Shooting star = grain of sand?

[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.

 

[Electron Spin]

I understood megacal's question very clearly. I wondered the same, Meg's question... How large and how far were these visible "shooting stars", from my observation point?
Davenn's earlier posts laid a good foundation, I think.

Throw all the definitions away!

I always guessed that they were about the size of a marble or so, but the distance was always a puzzlement for me, even though I always noted the inverse square law when observing it's tail or light, and all the little other obvious factors such as shape, angle of attack, velocity, composition and the like, now I know... using the ?relative? visual magnitude figure given in the Meteoroid Size Classification Chart.

DId I noticed that the visual magnitude figure is not based on anything?; no visual output such as luminosity, intensity, or the like, but no matter. The comparison can be made now.

I wanted to mention, that I have used meteoroid scattering, or ionization to communicate via RF for years, fun to do on the HF bands. Just another tool for an old key pounder.

I am really happy that megacal posed this question...I always wondered about how big and how far those little buggers were. Alas, my guesses were real close, so I am happy.

PS: I knew the figure for appx. 2.5 Km. for the candle/distance/human eye sensitivity thing too!. : )

 

[megacal]

Electron Spin,
glad it was profitable for you, as it was for me.

It just didn't seem reasonable for the average meteor to be created by such a small mass as a "grain of sand" even at 72km/sec.
B-B's. marbles, baseballs, yes.

Or to be able to see a candle 30mi away.
Just not intuitive.

 

[davenn]

baseballs, yes.

a baseball sized object will give an extremely bright trail that's a very significant meteor

 

[Electron Spin]

Megacal,
Yes it was and mfb's chart helped me anyway.

davenn, from an old pyrotechnician; A baseball sized rock with the right elemental compositions would give a nice colourful show I would hope! Very nice indeed. Someone correct me if I'm wrong (I know that you will ) without knowing the actual distance its kinda hard to know the true intensity, I believe. It appears to be a relativistic thing for the eyeballs.

Also, and off track, a good rule of thumb for estimating a rough line of sight distance for microwave antenna putter uppers is, Distance = $\sqrt 2ht +\sqrt 2hr$
Nice to know in a pinch I guess.

 

[DaveC426913]

I have never heard the streak itself - as distinct from the object creating it - being called a meteor.

I always understood that a meteor was the object. A meteor 20km long is non-sensical. A meteorite is a meteor that has reached the ground.

 

[Chronos]

I'm a little confused, is the OP questioning the reality of meteors or the physics? Meteors obviously exist, so what exactly is in doubt?

 

[Chronos]

The 500 eV energy calculation applies to individual atoms, not the entire mass of a meteoroid entering the atmosphere, so a massive meteoroid obviously produces a vastly brighter trail than a micrometeoroid.

 

[mfb]

Epsilon Eridani is a nice mag 3.7 star: clearly visible in a clear dark night sky, still visible from most cities. The visibility limit for perfect viewing conditions is usually given by mag 6, a factor 8 weaker. Epsilon Eridani's total power emission (including infrared+UV) is 1.3*10²⁶ W and its distance is 10.5 light years. To have the same brightness, assuming the same spectrum, a shooting star 100 km away needs a power of 130 W. If it lives for one second, this corresponds to 130 J of energy. The visibility limit is at 16 J. If the spectrum has more infrared or more UV it needs more energy, but we are still talking about hundreds of Joules.

A 1mg-object at 20 km/s has an energy of 200 J. At 40 km/s it has 800 J.

 

[megacal]

davenn,

a baseball sized object will give an extremely bright trail that's a very significant meteor

Yes, I didn't mean to imply it was the average. It would be spectacular.

YDaveC426913,

I have never heard the streak itself - as distinct from the object creating it - being called a meteor.
I always understood that a meteor was the object. A meteor 20km long is non-sensical. A meteorite is a meteor that has reached the ground.

I always thought a "meteor" was the object, too, but according to Wiki and the American Meteor Society, the meteor is the ionized gas
trail we see. So "meteor trail" is redundant (as someone said earlier), and Meteor Crater would be "Meteorite Crater".

Chronos,

I'm a little confused, is the OP questioning the reality of meteors or the physics? Meteors obviously exist, so what exactly is in doubt?

Does "OP" refer to the one who starts the thread?
If so, no, I'm not questioning the reality of meteors or the physics...just the common statement I'd seen that
meteors (assuming the average brightness) is due to a "grain of sand".

mfb,

Epsilon Eridani is a nice mag 3.7 star: clearly visible in a clear dark night sky, still visible from most cities. The visibility limit for perfect viewing conditions is usually given by mag 6, a factor 8 weaker. Epsilon Eridani's total power emission (including infrared+UV) is 1.3*1026 W and its distance is 10.5 light years. To have the same brightness, assuming the same spectrum, a shooting star 100 km away needs a power of 130 W. If it lives for one second, this corresponds to 130 J of energy. The visibility limit is at 16 J. If the spectrum has more infrared or more UV it needs more energy, but we are still talking about hundreds of Joules.
A 1mg-object at 20 km/s has an energy of 200 J. At 40 km/s it has 800 J.

I did a search, and found this:

How much one sand grain weighs? Let's assume that we are dealing with quartz grains. Quartz has a density of 2.65 grams per cubic centimeter. A grain with a diameter of 2 millimeters makes up only little more than four thousands of a cubic centimeter, and it weighs approximately 0.011 grams.

Then a grain of sand weighing 11mg (0.011g) would put out ~ 2000 J at 20km/s, and ~8000 J at 40km/s? And only 130 J are needed to produce a 1s 3.7 magnitude streak?

 

[mfb]

Then a grain of sand weighing 11mg (0.011g) would put out ~ 2000 J at 20km/s, and ~8000 J at 40km/s? And only 130 J are needed to produce a 1s 3.7 magnitude streak?

Yes. Clearly visible.

 

[Chronos]

The velocity of a meteoroid entering the atmosphere ranges from 11 km/sec to 72 km/sec. Even at minimum velocity, the kinetic energy of a meteoroid is around 6 x10⁴ joules per gram of mass. Only about 1% of the original kinetic energy of a meteoroid can be converted into visible light - re: http://abyss.uoregon.edu/~js/glossary/ablation.html.

 

[megacal]

mfb,
sorry, just realized that a mass of 11mg was based on a 2mm grain of quartz sand, and we have been talking about 1mm grain which may be
more or less dense than quartz.

But your calculation of 200 J to 800 J is for something 1/5 the size of a 1mm grain, right?
That seems much too small for a visible if not average meteor, but will research it more.

Only about 1% of the original kinetic energy of a meteoroid can be converted into visible light - re: http://abyss.uoregon.edu/~js/glossary/ablation.html.

So only 1% of the 200 J to 800 J becomes visible light, but not bright enough to
be seen as a meteor. (?)

 

[mfb]

1 mg is about 1/3 a cubic millimeter. A sphere with a diameter of 1 mm has a volume of 0.52 cubic millimeters.
Meteors don't have as much visible light as stars (relative to their power), see the post by Chronos, so my estimate was a bit too bright. If I take that 1% compared to the 42% for Epsilon Eridani, we need 5 kJ of energy for mag 3.7, and 700 J for mag 6. The 1mg-object (a quartz sphere with a diameter of about 1 mm) at 40 km/s is sufficient for mag 6.

 

[Electron Spin]

Epsilon Eridani is a nice mag 3.7 star: clearly visible in a clear dark night sky, still visible from most cities. The visibility limit for perfect viewing conditions is usually given by mag 6, a factor 8 weaker. Epsilon Eridani's total power emission (including infrared+UV) is 1.3*10²⁶ W and its distance is 10.5 light years. To have the same brightness, assuming the same spectrum, a shooting star 100 km away needs a power of 130 W. If it lives for one second, this corresponds to 130 J of energy. The visibility limit is at 16 J. If the spectrum has more infrared or more UV it needs more energy, but we are still talking about hundreds of Joules.

A 1mg-object at 20 km/s has an energy of 200 J. At 40 km/s it has 800 J.

Now this makes sense...

130 watts/sec = about Epsilon Eridani.

Yes, I would have guesses a mere 1mg object would have less light, but on second thought... I have seen a few gram sized objects with our common elements contained... about 500 wts. for a few seconds@ the avg. of ≈40 km/sec.

 

[megacal]

Conundrum deleted.

 

[mfb]

130 watts/sec = about Epsilon Eridani.

Either W or J/s. W/s doesn't fit. And you cannot assign that value to the star. It is scaled to the distance of a meteor.

 

[Electron Spin]

Did I get the mass wrong?
11mg = 0.011g = 0.000011kg ...but that was based on a 2mm diameter quartz grain, and we originally were specifying
a 1mm diameter grain, which (unless I'm wrong), should = 5.5mg.

Only 400 J for a gram, i.e. 1000mg??
According to mfb,

Sounds like a conundrum.

No, forget what I posted...I deleted it.. made a mistake..How did you copy that after I deleted it? Now that's magic!

And everybody's units are correct, but mine!

I'm outta' here! for now anyway!

Either W or J/s. W/s doesn't fit. And you cannot assign that value to the star. It is scaled to the distance of a meteor.

Snap! You got me mfb! W/s does not fit... is nonsense, sorry..!

 

[megacal]

How did you copy that after I deleted it? Now that's magic!

It's gone down a black hole.

 

[Chronos]

I hate to be repetitious, but, we have it on pretty good authority [NASA] that most meteors are produced by space debris about the size of a grain of sand [~2 mm]. Irrespective of the mathematical trivia, meteors are frequently observed in dark sky regions across the globe - sometimes over 100 an hour. I'm just saying who would know better or have any better reason to know than NASA?

 

[megacal]

I hate to be repetitious, but, we have it on pretty good authority [NASA] that most meteors are produced by space debris about the size of a grain of sand [~2 mm].

I just wish we had a way to fire a known mass, e.g. ~5mg (1mm grain of quartz) at 30km/s at the atmosphere to see
for ourselves. That's the only way to know for sure.
Otherwise, NASA is probably right.