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Watsisname
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UFO Sightings and Possible Explanations

14 Dec 2018 01:45

Stellarator wrote:
Source of the post I could even hear a hissing sound as it passed over.

This is something I find very interesting that is sometimes reported with very bright meteors.  

At first glance it seems impossible, because the meteor is very far away and it takes time for the sound of its passage to reach you (and if/when it does, it is as a boom rather than a hiss).  So usually these reports of hearing hissing meteors were thought to just be a psychological effect or something of that nature.

But apparently it is a real thing.  If the meteor is bright enough, the light or electromagnetic radiation may interact with the ground and somehow be converted to sound from your immediate surroundings, explaining how you hear it "in real time".  A study last year showed that this may happen by "photo-acoustic coupling", where the intense and rapidly fluctuating light of the meteor heats the surfaces of objects around you, which then creates faint sounds.  Personally I find this incredible and difficult to believe, but apparently it is supported by the math and by experiments.  The effect is weak, but in an otherwise quiet environment and with the right materials around you, it can be audible.  It's very cool.
 
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14 Dec 2018 02:10

The ones with sound are called bolides I think.

I've even heard of sounds reported (rarely) with Northern Lights!  Hissing and popping sounds.
 
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14 Dec 2018 03:14

The definition goes by brightness rather than sound: meteors that get brighter than Venus are called fireballs, and bolides are brighter than the full Moon.  Meteors big enough to reach bolide brightness do tend to produce an audible boom from passing through the atmosphere, and may also airburst.  It's probably true that they must also be in that size range to produce the audible sounds from the ground by the photo-acoustic coupling, assuming that's how it works.

Sounds associated with aurora have also been commonly claimed (some even claim to detect it in recordings).  Like hearing a meteor hiss, it can't be a direct sound, but must be a secondary effect produced in the listener's immediate surroundings.
 
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14 Dec 2018 18:40

Watsisname wrote:
Source of the post The effect is weak, but in an otherwise quiet environment and with the right materials around you, it can be audible.  It's very cool.

Indeed. I agree that it did not sound like a 'normal' sound, like a bug or machine hissing, but rather something subaudial that I could somehow hear. 'Ethereal' I think is the word to best describe it.

As for auroras making sound, my dad said he could sometimes hear auroras making crackling sounds like the ones described here when he saw them. This was years and years ago when he grew up in the northern parts of Canada.  He also thought it had something to do with electromagnetic interactions with certain parts of the immediate environment. Oddly, it was only there were ONLY green aurora out.
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15 Dec 2018 00:54

That is interesting. I wonder if it's neurological effect like synesthesia. I get a hissing sound with light intensity in dark rooms like an LED blinking makes me hear pulsating hissing sounds, especially with short wavelength light.  The LEDs on my PC case when it was in sleep mode used to make me hear hissing sounds in sync with it's blinking.  I believe this is a form of synesthesia


Anyway I have seen one really large fireball that produced a loud sonic boom that came about 5min later or so. Sounded like a supersonic jet flying over though there was no jets or planes and it lasted a few sec, the same length as the fireball.  It was very bright and green in colour.  Illuminated the ground as bright as a full moon I would say.
 
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15 Dec 2018 05:41

Watsisname wrote:
Source of the post meteors that get brighter than Venus are called fireballs, and bolides are brighter than the full Moon. 

Yes, that's a frequent definition, but it's not strictly followed.  Nor does it hint much whether it could be a meteorite dropping event, for which at least magnitude -8 (at 100 km distance), i.e. halfway between Venus and the full Moon, is a very rough lower limit (but much brighter bolides may drop nothing at all, so brightness alone isn't decisive).
Interestingly, the ancient Greeks also made a distinction between fireballs and bolides, but in a somewhat different way.  Pliny (1st century AD) explains: "There are also meteoric lights that are only seen when falling, for instance one that ran across the sky at midday in full view of the public when Germanicus Caesar was giving a gladiatorial show.  Of these are two kinds: one sort are called lampades (faces), which means 'torches', the other 'bolides' (missiles), that is the sort that appear at the time of the disasters of Modena.  The difference between them is that 'torches' make long tracks, with their front part glowing, whereas a 'bolis' glows throughout its length, and traces a longer path."
Fireballs then sounds like something falling in steeper and exploding whilst bolides are more of the Earth grazing kind.
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15 Dec 2018 21:29

vlad01 wrote:
Source of the post It was very bright and green in colour.  Illuminated the ground as bright as a full moon I would say.

I've never seen the green kind, just fiery orange. The one that passed over my town (and most of SW Canada) was not bright like the moon, but as bright as the rising sun. I'd say about Apparent Magnitude -15.
Last edited by Stellarator on 16 Dec 2018 21:28, edited 1 time in total.
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15 Dec 2018 22:18

I have seen a number of green ones. I believe they are bits of space junk. ie the metals producing the colours in satellite parts or similar as they turn into plasma.
 
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15 Dec 2018 22:56

I have never seen an UFO. I suppose I do not have enough fantasy to see one. A pity, really.
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16 Dec 2018 05:51

vlad01 wrote:
Source of the post I have seen a number of green ones. I believe they are bits of space junk. ie the metals producing the colours in satellite parts or similar as they turn into plasma.

Natural meteors contain a variety of metals and other elements, and can glow in all sorts of colors including green.  When I photograph the Perseid meteors, the most common colors are pink and green.  Bright ones also tend to leave a green wake that glows for a few seconds.  Here's an example of a fainter one, so we can be sure the color isn't affected by the sensor being over-exposed.  The meteor moved from upper left to lower right.  The saturation is boosted slightly for visibility.

Image


In this particular case, the green color at the beginning of the trail actually has nothing to do with the composition of the meteor itself.  It is instead from oxygen atoms in the upper atmosphere, excited by the meteor's passage.  It's also the exact same emission seen in green aurora. :)
In general, the colors of meteors and artificial space debris come from both their chemical composition and the glow of the surrounding air.  Brighter meteors and fireballs that reach deeper into the atmosphere can have a very complex spectrum:

Image


Many green spectral lines from magnesium, chromium, and iron, with more lines all across the spectrum -- even to ultraviolet.  This meteor was rocky rather than metallic.  The paper this figure comes from is also one of the most extensive studies of a meteor's spectrum I've seen and is pretty fascinating.  If anyone's interested I put the link here:  http://adsabs.harvard.edu/abs/1993A%26A...279..627B

How to tell if a meteor you see in the sky is natural or a piece of re-entering space junk?  
As midtskogen says, the best way is by how long it lasts.  Meteors come in fast and at any angle, so they usually burn up very quickly -- a few seconds or less.  The occasional grazing one may last longer as it crosses the sky.  Space junk on the other hand is much slower and always comes in shallow, so those are visible for much longer.

Re-entering space junk is also a lot less common to see than meteors.  You can expect a few meteors per hour even well outside of shower activity, while you may have to wait months for a piece of space debris to be visible burning up over you.
 
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16 Dec 2018 07:11

I have photographed a thousand bright meteors or so, and only one was visible for more than 30 seconds from my position, and if I remember correctly, it was visible for a minute and a half, first appearing over southern Sweden going west over the British Isles and disappearing west of Ireland.  These are Earth grazers coming in at a very shallow angle, sometimes dipping to around 60 km altitude and then bouncing off the atmosphere to reenter again elsewhere at a shallow angle later.  So these are rare, like space debris entries (the latter can be predicted, not the exact place where it will happen, but somewhere along a path around a few orbits).  Our meteor network has documented two Earth grazers lasting over 30 seconds and one satellite entry lasting more than one minutes.

I'm not an export on the colours, but it's probably not just a question of the material of the meteor/junk.  What we see is not really the object itself glowing, but the air surrounding it glowing.  So altitude, atmospheric composition, speed conditions should also matter.
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16 Dec 2018 12:16

midtskogen wrote:
Source of the post What we see is not really the object itself glowing, but the air surrounding it glowing.  So altitude, atmospheric composition, speed conditions should also matter.

Absolutely.  The glowing part of a meteor is a mixture of atoms from the object that have been vaporized, ionized, and mixed with the surrounding air which is also ionized, and this glowing region is much larger than the object itself.

It's also important to understand that the glow is not because the gases are hot, even though they are easily hot enough that you would think they should glow like a sun.  The glow is more closely related to a neon sign, made up of distinct spectral colors (emission lines due to electron jumps), than to a continuous thermal spectrum like sunlight.  This is because the gas density is too low at those altitudes to form a continuous spectrum.  It's the same reason the Sun's photosphere is white (continuous spectrum), while the less dense chromosphere above it is pink (an emission spectrum of mostly hydrogen and helium -- this is also how the element helium was first discovered by the way). :)  In meteors, which spectral lines we see depends on the composition of the meteor, the air, and their ionization level.

Since all these spectral lines are well known, taking a spectrum of a meteor can immediately determine which elements it contained (though not necessarily in what original relative abundance -- different elements get vaporized and mixed with the gas at different rates along its journey.)  There is also a "wake" of light emission following immediately behind the meteor, which has a similar spectrum as the head, but the lines have different relative intensities:

Image


At very high altitude, like where most Perseids start to become visible, the air pressure is low enough that a "forbidden" transition in oxygen atoms can occur.  It's the same transition which causes green aurora, and this is what makes the greenish trails that last for a few seconds after the meteor has already passed.  

As Perseids get deeper into the atmosphere this forbidden transition is shut off, and a pinkish emission takes over from ionized sodium and magnesium (magnesium lends white while sodium adds yellow), combined with red from ionized nitrogen and oxygen from the surrounding air.  Other meteor compositions can produce different colors; I just haven't studied them.

Some "exploding" meteors at this altitude can also leave a trail of smoke that glows reddish for up to several minutes and curls in the high atmosphere's winds.  It is very similar in origin to airglow, where single atoms combine and emit light through the chemical reactions.

All in all, the physics and chemistry behind meteor colors is highly complex and interesting, and you can learn a lot about them by studying their spectra.  This could actually be something really cool to combine with your automated meteor observing networks, if that's possible.
 
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16 Dec 2018 21:33

JackDole wrote:
Source of the post I have never seen an UFO. I suppose I do not have enough fantasy to see one. A pity, really.

It's not quite a question of imagination Jack Dole, but rather being at the right place at the right time in an odd situation. Your mind sort of goes blank in the moment, but then later rationalizes the event - if it even is something all that unusual. 99.99% of all these 'sightings' are perfectly explainable, either psychologically or physically.
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UFO Sightings and Possible Explanations

16 Dec 2018 22:33

Stellarator wrote:
JackDole wrote:
Source of the post I have never seen an UFO. I suppose I do not have enough fantasy to see one. A pity, really.

It's not quite a question of imagination Jack Dole, but rather being at the right place at the right time in an odd situation. Your mind sort of goes blank in the moment, but then later rationalizes the event - if it even is something all that unusual. 99.99% of all these 'sightings' are perfectly explainable, either psychologically or physically.

The ones that really interest me are the mass sightings that thousands of people (including police officers) see, as well as airline pilots and the military videos which they themselves cannot explain like the air force videos that came out at the beginning of the year.
 
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17 Dec 2018 03:05

A-L-E-X wrote:
Source of the post The ones that really interest me....

Mass sightings are interesting, but I do not think polices officers or pilots are less likely the to tell the truth, hallucinate or mistake something that they are unfamiliar with just because they are 'official' or a seemingly trust-worthy authority. The following video is a good case-study on the event I think you are referring to along the same lines of thought, by Armored Skeptic:

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