Shooting star = grain of sand?

[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 .