Meteor Section

What happens when a meteoroid enters the Earth's atmosphere ?

The English version. The Norwegian version.

When we humans admire a natural phenomenon, we want to know what causes it, what we are experiencing.

By K.Gaarder. Translated by L.T.Heen

Meteors depend on meteoriods. When a meteoroid collide with the atmosphere of the Earth, the mutual interaction between the two objects will give rise to four principally different phenomena.

Meteors

Typical meteors, which can be observed visually, with a TV camera or with a telescope, are caused by objects larger than 0.01 mm. The exact size limit depends on the speed of the object. Relatively luminous meteors (magnitude 0) are caused by objects with a diameter of 2 cm's at a speed of 15 km/s, or 1 cm at 30 km/s or 0.5 cm at a speed of 60 km/s.
   The data presented below are based on a speed of 15 km/s, a density of 3.5 g/cm3 and a vertical flight through the atmosphere.
   "Typical" meteors are between 0.05mm and 20 cm in diameter before hitting the atmosphere. As they approach the denser layers of the atmosphere, they are quickly heated. If the size is between 0.05 mm and 0.5 mm, the heat will penetrate the entire object. If the object is bigger than 0.5 mm, only the surface and a few tenths of a millimeter inward will be heated.
   When the temperature of the surface reaches about 2200 K, which usually happens at a height of about 80 to 90 km, material from the surface will start to sublimate* from the surface and surround the meteor. These gases will then loose some of their energy through excitation**.
   This phase of a meteor's life, where the melting surface gradually disappears due to evaporation, is also called the ablation phase.
   The meteor light consists mostly of discrete emission lines, where metals, espescially iron, contribute the main part. More than 90% of this light is caused by emission from sole atoms in the meteoroid material.
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* When a substance turns from a liquid into a gas, it is called evaporation (f. inst. from water to water vapour). If the substance turns from solid state into gaseous state, it is called sublimation (f. inst. from ice to water vapour).
** When an atom receives a certain amount of energy, it will go to a higher energy state. This process is called excitation. These states however, are unstable. When the atom returns to the ordinary energy state (the ground state), it will dispose of the extra energy by emitting a photon of radiation - for instance as visible light.

Meteorites

Another type of interaction between the meteoroid and the atmosphere takes place when the meteoroid is bigger than 20 cm in diameter (for a speed of 15 km/s). In this case the meteoroid doesn't have enough time to loose all its mass. The meteor will during the flight through the atmosphere slow down to a critical speed of about 3 km/s.
   The energy that normally would be transported to the surface of the meteor is not large enough anymore to keep the temperature above 2200K. Since the meteor still has some of its' mass left, the molten surface layer will cool and form a crust on the surface.
   When this happens, the meteor or the fireball (when it reaches magnitude -8 or brighter), will fade due to the low velocity. The velocity is not large enough to cause neither evaporation nor radiation. The remaining mass amounts to 10 grams or more.
   What's left of the meteor falls to the ground in dark flight, and the speed is reduced to that of a free falling body. The dark flight usually lasts for several minutes, in sharp contrast to the mere seconds' duration of the brilliant fireball.

Larger rocks

The third type of interaction between meteoroids and the atmosphere is a rare phenomenon. If the meteoroid is bigger than a few meters in diameter, the retardation through the atmosphere will be low. The body will disintegrate only slightly. It will hit the surface of the Earth with a velocity of several km/s, and make a meteorite crater. Evaporating substances will make the meteor shine until it hits the ground.

Micro-meteorites

The fourth type of interaction between meteoroids and the atmosphere, is the least interesting to meteor observers. If the size of the meteoroid is less than a few hundreds of a millimeter, it will slow down to a few km/s at a very large altitude in an extremely thin atmosphere.
   Such a meteoroid, with a speed of about 15 km/s, reaches its maximum temperature in a height of more than 90 kms, and subsequently the temperature will fall. The temperature will never become high enough for sublimation to occur from the surface. Since the necessary temperature to produce a light phenomenon is never reached, we will not observe a meteor. These specks of dust fall slowly and unchanged through the atmosphere and down to the ground.
   This dust from space of accounts for a great deal of the dust particles in the middle part of the atmosphere. It can be collected by different means, and the collections we have today provide ample material for scientific research.

Catch a falling star... Drawing by Marcel Vranek. Reproduced from the IMC Proceedings 1990.

Can meteors show colours ?

Yes, many meteors - and especially the brighter ones - can show magnificent colours. The colour depends on the materials that are ionized along the meteor trail (either atmospheric gases or the meteor material itself).
   The colour is not dependent on the temperature of the meteor, but of its contents. The meteor's spectrum can tell us about the meteor's composition, even though a detailed analysis is quite complicated. Sodium emission is very common in meteor spectra, and can perhaps explain why meteors often are observed to be orange in colour. But it can also be attributed to special excitation environments in the meteor, rather than an over-representation of Sodium. A meteor spectrum is generally dominated by iron and other heavy elements.

Background - meteors and their origin:

    [Meteors - review, definitions] [Origin of asteroids and comets] [Comets] [Origin of meteors] [Physical description of the meteor phenomenon] [Meteor showers and sporadic meteors]

Background - meteorites and craters:

    [Meteorites - review] [Localization of a meteorite fall] [Identification of meteorites] [Norwegian meteorites] [Asteroid- and cometary collisions] [Cosmic explanation of mass extinction ?] [The making of a meteorite crater] [Craters in the Nordic Region]

General:

    [Observations] [Results] [Meteor Section] [NAS] [Web]

Last modified: Fri Nov 22 17:17:44 MET 1996 by Lars Trygve Heen.