Meteoroid Impacts

Meteoroids regularly enter Earth's atmosphere at high speeds of around 50 km/s. Whether they are stopped by Earth's atmosphere, burn up, or plow into Earth's surface depends very much upon how their mass compares to the mass of the column of air that they would pass through. Scale height is a useful tool to help you identify both extremes in behavior.

Most treatments of meteors categorize the outcome into 4 categories (see Landstreet, Physical Processes in the Solar System, 2003, pp. 118). Realize that these bins are only an approximation and meteor behavior depends greatly upon its initial velocity, composition/density, and mechanical strength.

1. Very low-mass meteoroids – These meteoroids are smaller than 10^-5 m, have masses less than 10^-11 kg, and never display any meteor phenonemena. They are slowed by the low-density atmosphere at high elevations and they slowly drift down to Earth.


2. Low mass – This bin contains the typical meteors seen by the naked eye. They are roughly between 10^-5 m and 20 cm in size with masses up to a few kg. These still have substantial velocity when they encounter more dense regions of Earth's atmosphere and burn up in the atmosphere. Thus, although they typically produce a vibrant display, they don't impact Earth. These meteoroids are substantially less massive than the mass of the column of air that they would pass through (by at least a factor of 10).


3. Medium mass – This bin includes meteoroids between tens of cm and a few m (tens of kg to hundreds of metric tons). Their wide range of possible behaviors include substantial heating, sublimation, and often explosion as they lose their incoming velocity. The remants typically reach Earth, but at far more modest speeds determined by air resistance.


4. High mass – These meteoroids have sizes greater than ten m and fortunately for humans are quite rare. They are considerably more massive than the column of air that they pass through and plow into Earth's surface at high speed (by at least a factor of 10). They produce incredible meteor displays, huge explosions, and enormous craters.

We can often determine which category a meteoroid will be in by comparing the mass of the meteoroid to the mass of the column of air that it will pass through. The outcome of a meteoroid (for those in categories 1, 2, and 4) is readily apparent. Scale height is a useful tool in this process as it allows easy determination of the mass of air that a meteoroid would pass through.