METEOROID DEBRIS
As much as 1000 tons of cosmic debris-billions of microscopic particles- pepper the earth daily. We are aware only of those weighing a significant fraction of a gram, which produce the so-called shooting stars that flash across the sky. All but a few are too small to leave luminous trails. These solid particles are called meteoroids before they encounter the earth. Those large enough to survive flight through our atmosphere and land are called meteorites. And the luminous trails of the smaller particles that are completely vaporized in the atmosphere are called meteors. In order of increasing size and brightness, meteors are classified as (1) telescopic and radio meteors, (2) visual meteors, and (3) fireballs, or bolides.
Our atmosphere slows incoming meteoroids and transforms their kinetic energy into radiant and thermal energy. A meteoroid passing through the atmosphere leaves a wide, dense column of electrons stripped from the atoms and molecules in its path. As the ionized atoms regain their electrons, they deexcite, emitting photons that make the momentary luminous trail we see from the ground as a meteor, or shooting star.
Anything that remains of the meteoroid slowly filters down through the air as dust and solidified droplets of melted meteoroid.
When meteoritic particles encou nter the earth, they are moving anywhere from about 10 up to 72 kilometers per second, depending on their direction and the angle at which they stri ke the earth. The velocities convince us that meteoroids belong to the solar system, moving in independent orbits around the sun.
The normal observed rate for meteors is about 10 per hour over the entire sky. Why do we see fewer meteors before midnight than after midnight? During the even ing hou rs we are on the back side of the earth, facing the direction opposite to earth's orbital motion, and we see only the swift meteoroids overtaking us from the rear. During the morning hours earth's rotation has turned us so that we are facing in the same direction as its orbital motion. Hence we see those meteoroids that we overtake and those that meet us head on.
