Our Earth. A little planet in a huge space. It seems very unlikely that Earth could be hit by another cosmic body, such as a comet, an asteroid, or a meteoroid. However, the Earth is continuously hit by meteoroids and sometime also by an asteroid or a comet.
What is a meteoroid? What is a meteor? What is a meteorite?
According to the International Astronomical Union, a meteoroid is an interplanetary body, deriving from asteroids or comets, with a mass range from one thousand millionth of kg and ten million of kg. In other words, it is a body larger than a molecule, but smaller than an asteroid.
As a meteoroid enters the Earth's atmosphere, it collides with air molecules. At the heights where most meteors ablate, the mean free path of the air molecules is about 0.1-1 m. On the other hand, common meteoroid dimensions are of the order of 0.1-1 cm. This means that there is no hydrodynamic flow around the meteoroid and single air molecules impact on the body. If we consider a meteoroid a typical geocentric speed of 40 km/s, it can be found that air molecules impinge on the body with the same speed. The kinetic energy is about 8 eV per nucleon: a nitrogen molecule then has an energy of about 230 eV. The impact energy is readily transformed into heat, which makes atoms evaporate from the meteoroid. The collisions between free atoms and air molecules produce heat, light and ionization, i.e. a meteor. Since this transformation occurs throughout the flight, the meteoroid atoms are dispersed in a cylindrical channel along the path.
According to its dimensions, material strength, and atmospheric trajectory, a meteoroid can reach different zones of the atmosphere. Generally, the bodies that can reach the stratosphere are large enough to produce a bright meteor. If the magnitude is less than -4 we have a bolide. When the originary meteoroid is very large, on the borderline with asteroids, it can reach the troposphere. Generally these bodies exploded in the atmosphere (airburst). If a meteoroid survives to the interaction with air molecules and reach the soil, the remnant of the originary meteoroid is called meteorite. |
