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(meteorobs) A Message To Recent 'meteorobs' Subscribers, on the NE US Fireball
My name is Lew Gramer. I am a professional computer programmer and amateur
astronomer, and I administer this 'meteorobs' email forum.
I have noticed a number of new people just joining our list in the last 24
hours, presumably interested in gleaning reports of the unusually bright
fireball apparently witnessed over parts of the NE US yesterday, 23 July.
Welcome! You should be aware that we are a mailing list devoted to both the
SCIENCE and amateur observation of meteors. We hope you will remain with us
for some time, even after the fireball furor dies down, as we are nearing a
major annual event in meteors, the "Perseid" meteor shower of early and mid
August. This event is of considerable public interest, and often newsworthy
in its own right... And beyond that, very soon (November) many parts of the
world - including the Eastern US - hope for a once-in-a-generation event, a
meteor storm (see definition below) from the annual Leonid meteor shower.
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NOTE ON 'meteorobs':
We are NOT a list for discussing meteorite hunting, though that topic may
often legitimately relate to meteor science... Nor is 'meteorobs', in any
way, a forum for discussion of alien sightings, extraterrestrial life, or
general topics of astronomy or space science. PLEASE RESPECT THIS FOCUS!
[We do have the capability to moderate posts from a user, and in extreme
cases, to temporarily or even permanently remove that user from the list,
for ignoring our community's rules of courteous, on-topic discussion.]
Additionally, note that anything appearing on 'meteorobs' is considered
by our global readership, to have entered the public domain, regardless
whether specific provisions reserving rights, may be included within the
text: therefore be cautious what you choose to post on this public forum.
Finally please note that, as a community, meteor observers rely on an ever
expanding public awareness of, and DIRECT involvement in our endeavor! So
please don't hesitate to ask questions, or to cite any Web sites or other
references you see on our list, properly attributed. And again, welcome.
==========================================================================
ON FIREBALLS:
For a more extensive discussion of why seeing a fireball does NOT mean
that a meteorite will necessarily be found, please see the Web page:
http://www.tiacdot net/users/lewkaren/meteorobs/fireballs.html
And for a more authoritative source on all the above topics (often MORE
authoritative than your local professor of astronomy!), please see the
Web site of IMO - the International Meteor Organization:
http://www.imodot net
Here are Web sites of some communities of meteor observers specifically
focused here in North America. Members of these groups will be pursuing
further reports of this event over the next several days and weeks:
http://web.infoavedot net/~meteorobs/
http://www.amsmeteors.org/
http://wwwdsa.uqac.uquebecdot ca/~mhiggins/MIAC/
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DEFINITION OF METEOR TERMS:
By the way, just to ensure that all posts you read on 'meteorobs' are well
understood, here is a summary of terminology used in the study of meteors:
meteor - flash of light in the sky, caused by material entering earth's
atmospheric blanket at high enough speed to heat a column of air around
the path of entry. Also known as a "shooting star", "falling star", or
in some rural areas a "wishing star". Note that when you see a meteor,
you are almost NEVER watching anything "burn" in the air: rather, you
are seeing the kinetic energy of a particle which is really quite small,
being converted by friction into electromagnetic radiation, and causing
the air around it to glow from recombination. Note also, that not all
meteors are caused by natural material: a screw or space hammer, or a
rocket stage or satellite section, is traveling fast enough to produce
a lovely (often very slow, sometimes bright) meteor, when it reenters.
fireball - a very bright meteor, pure and simple! Generally, any meteor
which is as bright as the brightest planets or brighter, is a fireball.
Fireballs are individual occurrences, lasting no more than some seconds
or in very rare instances - usually in the case of artificial materials
reentering the Earth's atmosphere - a minute or so.
[Note that the glowing column of air left behind by a fireball may last
far longer than the visible meteor itself! Such "afterimages", known as
"persistent trains", have been known to remain visible for up to an hour
or more, gradually diffusing and being deformed by upper atmosphere wind.
When a witness describes an event as "remaining visible" several minutes,
they are quite likely to be describing this train, not the fireball...]
daytime fireball - a meteor which is so unusually bright, it can actually
be witnessed from the ground in full daylight. These are extremely rare,
simply because very bright fireballs are very, very rare: in general, the
brighter the fireball, the more rare it is. Such daytime fireballs may be
caused by surprisingly small particles (and often very fragile "cometary
debris"), depending on their speed relative to earth, and angle of entry.
Note that fireballs aren't the only celestial object visible in daylight
of course: the moon when it is up, is readily visible in full sunlight,
as are the very brightest planets, under just the right conditions.
meteor shower - this is an event which extends over one or (usually) many
nights, and is NOT an isolated or instantaneous event. Instead, a shower
is an event during which there is a measurable increase in the number of
meteors to be seen in the sky from a particular body. Meteor showers are
very common (though often lovely) events, which often occur more or less
predictably during the same dates every year. 'Fireballs' (see above) are
sometimes associated with a meteor shower, but often not. Note that there
are (to my knowledge), no established reports IN HISTORY of a *METEORITE*
every being associated with any known meteor shower. Meteor showers often
are caused by streams of fragile comet debris ("rivers of dust" orbiting
the sun much like earth does - see above under 'comet'). Such debris is
simply too fragile (as a rule) to be able to leave a solid meteorite.
meteor storm - a brief period (generally less than one hour) during which
a meteor shower becomes extremely active - sometimes displaying thousands
of shooting stars in that one brief period. Meteor STORMS are much rarer
than the showers that cause them, and may only happen once a generation.
Storms may sometimes happen to produce many bright fireballs, but are in
fact much more likely to consist of MOSTLY meteors which are very FAINT.
[And once again, meteor storms have never in recorded history been known
to be associated with a *METEORITE*. These are very fragile particles!]
comet - an object in space, which orbits the Sun just like the Earth,
other planets, and the 'asteroids'. The stable, enduring portion of a
comet, its "nucleus", is very small (less than 100 km) and is believed
to consist of a mix of volatiles (ice) and interplanetary dust, melded
together to form a "dirty snowball". If a dirty snowball's orbit brings
it near the Sun, the volatiles begin to sublimate (melt straight into a
gas), causing a halo of dusty debris to form (the "coma"). As the comet
orbits, some of this debris is "pushed away" from it by solar radiation
(the comet's "ion tail"), and the heavier material is "left behind" in
an orbit similar to the comet nucleus, forming a long debris trail (the
comet "dust tail"). This latter debris trail may be the source of a...
meteoroid - the particles of material in interplanetary space which are
the cause of most 'meteors' (see above). These often come from comets,
and are extremely fragile ("celestial dust bunnies"). But occasionally,
they may also come from asteroids, or even moons or planets in our solar
system, and may be much more dense and less fragile. In such rare cases,
and under even rarer circumstances of entry, a 'meteor' may produce a...
meteorite - particle which survives fiery entry into Earth's atmosphere,
without disintegrating, and actually finds its way to the surface. These
meteorites almost NEVER hit the earth with high velocity: instead, they
are far more likely to be entirely slowed by entry into our atmosphere.
This is much like throwing a stone into a clear pond: at first, it will
travel in the direction you tossed it. But very soon, it will be slowed
down by friction with the water, to the point where it finally begins to
fall straight down at a much slower speed, and simply "settles" to the
bottom of the pond under gravity. Our atmosphere is just such a pond!
[Final Note: 'bolide' is a non-specific term for a fireball that appears
to "explode", i.e., either fragments, ends in a bright "terminal burst",
or perhaps causes a sonic boom or other audible effect. This term is not
in common use among meteor observers, as it isn't entirely well defined.]
==========================================================================
Clear skies to all, and if you have any questions, please don't hesitate
to post them to our list! Answering questions from members of the public
interested in learning more about meteors is one reason why we're here.
Lew Gramer, 'meteorobs' List Administrator
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