[meteorite-list] Witness information that is more helpful than color

From: Matson, Rob D. <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Wed, 4 Nov 2015 23:48:26 +0000
Message-ID: <4A4FA25E4DFE584AA580F4F069F9B440B5204D16_at_EMP-EXMR104.corp.leidos.com>

Hi John,

I think there are definitely things that can be learned by looking at the spectroscopy
of fireball emissions, but of course such data are rare. And human eyeballs/brains
are a poor substitute. We don't have the necessary spectral resolution, and of course
the optical response is far from flat. It is perhaps not coincidental that green-blue
(0.498 microns) is the peak of our scotopic response. Deep red (>0.63 microns)
sensitivity is almost non-existent in scotopic vision, so even if a fireball had a
significant red component, a much smaller green component would swamp it just
due to our spectral response.

Since nothing really diagnostic can be learned from a witness's perception of a
fireball's color (as far as the meteoroid's composition is concerned), I see little
point in asking them or encouraging them to report it. The next best thing
that a novice witness can report (other than an accurate time and duration)
is the slope of the meteor track relative to the horizon -- perhaps using a
clockface analogy to avoid scary geometry. If I know the approximate fall
zone reasonably accurately, a distant observer's slope approximation can
greatly narrow down the true flight bearing, even without azimuth information
(which can already be inferred from their location relative to the fall with
greater accuracy than they can report).

Mike Hankey has put together some very nice tools on the AMS site for
amateurs to contribute useful information, concentrating on those things
that non-technical people are reasonably good at. With enough witnesses,
the average solution can sometimes be fairly accurate, even if the individual
reports are all over the place. --Rob

-----Original Message-----
From: Meteorite-list [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of kashuba via Meteorite-list
Sent: Wednesday, November 04, 2015 2:16 PM
To: 'Rob Matson'
Cc: 'meteorite-list'
Subject: Re: [meteorite-list] Very Bright Fireball Over Europe on Halloween Night

Rob, Marco,

OK, so color isn't important. But why the different colors? Not green can't mean no oxygen. Is the green overwhelmed by other colors? Why?

- John

John Kashuba
Bend, Oregon

-----Original Message-----
From: Meteorite-list [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Rob Matson via Meteorite-list
Sent: Wednesday, November 04, 2015 12:54 AM
To: 'meteorite-list'
Subject: Re: [meteorite-list] Very Bright Fireball Over Europe on Halloween Night

HI All,

Marco took the words out of my mouth. Getting tired of hearing that a green meteor tells you anything about its composition. I know that it's natural for people to think the most important thing they can report about a meteor is its color, but I wish various broadcast media would do the public a service and disabuse them of this notion. It would be far better if witnesses could be trained to get in the habit of counting the duration accurately, and noting the exact time of the meteor to the nearest minute. Seeing as how almost everyone has a cell phone these days, and all cell phones have accurate clocks, there really is no excuse to get the time wrong. Yet even a casual browse of the AMS fireball site reveals that people clearly don't think getting the time right is important. And even more obvious is that most people have no business reporting anything about fireball starting and ending bearings and elevation angles. --Rob

-----Original Message-----
From: Meteorite-list [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Marco Langbroek via Meteorite-list
Sent: Wednesday, November 04, 2015 12:06 AM
To: meteorite-list at meteoritecentral.com; baalke at zagami.jpl.nasa.gov
Subject: Re: [meteorite-list] Very Bright Fireball Over Europe on Halloween Night

> A lot of folks say it looked green to them, which means it may have
> been metallic;


It is a perpetuated misunderstanding that meteor colours are primarily due to their composition. It's a science myth inspired by High School Bunsen burner

experiments that appears hard to kill.

While composition in some cases does have some influence on the colour, it is actually the composition of the atmosphere that is usually dominant for our perception of meteor colours.

That certainly is true for green colours. Meteor spectra show that meteors usually are very strong at the "forbidden" Oxygen line at 5577 Angstrom
(557.7
nm). This line is due to atmospheric Oxygen, the same atmospheric Oxygen exitation line also responsible for the green colours of Aurora.

So green meteor colours are likely atmospheric in origin and say little about the meteoroids' composition.

- Marco

-----
Dr Marco (asteroid 183294) Langbroek
Dutch Meteor Society (DMS)

e-mail: dms at marcolangbroek.nl
http://www.marcolangbroek.nl

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Received on Wed 04 Nov 2015 06:48:26 PM PST