# Stargazing Shooting star = grain of sand?

1. Jan 3, 2017

### 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 travelling 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

2. Jan 3, 2017

3. Jan 4, 2017

### Staff: Mentor

Thread re-opened. This is a good question.

4. Jan 4, 2017

### davenn

Hi megacal

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.

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

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

you can do the maths, here's the formula

KEJ = 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 piece

Dave

5. Jan 4, 2017

### jbriggs444

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.

6. Jan 4, 2017

### davenn

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

7. Jan 4, 2017

### Staff: Mentor

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?

8. Jan 4, 2017

### davenn

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

9. Jan 4, 2017

### Staff: Mentor

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

10. Jan 4, 2017

### davenn

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

11. Jan 4, 2017

### 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 lenght 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 travelling 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?

12. Jan 4, 2017

### davenn

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

13. Jan 4, 2017

### Staff: Mentor

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.

14. Jan 4, 2017

### davenn

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

15. Jan 4, 2017

### Staff: Mentor

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.

16. Jan 4, 2017

### davenn

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

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

17. Jan 4, 2017

### 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.

18. Jan 4, 2017

### Staff: Mentor

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.

19. Jan 4, 2017

### Staff: Mentor

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).

20. Jan 4, 2017

### Bystander

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