[HN Gopher] Fireball over Iceland
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Fireball over Iceland
Author : politelemon
Score : 187 points
Date : 2023-09-17 08:19 UTC (14 hours ago)
(HTM) web link (apod.nasa.gov)
(TXT) w3m dump (apod.nasa.gov)
| rwmj wrote:
| Even after reading the FAQ (which is excellent:
| https://www.amsmeteors.org/fireballs/faqf/) I'm not quite clear
| why a meteor would be a fireball versus not being one. Is it
| simply about size, or something in the composition?
| naavis wrote:
| It is just about the brightness. Answer number 1 in the FAQ
| states:
|
| > A fireball is another term for a very bright meteor,
| generally brighter than magnitude -4, which is about the same
| magnitude of the planet Venus in the morning or evening sky. A
| bolide is a special type of fireball which explodes in a bright
| terminal flash at its end, often with visible fragmentation.
|
| Or was your question why are some so bright? That is mainly a
| function of size and speed.
| rwmj wrote:
| Could it also be composition? eg. Mainly water vs mainly rock
| vs rock with particular metallic impurities?
| jl6 wrote:
| Meteors are very small. The bright light comes mostly from
| the molecules of atmosphere in front of the meteor being
| compressed, heated, and ionized[0] - essentially from the
| friction of the high-speed meteor entering the atmosphere.
|
| I guess if the meteor was a ball of pure magnesium, it
| might glow brighter, but there isn't so much oxygen up at
| that height, so my no-calc-gut-feel estimate is that this
| would contribute orders of magnitude less brightness than
| the glowing atmospheric molecules.
|
| [0] https://www.amsmeteors.org/meteor-showers/meteor-faq/#3
| uoaei wrote:
| To the extent it's about composition it's probably mostly
| about how long the meteor holds together so it can retain
| velocity and the surface can stay hot enough to glow. A
| bunch of small pieces decelerate faster than one big piece.
| Compositional changes would only change the color somewhat
| but likely not enough to be worth commenting on.
| alfiopuglisi wrote:
| Mostly size. Ordinary shooting stars are grain-of-sand sized,
| and what you see is actually the superheated air that they
| displace coming in at 10+ km/s. Something slightly bigger, like
| a rock a few cm wide, can cause a blinding fireball.
|
| Speed also counts. It can range from 8 km/s (a "stationary"
| object, relative to us, falling into Earth's gravity well) to
| about 70 km/s (something in a retrograde orbit in the Earth's
| plane). That's a factor of almost 80 in energy to be dissipated
| mcpackieh wrote:
| From Britannica:
|
| > _Meteors are the result of the high-velocity collision of
| meteoroids with Earth's atmosphere. A typical visible meteor
| is produced by an object the size of a grain of sand and may
| start at altitudes of 100 km (60 miles) or higher. Meteoroids
| smaller than about 500 micrometres (mm; 0.02 inch) across are
| too faint to be seen with the naked eye but are observable
| with binoculars and telescopes; they can also be detected by
| radar. Brighter meteors--ranging in brilliance from that of
| Venus to greater than that of the full Moon--are less common
| but are not really unusual; these are produced by meteoroids
| with masses ranging from several grams up to about one ton
| (centimetre- to metre-sized objects, respectively)._
|
| I think if you consider light pollution around populated
| areas and the fact that most people don't spend much time
| looking at the sky carefully, most of the shooting stars
| actually noticed by most people are larger than sand,
| probably more like tiny gravel than sand.
| camillomiller wrote:
| How big would such a meteor be, more or less, to generate such
| glow? Is there a way to estimate it?
| ls612 wrote:
| 10 meters was the size of the Chelyabinsk meteor which caused
| significant damage on the ground from its air burst. I'd guess
| significantly under that size then since this doesn't appear to
| have done anything other than make a pretty light.
| Aachen wrote:
| One metre does it
|
| I saw this one from 500km away and it was easily the most
| impressive thing I've seen in my life:
| https://en.m.wikipedia.org/wiki/2023_CX1 It says that was 1m in
| mean diameter, but also weighed 1Mg, which seems like a lot but
| when I look it up, a spheric m3 of aluminium is already heavier
| than that (1.4Mg) and iron would be 3x heavier still
| ericbarrett wrote:
| In a peer comment's link to the video, the videographer
| estimates this one was the size of a tennis ball. A typical
| meteor you might catch out of the corner of your eye on a dark
| summer night is the size of a rice grain.
| ethanbond wrote:
| That's bonkers
| User23 wrote:
| Good old p=mv. And that v can be pushing 70 km/s thanks to
| the combined motion of the Earth and the meteoroid.
| bboreham wrote:
| Would like to know what time this happened - I was in Iceland
| looking up at the sky that night, but didn't see the meteor.
| lucb1e wrote:
| 22:35, presumably Icelandic timezone because the comment (at
| https://youtu.be/GHHitRCagcE) containing this timestamp says
| "We were in Hofn watching the northern lights when we saw it"
| robga wrote:
| Video https://youtu.be/GHHitRCagcE
| mcv wrote:
| Sounds like they were just about to pack up and leave, and then
| this happens. Spectacular, and incredibly lucky that they got
| to see it like that. Not to mention those northern lights look
| great. I should go to Iceland some day.
| Aachen wrote:
| The video shows northern lights for the most part. I like that
| it's a video and you can see people move stuttery/stop-motion.
| Typically, northern lights are posted as picture and you have
| no idea what it was really like irl. I saw it once in Finland
| and thought it was clouds very faintly moving against the night
| sky. "Too bad," I thought, "if there were lights then this
| obscured them". The next morning I saw pictures where it looked
| like the most spectacular thing: those "clouds" _were_ northern
| lights! Here at least, it 's obvious that the exposure time per
| frame is cranked up because we don't perceive people as
| stuttering during the night.
|
| (Later I saw the lights properly in Iceland, but it was still
| just a faint green haze. Can be mistaken for city glow at a
| glance, but if you look for a bit it becomes clear that it's
| moving slightly and too green. Again less than what one might
| be lead to expect)
| stavros wrote:
| That's not always the case. I saw them in Iceland and yes,
| they were faint one day (such that only my camera could see
| them), but another day they were really bright, and they
| looked like waves in the sky, changing very quickly. It
| depends on the day.
| amatecha wrote:
| Aurora that looks grey is simply too feint for your eyes to
| perceive the colour[0][1]. One way you can confirm whether
| what you're seeing is aurora or not, is to take a picture
| with a digital camera. Camera sensors can discern the green
| (or whatever) hue, whereas the cones (which perceive colour)
| cannot pick up such dim light.
|
| Once the aurora becomes stronger, you can absolutely see
| shockingly-vibrant green, pink, purple, and other colours,
| literally streaming/swirling/dancing across the sky. It's an
| unforgettable experience. I hope you have the chance to see
| this at full-strength sometime, and I absolutely recommend
| seeking it out if possible. (extra-recommended to watch while
| sitting in an outdoor hot tub at midnight in -10*C in
| Iceland, of course!)
|
| [0] https://www.neworld.com/blog/2018/aurora-borealis-and-
| our-ey...
|
| [1] https://futurism.com/how-we-see-the-aurora-borealis-
| camera-v...
| mlindner wrote:
| I've seen the northern lights once before with my own eyes
| and no you cannot mistake them for "clouds" or "city lights".
| You unfortunately probably saw a very faint version, probably
| low to the horizon.
|
| And no I wasn't even in the far north. It was during a solar
| storm and I was in my own backyard in southern Michigan.
| bjornasm wrote:
| Don't go into the trap to think that all northern lights are
| that faint. Iceland are pretty far South in many ways, in
| addition the intensity varies. Sometimes it is just
| spectacular, a lot more spectacular than on pictures.
| patall wrote:
| Sorry mate, but you did not see them properly. In February,
| we had some that looked like in the video to the naked eye.
| And I was in the outskirts of Stockholm, so neither that
| remote nor that far north. Obviously doesn't happen that
| often, though the locals weren't bothered much.
| Aachen wrote:
| I know it gets better than what I saw in general, but this
| was about three months of being in Finland and Iceland
| cumulatively (both times during fall). Figured that's the
| best most people can hope to see, at least without
| specifically going very far north at the right time of year
| for this specific purpose, and so the pictures just aren't
| representative of what you are likely to see while in these
| countries
| hydrok9 wrote:
| I live in Winnipeg, way south of Finland, and I've seen
| spectacular northern lights not far from here.
| bjornasm wrote:
| >Figured that's the best most people can hope to see(...)
| so the pictures just aren't representative of what you
| are likely to see while in these countries
|
| Its quite wrong to assume your three month experience is
| representative for what you would expect to see of
| northern lights in Iceland and Finland.
| Aachen wrote:
| I don't think most people's holidays are going to be
| longer? Not sure what makes you say that. I understand
| one might get lucky or unlucky, are you saying my 3
| months happened to be extremely unlucky and therefore
| it's "quite wrong" that this can be extrapolated to an
| average experience when visiting the countries?
| dagurp wrote:
| They always look better in pictures. Even a cheap camera
| phone will take a good picture of them.
| jharohit wrote:
| 1:06 - what is moving in the top right quadrant of the video
| amongst the stars?
| alfiopuglisi wrote:
| Probably a satellite in low-earth orbit. They only take a few
| minutes to go from one horizon to the other. Since it is very
| bright, I am guessing it is the ISS.
|
| Update: Iceland is too far north (64degN) to see the ISS (max
| 51.6degN). Too bad.
| lucb1e wrote:
| Was about to say, northern lights are not in the south as
| seen from Iceland so ISS would not be probable. It should
| be possible to figure out what high-inclination object was
| in the sky at the right time of day (it needs to reflect
| sunlight) on September 13th as the video description says
| it was shot on
|
| Edit: _or based on the fireball timing_ , mister detective
| here just realised. The comments even say "Is it possible
| that it was on 12.09.2023 at 22:35? We were in Hofn
| watching the northern lights when we saw it" and a reply
| confirms it
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