What percentage of meteors are visible to the naked eye?

Question

I read on a box of Lucky Charms that there are nearly a million shooting stars every day. My initial reaction was, "that's obviously false." However, upon further research I discovered that "shooting stars" are just meteors, and there are, in fact, millions of meteors in our atmosphere daily.

However, according to the Wikipedia article linked above, most meteors are about the size of a grain of sand. I suspect that you couldn't actually see a meteor that small, and I personally wouldn't consider it a "shooting star" if it weren't visible.

So that has me wondering: what percentage of meteors are visible to the naked eye?

I assume whether a meteor is visible depends on factors such as time of day and light pollution, in addition to the size of the meteor. If possible, I would appreciate an answer that takes those factors into account. If that's not feasible, I would at least like to know what percentage of meteors are big enough to be seen in the middle of the night with minimal light pollution.

Note: This question/answer indicates that there are many meteors too faint to see, and that small meteors are more numerous than large meteors (I think it indicates that small meteors are exponentially more numerous, but I'm not sure because my math is rusty). This suggests that a high percentage of meteors cannot be seen with the naked eye. I would like to know just how high that percentage is.

Answer 1

Meteors caused by small objects are visible.

Most visible [meteors are produced by meteoroids, which] are between 1 mm and 1 cm in diameter. For example, a meteor of magnitude +5, which is barely visible with the naked eye in a dark sky, is caused by a meteoroid of 0.5 mm in diameter and weighs only 0.00006 grams.

Meteors are atmospheric phenomena and only visible in a local area. A back-of-the-envelope calculation¹ suggests that even at a rate of only one visible meteor every 10 minutes at a particular location, there would be over a million potentially visible meteors worldwide per day.

Meteors are caused by the ionization trail produced when an object the size of a grain of sand enters the upper atmosphere. You can't see the object, what you see is the glowing trail.

Surely there are smaller dust grains that don't produce visible meteors. Less bright meteors are more common than brighter ones, but this probably doesn't scale, but not exponentially more common.

However we don't need to suppose that the author of the claim was counting these invisible "micrometeors". The claim can be true even without it.

¹ _{The surface area of earth is $4\pi×6400^2$ square km. If you can see a meteor in a 100km radius, you can see an area $2\pi×100^2$ square km. Divide the first by the second and multiply the number of 10 minute intervals in a day. Adjust parameters to taste.}

Answer 2

The naming must first be defined.

• Meteoroid: A rock orbiting the Sun.
• Meteor: When a meteoroid enters a planet's atmosphere it becomes a meteor. This is what we see streaking across the sky, mostly at night, and are indeed very small. Related, there could also be a 'fireball' This would be a larger rocks .
• Meteorite: If the rock survives the passage through the atmosphere to hit the ground, it will be a meteorite.

The International Meteor Network presents this chart for reported meteor observations.

Most meteors observed are from showers which are the result of Earth crossing orbit with that a comet. A comet's tail residue is scattered along its orbit, which residue is mostly dust and millimetre sized rocks. Meteor showers can also be studied and will show that they have a common radiant after which we normally name them. The Perseids' radiant is in the constellation of Perseus.

The size distribution of asteroids (chemical composition is different from comets however) can be seen here. We see that there are indeed far fewer larger ones than smaller ones. When asteroids collide they do shatter and the distribution of the ejecta can be assumed to be somewhat similar to what we observe here on Earth with the debris when we break rocks - a few larger ones and a lot of smaller ones, including dust. Meteors of such origin would appear in the sky in areas away from the field a known radiant, or if appearing in the same field will more than likely be moving in a non-radial path.

So we see that the source for smaller meteors is more abundant. Now, what makes them visible has a number of factors - time of day, size and chemical composition. The colour of a meteor indicates the most abundant chemical in the meteor and we know that not all chemicals burn with same intensity.