Perseid questions

[Matt F. Cary]

I watched the Perseids on Thursday night/Friday morning from 2330 to 0500
PDT (subtract 7 for UT). I was in Shingle Springs about 30 miles east of
Sacramento, California.

There was some light pollution, but Andromeda was barely visible to the
naked eye.

At one point , I saw a brilliant blue meteor very near the Eastern horizon
and it made me jealous of whatever lucky person had that one go overhead,
where it must have been one of the brightest of the night. This got me
thinking about how far away that might be which led me to the following
question:

* At what altitude do meteors become visible? *

Two people I was viewing with saw one that was obscured from my view by
the house, but I could see the light the meteor threw off over the roof
and the burst it gave at the end. It lit up a fair protion of the sky;
maybe 15 degrees. This brought up my second question:

* What defines a fireball? *

The meteor described above would seem to be a clear example, but we
saw others that were quite bright. Is there a specific definition
or is it one of those things where you know one if you see it and if
you have to ask, it wasn't?

I looked through the FAQ for both questions and either it wasn't there
or I missed it.

This was the best meteor viewing I have seen; definitely better than last
year. We saw many that left persistent trains and there were very few
small ones that were barely visible.

I crudely estimated that the rate where we watched was well in excess of
200/hr extrapolating for a full-sky view from 0330 to 0430.

Matt Cary ca...@nas.nasa.gov

[Don Stimpson]

Matt F. Cary <ca...@win22.nas.nasa.gov> writes:

> * At what altitude do meteors become visible? *

This is my first post on internet. I've really enjoyed the
discussions in sci.astro and hope to contribute.

Ian Halliday et.al. Meteoritics 24, 65-72 (1989) "The typical
meteorite event, based on photographic records of 44 fireballs"
is a good reference.

The next paper (Halliday et.al. same journal & vol. pgs 173-178)
states that 9 fireballs per year drop >=1 kg of meteorites in an
area of 1 million square miles (equivalent to most of the mid
west!). Influx for a million square miles is 54 kg with masses
between 0.01 and 100 kg. On average!

Following data is from 44 events recorded by the Canadian camera
network from 1971 to 1984 (network stopped in March 31, 1985).

Average numbers are:

Duration: 4.2 seconds

Beginning height: 72km (44.7 miles)

Ending height: 31 km (19.3 miles)

Initial velocity: 15.2 km/s (34 kmph)

Terminal velocity: 8.2 km/s (18 kmph)

Peak Brightness: -9 (range -7 to -15)

A fireball(=bolide) is a bright meteor(=shooting star), no hard
cutoff that I know of. Oliver in the book "Meteors" says at
least as bright as Jupiter or Venus or mag. ca. -2 to -4.
--
all the best, don
STIM...@delphi.com
_________________________________________________________________
meteorite discovery center
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Don Stimpson `\)| Meteorite Discovery
Sheila Knepper `\| through Education
Gurnee, Il 60031 * and Exploration
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[Malcolm Currie]

Matt F. Cary <ca...@win22.nas.nasa.gov> writes:

> * At what altitude do meteors become visible? *

Around 100km (62 miles) for most meteors, though as Don Stimpson points
out, larger bodies that cause bright fireballs can penetrate much deeper
into the atmosphere.