Stargazing How Can a Grain of Sand Create a Bright Meteor?

[megacal]

Hi,
I frequently see statements that meteors are due to a grain of sand,

"Most meteors typically measure 1m across and 20km long, and consist of a cylinder of excited atoms and molecules. They are normally seen between 120 and 80km above Earth's surface.

To produce a meteor, a meteoroid needs only a mass of one millionth of a gram, but needs to be traveling at a tremendous speed: anywhere between 11 and 74km/sec" -Astronomy Today​

How is it possible for something with so little mass create so large & bright a meteor just from
atmospheric friction? Can you please include the math?

Thanks,
Cal

 

[berkeman]

Thread closed temporarily for Moderation..

 

[berkeman]

Thread re-opened. This is a good question.

 

[davenn]

Hi megacal

"Most meteors typically measure 1m across and 20km long, and consist of a cylinder of excited atoms and molecules.

I read that statement in the link you provided
I suspect a serious typo there. They should have said asteroids NOT meteors.
Rather MOST meteors are very small, grains of dust to larger objects of larger size up to a few 10's or 100's of kg's.

To produce a meteor, a meteoroid needs only a mass of one millionth of a gram, but needs to be traveling at a tremendous speed: anywhere between 11 and 74km/sec"

this is essentially correct ... 30 km/s isn't an uncommon speed

How is it possible for something with so little mass create so large & bright a meteor just from
atmospheric friction? Can you please include the math?

even an object less than 1 gram will produce a very bright flash

you can do the maths, here's the formula

KE_J = 1/2 mv²

that is ... kinetic energy released ( in Joules) = 1/2 x (mass in kg) x (v in m/s)²

now you can plug in some figures and see how you go

start with a 1kg rock, then try a 1 g sized pieceDave

 

[jbriggs444]

I suspect a serious typo there. They should have said asteroids NOT meteors

The article is careful to distinguish between a "meteoroid" (the object which speeds into the Earth's atmosphere) and a "meteor" (the bright path that it leaves behind). It is the bright path which is referred to as being perhaps 1 m in diameter and 20 km in length. No confusion is possible between such a path and an asteroid.

 

[davenn]

It is the bright path which is referred to as being perhaps 1 m in diameter and 20 km in length

that ISNT what was stated above which was quoted from the article

the paths across the sky are going to be many many 10's of km long

they really did boo boo in their description

Dave

 

[fresh_42]

that ISNT what was stated above which was quoted from the article

the paths across the sky are going to be many many 10's of km long

they really did boo boo in their description

Dave

Yes, but 1 m in diameter for the stone itself? I also was confused by this figure and the word meteor instead of tail? I'm still confused. And why bother our atmosphere when we talk about meteors or its tails?

 

[davenn]

but 1 m in diameter for the stone itself?

not uncommon, that is a respectable sized meteor
the Chelyabinsk meteor of 2013 over Russia was around 20m in diameter with a mass of 12000-13000 metric tonnes

 

[fresh_42]

not uncommon, that is a respectable sized meteor
the Chelyabinsk meteor of 2013 over Russia was around 20m in diameter with a mass of 12000-13000 metric tonnes

Yes, of course, but not 20 km long at the same time? Spaghetti sized meteors?

 

[davenn]

Yes, of course, but not 20 km long at the same time? Spaghetti sized meteors?

exactly and that's why I have question that article and the way it is written ! any object 20km long is an asteroid !

 

[Bandersnatch]

@davenn , I agree with jbriggs - there seems nothing wrong with the quoted part. They say that a typical (so, a small one) meteoroid (so, a rock) will produce a metor (so, a trail) of that size. It also states that brightness and length of a meteor depends on the size of the meteoroid.

What the article is not clear about, is in stating what size is a 'typical' size of a meteoroid. I'm not sure if it's correct to assume that a 1 microgram meteoroid mentioned in the next paragraph is supposed to refer to the 1mx20km meteor.In any case, a 1 microgram meteoroid traveling at 74km/s carries 2.5 Joules of energy, so even if it does disintegrate over such a long path, it'd be invisible to a naked eye (but could be detectable by other means: https://en.wikipedia.org/wiki/Meteor_burst_communications )

The question worth exploring here is: how long a meteor (i.e. a trail) does a 1 microgram meteoroid produce?

 

[davenn]

I agree with jbriggs - there seems nothing wrong with the quoted part.

reading a bit, it seems it is the common expression to call the trail the meteor, rather than the object itself
that wasn't the way I was originally taught

I will have to change my thoughts to the new fangled ways Dave

 

[fresh_42]

I will have to change my thoughts to the new fangled ways Dave

Nope. This is a stupid convention. Asteroids are stones, meteors are also stones nearing the sun, and meteorites are stones hitting the earth. I don't see any advantage of calling some icy gas a meteor. If at all then a comet. But even this is edgy.

 

[davenn]

Nope. This is a stupid convention. Asteroids are stones, meteors are also stones nearing the sun, and meteorites are stones hitting the earth. I don't see any advantage of calling some icy gas a meteor. If at all then a comet. But even this is edgy.

?? don't understand what you are driving at there and why you quoted me ?

 

[fresh_42]

?? don't understand what you are driving at there and why you quoted me ?

I probably misunderstood your change of thoughts by a change of wording. I simply can't understand why the tail or coma or, whatever the English word is, can be called meteor. It's like calling the wet street rain. Sorry, wasn't meant to correct you.

However, it still puzzles me that a burning grain should be visible by the naked eye. And an object of only 1 m in diameter should be visible from several thousands miles apart. Or did this also only refer to what happens in the atmosphere? Might be my bad English that I didn't understand the Astronomy Today article very well.

 

[davenn]

I probably misunderstood your change of thoughts by a change of wording. I simply can't understand why the tail or coma or, whatever the English word is, can be called meteor. It's like calling the wet street rain. Sorry, wasn't meant to correct you.

ahhh ok ...
well as everyone above was correcting me in what I said about the article

"Most meteors typically measure 1m across and 20km long, and consist of a cylinder of excited atoms and molecules.

and stating I was wrong. I went searching and it seems that these days they refer to the trail as the meteor, not the actual object that is streaking across the sky
This was unknown to me, but several listing seem to agree with that quoted section from that Astronomy Today article.

So they cannot be called meteor trails as that would be the same as calling them meteor meteors
the streak of light is called a meteor.

And therein I have to change the way I was taught

Dave

 

[Bandersnatch]

Meteor literally means 'celestial phenomenon'.
Meteoroid means 'meteor-like', with ending reflecting usage in asteroid (star-like) and planetoid (planet-like) so that all can be easily understood to refer to space rocks. It is a rock that becomes a meteor.
Metorite, similarly, has an ending similar to minerals like hematite, cassierite, bauxite, etc. This is consistent with it being found where all other minerals are found - on Earth. It is a remnant of a rock that became a meteor.
All three taken together describe the three stages of the physical phenomenon in a nice and clear fashion.

Comets are icy space rocks that produce a coma when they get close to the sun. A comet could conceivably produce a meteor, but it would not be produced by the same interaction as coma (friction/ram pressure vs evaporation and dispersion by solar wind).

In the end it's all about having sufficiently unambiguous lingo so as not to cause confusion - such as this thread illustrates ;)The question of how big a meteor can a 1 microgram meteoroid produce is still an interesting one, though.

 

[fresh_42]

Well, I won't adopt this. It is silly. See my example with the wet street above. Will they start to call wet asphalt rain? You just taught me that the gas tail is actually called trail. I will learn this, not this modern nonsense.

 

[fresh_42]

It is a rock that becomes a meteor.

So does it only apply to objects that are totally dissolved? Does a "meteor" necessarily stop to exist or does it turn into - what? a rock again, once it's on its way back? And I have my doubts that objects of the magnitude of grams can be seen. I think there is simply not enough time, so one would need to have a controlled environment to see some, which is not what the article suggested (IM-possibly wrong-O).

 

[Bystander]

Twenty km over 70 km/s equals how long a time? Luminosity is how much?

 

[davenn]

Or did this also only refer to what happens in the atmosphere?

yes

small grains a tiny fast flash
larger masses produce a fireball (bolide) that can be visible by people up to maybe 1000km or so apart
there have been meteors here in Australia, in recent years, that have been visible by people with over 500km ground separation

Dave

 

[davenn]

A comet could conceivably produce a meteor,

could ?
Do produce meteors ... all the annual meteor showers are the result of material from comets

 

[megacal]

Thanks for all the replies. I guess I wasn't the only one who was confused by the terminology.
per Wiki, "...a meteoroid enters the atmosphere to become visible as a meteor and impact the Earth's surface as a meteorite"

In any case, I've seen said many times that a visible trail (shooting star, meteor) is due to something the size of
a grain of sand.

How did they calculate that? It seems impossible for a grain of sand to create a meteor/trail
that can be seen e.g. 100mi or even 50mi away.

KEJ = 1/2 mv2
that is ... kinetic energy released ( in Joules) = 1/2 x (mass in kg) x (v in m/s)2

Does the equation account for a meteor trail 1m x 72km created by a grain of sand?

How do you know the calculated amount of energy equates to the visible trail?

How do you calculate the mass of the meteorite if all you have is the visible trail?

 

[davenn]

Does the equation account for a meteor trail 1m x 72km created by a grain of sand?

no, that formula is for working out the kinetic energy of whatever mass object traveling at whatever speed

 

[Bystander]

http://astronomyforchange.org/where-are-the-five-brightest-stars-in-our-sky/
Threshold for the "average" human is M6.5. What is that versus incandescence at 50-100 km altitudes?

 

[berkeman]

1 microgram meteoroid

But a grain of sand (which was the OP's question) is more than a microgram...

http://hypertextbook.com/facts/2003/MarinaTheodoris.shtml

0.67–23 mg

 

[davenn]

How do you know the calculated amount of energy equates to the visible trail?

How do you calculate the mass of the meteorite if all you have is the visible trail?

I don't think either of those Q's could be answered without more info on the object

size, mass, what it is made of, the angle it hits the atmosphere and velocity ... all of those variables would affect the trail length

 

[NascentOxygen]

Can we say that with the upper atmosphere being rarified air, the consequent fewer collisions per meter means that the particle from space will penetrate much farther before it has expended its energy or burnt up—compared to what we'd expect to experience were it to occur near ground level?

 

[megacal]

Can we say that with the upper atmosphere being rarified air, the consequent fewer collisions per meter means that the particle from space will penetrate much farther before it has expended its energy or burnt up—compared to what we'd expect to experience were it to occur near ground level?

Yes, that's true, but is it provable that even a large grain of sand, e.g. 1 mm³ can produce a light trail (meteor) that is visible over
at least 1 degree of arc e.g. 50km (30 miles) directly overhead on a clear moonless night?

Has it been proved, directly or indirectly, or is it just a rough guess?

BTW, just found the American Meteor Society web site, and will ask them about it,
but would still appreciate any data or reference that would support
the "grain of sand meteor" statement to finally know if there is a grain of truth to it.

 

[oz93666]

Yes, that's true, but is it provable that even a large grain of sand, e.g. 1 mm³ can produce a light trail (meteor) that is visible over
at least 1 degree of arc e.g. 50km (30 miles) directly overhead on a clear moonless night?

Has it been proved, directly or indirectly, or is it just a rough guess?
.

The way to answer this is first to calculate the KE of the grain of sand ...1/2 m v2 in joules ... assume all that energy is expended . But how much is heat and how much visible light , that's the tricky part... If you had a rough figure for the light output in Joules , you could compare it with , say the light from a planes headlight , taking into consideration the meteor emits in all directions ... my instinct tells me you would never see it ... need perhaps a baseball size meteor to see the light from Earth's surface.

 

[megacal]

Yes, my gut tells me (I know it's not scientific) that it must be at least the size of a golf ball.
A "grain of sand" seems way too small to create a visible trail.

It may BE just a grain of sand, but hope to get a definitive answer from someone here, or the Meteor Society, JPL or NASA.

 

[jbriggs444]

Intuition can be a poor guide on such questions. One can see a lit candle some 30 miles away. A grain of meteoric sand would be much brighter than that.

 

[oz93666]

Intuition can be a poor guide on such questions. One can see a lit candle some 30 miles away. A grain of meteoric sand would be much brighter than that.

Are you sure it would? Just how would you know what % of the energy is visible light ?

Who wants to calculate the KE of a 1mm diameter grain of sand meteor ? ...any takers?

 

[jbriggs444]

Are you sure it would? Just how would you know what % of the energy is visible light ?

Does not matter. The higher the temperature, the greater the radiation at any particular frequency. A grain of sand at interplanetary speeds is much hotter than the incandescent carbon in a candle flame.

 

[oz93666]

The more I think about this question , I realize how little we know ! ...I'm not even certain a grain of sand would reach red hot ... it's surface area is very high in proportion to it's mass , this will mean it will radiate heat away very quickly per gram(equivalent to KE) of material ... it will certainly not penetrate very far into the atmosphere , probably be brought to rest very high up in low density atmosphere, ... it's not at all clear how far it will travel , a very short distance for sure .

 

[BenAS]

According to this website
http://www.amsmeteors.org/meteor-showers/meteor-faq/ most meteoroids are between the size of a grain if sand and a small pebble and weight less than 1-2 grams. The light we see is caused by the KE ionizing atmospheric molecules.

 

[oz93666]

A search gives meteor speeds at 11 km/sec to 72 km/sec ... suppose we were fire a grain of sand at sea level at 40km/sec (into the atmosphere ) ...what would we see ? how far would it travel? I would guess a small bang and a flash ... the flight path might be less than 50 meters ..

What about 1% atmosphere ... I would guess the flight path would be a Km ... it might be emitting light from 50m till 950m

Coming from space our grain of sand would start to slow down at the slightest hint of atmosphere ,if traveling in a direction towards the center of the Earth , I would bet it would be stopped in less than a second , perhaps a tiny flash of light.

 

[Bandersnatch]

A 1g particle traveling at 74km/s carries 2.5 MJ of kinetic energy. Even at low efficiency, converting that to light within one second is more than a tiny flash.

 

[NascentOxygen]

If we had a means for sampling the relative sizes of particles entering the atmosphere it may be possible to correlate the frequency that shooting stars are observed with the particle sizes measured during that time. For example, suppose that over an average 24 hour period it is estimated that only 1 or 2 baseball sized or larger particles enter the atmosphere, then clearly it must be much smaller particles causing the myriad shooting stars witnessed during that period. [emoji95] [emoji94]

During the time that the ISS has been sweeping through particle-riddled inner space, have any baseballs actually cannoned through some part of the structure or its expanse of arrays?

We could set up a maths problem in reverse: Suppose it takes a particle of baseball size or larger to create a significant meteor trail, use your observed count of shooting stars to estimate how many such particles might already have been expected to have hit the ISS during the time it has been orbiting.

 

[megacal]

Intuition can be a poor guide on such questions.

Intuition should not be dismissed...it's based on life experiences, and if intuition tells you something is possibly amiss,
it's worth confirming or proving it wrong.

One can see a lit candle some 30 miles away.

Have you personally seen a candle from 30mi (at sea level)?
Maybe a Roman candle.

 

[jbriggs444]

Intuition should not be dismissed...it's based on life experiences, and if intuition tells you something is possibly amiss,
it's worth confirming or proving it wrong.

Have you personally seen a candle from 30mi (at sea level)?

Are you saying that you disbelieve the figure? Have you run the test?

 

[megacal]

Of course not.
You made the assertion, so I asked if you had actually seen for yourself a candle 30mi distant.
If you have, you must have exquisite eyesight

Yes, I doubt it's possible at sea level , but if you have seen it, then I take you at your word.

The question remains, are most visible meteors created by of a grain of sand?

 

[megacal]

(removed contentious post...I didn't come to argue...my apologies to all those who
tried to answer my original question for the digression)

BTW, still waiting for a reply from the American Meteor Society & JPL.
WIll probably ask Bob Berman about it as well.

 

[Chronos]

For the aficionado, the IAU [International Astronomical Union] is generally credited with managing astronomical nomenclature. The IAU is the same body that notoriously demoted Pluto from its official designation as a planet - much to the chagrin of Disney fans and astrologers across the globe. They ponder such issues to tedious length and detail sufficient to compare to an act of Congress. Their classification scheme is outlined here; http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2009.01009.x/full, Meteorite and meteoroid: New comprehensive definitions. ISO 14688 is the generally accepted authority on sand classifying it as granular material in the size range from .063 mm to 2 mm. At 2 mm a sand grain is a veritable boulder and would really sting if fired at the velocity of a 9 mm slug - which is very much slower than the average meteoroid entering the atmosphere [around 50 km/sec]. The energy of meteoroid atoms evaporated via atmospheric collisions is around 500 electron volts - more than sufficient to ionize air molecules. For further boring details, see http://articles.adsabs.harvard.edu/...=0&data_type=GIF&type=SCREEN_VIEW&classic=YES

 

[Bystander]

For history of war buffs, just got back from googling "blackout, blackout in ww ii," ad nausem for the apocryphal "glow of a cigarette" stories, and have, so far, drawn a blank; anyone got any other suggestions for "visual purple/rhodopsin" threshold searches?

 

[megacal]

The energy of meteoroid atoms evaporated via atmospheric collisions is around 500 electron volts - more than sufficient to ionize air molecules.

Thanks for that input. That helps answer the question...it's not the material from the meteoroid, but the ionized gas around it that's visible.

What is the mass of the meteoroid that generated 500 ev?
The AAJ article doesn't specify the mass. I read the first page, and skimmed the next 3 without finding mass mentioned...it's mainly about how to measure
the trails...(or did I miss something?)

BTW, found this illuminating article about the visibility of a candle.
"...the farthest distance a human eye can detect a candle flame is 2.76 kilometers."

 

[mfb]

Intuition should not be dismissed...it's based on life experiences, and if intuition tells you something is possibly amiss,
it's worth confirming or proving it wrong.

Life experience doesn't help for objects moving at tens of kilometers per second.

During the time that the ISS has been sweeping through particle-riddled inner space, have any baseballs actually cannoned through some part of the structure or its expanse of arrays?

No. But grain-sized impacts happened. They can lead to notable damage if they have a diameter of 1 mm or more. This presentation has some examples on slide 5. Slide 8 in the presentation gives an impact depth (aluminium projectile vs. aluminium shielding) of 4 times the projectile diameter. The ISS has the best shielding ever, but its limit are centimeter-sized objects (see slide 24). A baseball sized object would easily penetrate the shielding, with potentially lethal results if it hits the pressurized volume.Objects starting at a size of ~1mm produce meteors visible to the naked eye - see the size chart here.

 

[oz93666]

Objects starting at a size of ~1mm produce meteors visible to the naked eye - see the size chart here.

That chart must be the final word ... just visible at Imm size..

Interesting to note ..."Meteors become visible between about 75 to 120 km above Earth. They usually disintegrate at altitudes of 50 to 95 km "

At the altitude small meteors are burning up (95Km) the atmosphere has one millionth the density of air at sea level !

 

[mfb]

This is closely linked to the penetration depth considerations for spacecraft s.
The area density of the atmosphere above 50 km is about 10 kg/m², at the density of rock that would be a layer of 3 mm.
The area density of the atmosphere above 80 km is about 0.15 kg/m², at the density of rock that would be a layer of 50 µm.
Atmospheric mass densities estimated based on this chart.

 

[megacal]

mfb,
thanks for the size chart link. It seems to correlate with the info from AMS below, which is definitive for me.

AMS Director, Dr. David Meisel replied to my original question:

Folks:
Object mass, not diameter (i.e. size), is often the parameter of choice in various ablation models of meteors and spacecraft . But these have to be calibrated using experiments. In the 1960’s there were several multistage rocket experiments where weighed samples of various materials were launched down into the atmosphere at various speeds. And of course there have been literally thousands of rocket and spacecraft entries that verify the masses at the heavy end of the scale. These were and are being followed up at small sizes by modified nuclear accelerator experiments in both the US and in Europe. Since the theories (as also the measurements) are based on mass, the actual size will depend on the density which can range from fluffy 0.1 gm/cc (Leonids) to rocky 3 gm/cc.(Geminids). How bright a given mass is when it becomes a meteor depends on its velocity. So here is where the math and numbers come in. Sorry.

A grain of sand (coarse) is 3 gm/cc and perhaps averaging 1 mm in diameter. The mass is then ~ 5 mg. A zero magnitude (Geminid) meteor is in the 1 gm range for an average velocity of 35 km/sec Conversion to magnitudes gives 8.3. For a slower velocity ~20 km/s, the value is 9.5 magnitude. For a Leonid, 72km/s the magnitude rises to only 6.7 . So a grain of sand is a size under estimate for visual meteors, but is about right for the average of radio scatter detected meteor. For visual meteors, one has to go to glass beads (6mm are used for jewelry, Marbles are an overestimate for average visual meteors. ) 3.6 mag at 20 km/sec., 2.4 mag @ 35 km/s, 0.9 mag @ 72 km/s. For Leonids, the velocity does a lot. A grain of sand mass, 6.5 mm diameter “dust bunny” gives 4.3 mag. For fireballs, the mass goes up from marbles.