Space Debris III: Meteors
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By Lesa Moore
In previous issues, we have looked at Near-Earth Asteroids, Main Belt Asteroids and Meteorites. Before an object can fall to Earth as a meteorite, it must first survive its plunge through the atmosphere.
Meteors are commonly thought of as tiny particles "burning up" in the atmosphere. However, when we discuss meteors, we are referring not to the particles themselves, but the associated flashes of visible light and atmospheric ionisation effects. "The ablating meteoroid not only leaves behind it a train of excited atoms that de-excite to produce the brief blaze of light - a visual meteor - but it also produces a column of ionised atoms and molecules (both atmospheric and meteoric) that can act as reflectors of radar pulses transmitted from ground-based telescopes - a radio meteor" (Collins).
Velocities
Particles which result in meteors enter the Earth's atmosphere at speeds generally between 11 and 73 kilometres per second. The variation has little to do with the meteoroid's intrinsic velocity, but is mainly dependent on whether the meteoroids are travelling in a similar direction to the Earth, or colliding with us head-on as we orbit around the Sun. The exception to this is the case of extrasolar meteors, which may collide with Earth with relative speeds in excess of 73 kilometres per second.
Meteor Showers
The Earth's revolution around the Sun is a key factor in the occurrence of meteor showers. We observe these displays when the Earth moves through a meteoroid stream left by a periodic comet. Each shower can be attributed to a particular comet, and showers tend to be richer if the comet has visited recently, producing a meteor storm. The greatest meteor storm recorded was on the night of 16th Nov. 1966, when observers reported "upwards of 144,000 meteors an hour" in the most intense hour of the Leonid shower (Stephens). This year could be another spectacular year for a Leonid meteor storm.
References:
Collins Dictionary of Astronomy, 1994.
Stephens, S., The Biggest, The Brightest, The Best, Astronomy, June, 1997.
