How Can a Grain of Sand Create a Bright Meteor?

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Discussion Overview

The discussion centers on the phenomenon of meteors, specifically addressing how small objects, such as grains of sand or tiny meteoroids, can create bright meteors when entering Earth's atmosphere. Participants explore the physics behind this process, including the necessary speed and mass of the meteoroids, and the definitions of related terms like meteoroid, meteor, and meteorite.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that meteors can be produced by very small meteoroids, with a mass as low as one millionth of a gram, traveling at high speeds between 11 and 74 km/sec.
  • There is a discussion about a potential typo in an article that describes meteors as being 1m across and 20km long, with some arguing that this description is misleading and should refer to asteroids instead.
  • Participants present differing views on the definitions of meteoroids and meteors, with some asserting that the term "meteor" should refer to the bright trail produced rather than the object itself.
  • One participant calculates kinetic energy using the formula KEJ = 1/2 mv², suggesting that even small meteoroids can produce significant energy upon entering the atmosphere.
  • There is confusion regarding the visibility of small meteoroids and the size of the meteor trails they produce, with some questioning how a small object could be visible from great distances.
  • Some participants express frustration with the terminology used in the discussion, particularly the use of "meteor" to describe the trail rather than the object itself.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions of meteoroids and meteors, nor on the accuracy of the quoted article. Multiple competing views remain regarding the size and visibility of meteors and the terminology used to describe them.

Contextual Notes

There are unresolved questions about the definitions of terms and the implications of the article's descriptions. Participants express uncertainty about the relationship between the size of meteoroids and the brightness of the meteors they produce.

  • #61
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.
 
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  • #62
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.

Chronos said:
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. (?)
 
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  • #63
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.
 
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  • #64
mfb said:
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.
 
  • #65
Conundrum deleted. :cool:
 
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  • #66
Electron Spin said:
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.
 
  • #67
megacal said:
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. :confused:
No, forget what I posted...I deleted it.. made a mistake..How did you copy that after I deleted it? Now that's magic! :nb)

And everybody's units are correct, but mine! :olduhh:

I'm outta' here! for now anyway!
mfb said:
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..! o:)
 
  • #68
Electron Spin said:
How did you copy that after I deleted it? Now that's magic! :nb)
It's gone down a black hole. :biggrin:
 
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  • #69
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?
 
  • #70
Chronos said:
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. :wink:
 
  • #71
NASA has a super rifle they use for such things, it can fire projectiles over 17,000 mph [around 8 km/sec]. They use it, among other things, to test shielding of spacecraft against meteoroid impacts.
 
  • #73
BenAS said:
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.
I'm late to the thread, but I was going to say this. The only explanation for why something seemingly incapable of being visible due to its low energy output (that I can think of) is ionization of surrounding particles. This would not only create a larger area of light, but a wider spectrum as well.
 
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  • #74
So far there's a lot of arm waving but no actual math. So here goes. Imagine a speck of rock, density 3000 kg/m3. It's 1mm across, with a volume 10-9 m3 and mass 3 x 10-6 kg. If it hits at Earth's orbital velocity, 30 km/sec (30,000 m/sec) then its kinetic energy is 1/2 mv2 = 1/2 x 3x10-6 x (3x10^4)^2 = 4.5 x 10^2 joules. Now, if it takes a second to flame out (typical for the small meteors I've seen), then it's radiating 450 joules/sec = 450 watts. Crudely, auto headlights are around 50 watts and can be seen many miles away at night. So our meteor is about ten times that.

How do we convert our meteor to actual brightness? Incandescent lights are very inefficient. But then again, our meteor is also emitting due to incandescence, although more efficiently since it's a lot hotter. The Sun delivers 1361 watts per meter squared on earth. The apparent magnitude of the Sun is -26.7. Sirius is just about 25 magnitudes fainter or 5 steps of 5 magnitudes or one ten billionth as bright, so its energy flux on Earth is 1.36 x 10^-7 W/m2. Now, if our meteor is 100 km away (slant range) it will emit 450W/4pi x 100,000 x 100,000)m2 = 3.6 x 10-9 W/m2 at the observer's location. It's about 1/38 as bright as Sirius, or about 4 magnitudes, about M = 2.5. That's roughly as bright as a star in the Big Dipper.
 
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  • #75
Steve Dutch said:
So far there's a lot of arm waving but no actual math.
Did you miss page 3 and 4? We have those calculations already, with the same approach but with more accurate resuts: Comparing the shooting star power to a real star gives an error of a factor ~40 in brightness.
 
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  • #76
Steve Dutch said:
So far there's a lot of arm waving but no actual math. So here goes. Imagine a speck of rock, density 3000 kg/m3. It's 1mm across, with a volume 10-9 m3 and mass 3 x 10-6 kg. If it hits at Earth's orbital velocity, 30 km/sec (30,000 m/sec) then its kinetic energy is 1/2 mv2 = 1/2 x 3x10-6 x (3x10^4)^2 = 4.5 x 10^2 joules. Now, if it takes a second to flame out (typical for the small meteors I've seen), then it's radiating 450 joules/sec = 450 watts. Crudely, auto headlights are around 50 watts and can be seen many miles away at night. So our meteor is about ten times that.

How do we convert our meteor to actual brightness? Incandescent lights are very inefficient. But then again, our meteor is also emitting due to incandescence, although more efficiently since it's a lot hotter. The Sun delivers 1361 watts per meter squared on earth. The apparent magnitude of the Sun is -26.7. Sirius is just about 25 magnitudes fainter or 5 steps of 5 magnitudes or one ten billionth as bright, so its energy flux on Earth is 1.36 x 10^-7 W/m2. Now, if our meteor is 100 km away (slant range) it will emit 450W/4pi x 100,000 x 100,000)m2 = 3.6 x 10-9 W/m2 at the observer's location. It's about 1/38 as bright as Sirius, or about 4 magnitudes, about M = 2.5. That's roughly as bright as a star in the Big Dipper.

Yes we all have done the math. As you applied 3 milligrams of space stuff hitting us perpendicular and at Earth's orbital velocity. Applying our dear KE formula we all love so dearly. Most of us just do it in our heads with nice round numbers as you used, nice indeed.
The arm waving helps us lose some weight so we remain attractive and tan with about 1.4 kw as a tanning agent as you indicated. :cool:
mfb said:
Did you miss page 3 and 4? We have those calculations already, with the same approach but with more accurate resuts: Comparing the shooting star power to a real star gives an error of a factor ~40 in brightness.
With your calc. being one of the complete ones, unlike mine mfb. :redface:
 
  • #77
Gary Weller said:
I'm late to the thread, but I was going to say this. The only explanation for why something seemingly incapable of being visible due to its low energy output (that I can think of) is ionization of surrounding particles. This would not only create a larger area of light, but a wider spectrum as well.

Good point Gary. The wider perceived bandwidth or visible spectrum due to more or added elements ionizing in the surrounding Earth's atmosphere than the original rock/ice chemistry. You just can't beat 'fly by the wire gas spectrometry' (if there is such a thing).
 
  • #78
This is all basic physics, unless I missed something relevant. The KE, and physics, of meteoroids entering the atmosphere is understood well enough to allow to return astronauts to earth. Next question.
 
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  • #79
mfb said:
If the spectrum has more infrared or more UV it needs more energy, but we are still talking about hundreds of Joules.
Gary Weller said:
I'm late to the thread, but I was going to say this. The only explanation for why something seemingly incapable of being visible due to its low energy output (that I can think of) is ionization of surrounding particles. This would not only create a larger area of light, but a wider spectrum as well.
One of the most fascinating talks I had the pleasure of attending in grad school was given by Dudley Herschbach on this very topic. He went through the basic kinematics and worked out the blackbody spectrum for an average meteor and concluded that, if that's all there is, then we shouldn't be able to see meteors. Then he delivered my all-time favorite line: "If it isn't physics, it must be chemistry!" and went on to explain that a good chunk of the light that we see comes from the sodium D line (smack dab in the middle of the visible spectrum). The talk then meandered its way through the existence of sodium and iron layers in the upper atmosphere (http://www.albany.edu/faculty/rgk/atm101/sodium.htm). Research in atmospheric chemistry has been focused for so long on things like ozone/CFCs, carbon dioxide, and acid rain, that his talk was a nice break from the doom and gloom often associated with the field.
 
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  • #80
TeethWhitener,
thanks for adding that fresh info to the conversation from the chemist's perspective.
Would love to have been a fly on the wall to hear Dr.Herschbach's presentation. :wink:
 
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  • #81
megacal said:
TeethWhitener,
thanks for adding that fresh info to the conversation from the chemist's perspective.
Would love to have been a fly on the wall to hear Dr.Herschbach's presentation. :wink:

Me too megacal, me too!
TeethWhitener, See the later portion of post #77 and #73 ... There was some spectro suggested there. :blushing: