grain of rice.


  #8  
Old May 2nd 04, 03:39 AM
|-|erc
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"Kilolani" wrote in
wrote:

For a football sized mass to
reach the ground it would have to start out several feet across.
(Guesstimate!)


How would something that was several feet across be football sized?

Damn, I sure don't want to play on YOUR team!


I believe his implication is that it would have to be several feet across
when it entered the atmosphere in order to survive and hit the Earth as a
"football sized" mass. Unfortunately, he is incorrect in his basic premise.
Football sized and larger meteorites are not striking the Earth on a daily
basis. The size of most meteors is somewhere between a grain of sand and a
grain of rice.


You swapped from meteorite to meteor there. But the moon has millions of craters
from reasonable sized collisions. Nothing is reaching Earths ground (regularly)
so my implication was meteors are mostly *less* than football sized.

Are you sure a grain of sand will ignite a visible trail across the sky?

What is the smallest sized meteorite that will become a meteor and hit the ground?
These occur on a decade or century basis, (second guesstimate).

Herc



  #9  
Old May 2nd 04, 04:13 AM
TMG
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|-|erc wrote:
"Kilolani" wrote in

wrote:


For a football sized mass to
reach the ground it would have to start out several feet across.
(Guesstimate!)


How would something that was several feet across be football sized?

Damn, I sure don't want to play on YOUR team!


I believe his implication is that it would have to be several feet across
when it entered the atmosphere in order to survive and hit the Earth as a
"football sized" mass. Unfortunately, he is incorrect in his basic premise.
Football sized and larger meteorites are not striking the Earth on a daily
basis. The size of most meteors is somewhere between a grain of sand and a
grain of rice.



You swapped from meteorite to meteor there. But the moon has millions of craters
from reasonable sized collisions. Nothing is reaching Earths ground (regularly)
so my implication was meteors are mostly *less* than football sized.



The term meteorite means it gets all the way to the ground. Meteors do
not. It's estimated that the earth gains ~10^8 kg per year from
non-terrestrial infall. I'll leave it to the studious reader to
calculate the impact on earth's gravity, tides, rotation, frame drag, etc.


Are you sure a grain of sand will ignite a visible trail across the sky?


Yes - plasma glow really lights up a dark night sky. Light pollution is
why most of us don't see this as a common event.


What is the smallest sized meteorite that will become a meteor and hit the ground?
These occur on a decade or century basis, (second guesstimate).


[other way around - 'ites are the ones that hit the ground]

Depends on the angle of attack relative to the atmosphere. Dust floats
in constantly. As to the strike size relative to the start size, there
is a graph at: http://www.tulane.edu/~sanelson/geol204/impacts.htm
that will give a good approximation.

Herc


regards
  #10  
Old May 2nd 04, 04:42 AM
Gistak
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On 5/1/04 10:39 PM, in article
, "|-|erc"
wrote:

"Kilolani" wrote in
wrote:

For a football sized mass to
reach the ground it would have to start out several feet across.
(Guesstimate!)


How would something that was several feet across be football sized?

Damn, I sure don't want to play on YOUR team!


I believe his implication is that it would have to be several feet across
when it entered the atmosphere in order to survive and hit the Earth as a
"football sized" mass. Unfortunately, he is incorrect in his basic premise.
Football sized and larger meteorites are not striking the Earth on a daily
basis. The size of most meteors is somewhere between a grain of sand and a
grain of rice.


You swapped from meteorite to meteor there. But the moon has millions of
craters
from reasonable sized collisions. Nothing is reaching Earths ground
(regularly)
so my implication was meteors are mostly *less* than football sized.

Are you sure a grain of sand will ignite a visible trail across the sky?


From space.com:

--
Many shooting stars are produced by grit no larger than a grain of sand.
Some of the more spectacular ones are pea-sized and the really stunning (but
very rare) fireballs are the size of an orange or larger.

http://www.space.com/scienceastronom...rs_000809.html
--

What is the smallest sized meteorite that will become a meteor and hit the
ground?


A meteorite is a meteor that has struck earth's surface.

P

 




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