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Meteors

The words meteoroid, meteor and shooting star are terms used to described solid particles of interplanetary material at various point in their life-cycle.

A meteroid becomes a meteor - a term derived from the ancient Greek meteron/meteoros meaning "something found, or happening in the air"- only when it enters an atmosphere. Those meteors with enough mass and velocity (and therefore energy), to produce a burst of visible light are known as shooting stars. The smallest particles able to produce shooting stars are about 0.01 mm in diameter.

There is no upper limit to meteoroid size, and several meteors with diameters in the range of kilometres have impacted with the Earth at various points in it's history. Information on their (typically catastrophic) results can be seen on geology.com. Thankfully though, typical meteoroids sizes are of the order of several millimetres – with a size range of between 0.05 and 200 mm (see Ceplecha et al.1998).

Most meteoroids are thought to originate from short-period comets and to a lesser extent asteroids. As the orbit of the comet approaches within 1.5 astronomical units (AU) of the Sun, sublimation of surface ice liberates embedded particles and produces a source of particles that are then pushed away from the parent body by gas and radiation pressure. These ejected particles form a stream known as a meteor stream that share the Keplerian orbit of the parent body. Over many orbits this process creates a meteor stream outlining the orbit of the parent body. The approximate width of an average stream being between 0.05 - 0.1 A.U. (7.5 - 15 million km).

However, other processes act upon the meteor stream scattering the particles into the general population of interplanetary dust. Therefore to persist a meteor stream must be constantly re-seeded with new material from the parent body. As a result of these scattering processes the majority of observed meteors do not originate as part of a particular meteor stream, but are caused by meteoroids belonging to the general interplanetary dust population. It is estimated that the total mass of particles colliding with the Earth is approximately 4 - 200 tons per day.

A meteor shower is a period of enhanced meteor activity, and occurs when the Earth passes through a meteor stream. If the meteor stream has a high number density then an intense meteor shower such as the Leonids will occur. A particular Meteor shower will occur at roughly the same time every year and marks the intersection of the Earth's orbit with the meteor stream. The largest showers can produce several hundred visible meteors per hour. The meteors of a particular meteor shower appear to 'radiate' from a particular region of the celestial sphere which is several degrees across. This effect is due to the observer being inside the stream. The constellation in which the shower radiant appears is used to name the shower. For example, the radiant of the Leonids appears in the constellation of Leo.

Anatomy of a Meteor

The radiant on the Leonid meteor shower. Taken by Juraj Toth of Modra Observatory The average meteoroid is not very dense or very large. This prompts the question; where does the energy required to produce the meteor trail come from? The answer comes from the high entry speed of a meteoroid into the atmosphere. The entry speed of meteoroids is between 11.2--72.8 kms-1. The lowest possible entry speed is 11.2 kms-1. This lower limit corresponds to the infall of a meteroid that was initially at rest at infinity to the Earth's surface. The upper limit is 72.8 kms-1 is the result of a 'head-on' collision between a meteoroid and the Earth. The Earth's orbital speed is 30.3 kms-1 and the solar system escape velocity (the maximum speed a gravitationally-bound solar system body can travel at 1 A.U.) is 42.6 kms-1. The sum of these two values yields the maximum possible "head-on" collision velocity of 72.8 kms-1.

When a meteoroid enters the denser parts of the atmosphere, friction causes the temperature of the meteoroid to increase rapidly. For particles less than 0.01 mm in size the drag caused by the atmosphere slows the meteoroid to free fall velocities and the temperature does not rise above the meteoroids melting point. The frictional heating only affects a surface layer a few tenths of a millimetre thick. This is due to the thermal properties of meteoroids and the very quick 'burst' of frictional heating (i.e. the heat at the surface of the meteoroid does not have time to radiate through more than a few tenths of a millimetre). The effect of the rapid increase in surface temperature depends upon the size of the meteoroid. Particles below 0.5 mm in size are heated right through, while only the surface of larger bodies is affected by atmospheric entry. At heights between 80--90 km the surface temperature of the meteoroid reaches about ~2000 K. At this temperature the surface material of the meteoroid starts to ablate and sublimate, producing a tail of hot vapourous material. The de-excitation of these hot molecules and atoms, via inelastic collisions with the surrounding atmosphere produces a trail of free electrons and ions. It is this de-excitation of this hot material that causes the familiar streak of light seen as a 'shooting star'. In the context of this study, the formation of a trail of free electrons behind the meteor is important, as it is this trail that scatters the radio waves from the meteor radar.

A typical ionised meteor trail is a long cone a few tens of kilometres in length, spanning a height range of 10--15 km. The radial distribution of ionisation within the trail comprises a diffuse cloud of ionisation surrounding a high density core. The initial radius of the trail is defined as the r.m.s. position of the ablated ions when thermal equilibrium has been reached. This typically takes less than 1 millisecond and involves ~10 collisions. The initial radius of the trail depends on the entry speed of the meteor and the density of the surrounding atmosphere (actually the atmospheric mean free path). Due to the mean free path of the atmosphere increasing with height the initial radius of the trail also increases with height. Typically the initial radius of the trail will be 0.40, 0.6 and 1.0 m at 80, 90 and 100 km respectively.

Immediately after the trail is formed the electron number density decreases because of ambipolar diffusion and atmospheric turbulence, which increase the volume of atmosphere through which the ionisation is distributed; recombination and ionic reactions also decrease the total ionisation present in the trail. Studies have shown that the most important process involved in reducing the electron number density is ambipolar diffusion.

Links and References

Meteors

Meteorites on Wikipedia
More on Meteorites
The International meteor organisation

Comets

A short tutorial on Comets
An excellent tutorial on Comets

Journal Papers (may need a password to view)

Ceplecha et al., 1998
Special issue of Atmospheric Physics and Chemistry: Meteors in the atmosphere

1 Comment

  1. Anonymous

    < What the finders found >The only shwores you need a calendar for (hopefully)If you're curious when you should be looking skyward for that next downpour of space rocks, look no further than . Complete with dates, peak dates, and moon phases (the less light there is in the sky, the more meteors you'll see), along with lots of general astronomy and stargazing info.Another good reference for shwores and other celestial events such as eclipses, comets, and planetary happenings is , which is chock-a-block with all sorts of fascintating star and sky gazing occurrences., including peak activity dates for each of the shwores.Things you'll want to consider for meteor shower viewing:If you live near anything remotely resembling a major metropolitan area, you'll probably need to get at least a little bit outside of town to see anything but the brightest ones. Light pollution covers up the rest.Dress for the weather. You're going to be out in the dark, usually predawn hours. Bring some bug spray if it's summer. Bundle up and bring a thermos of something hot in the winter. Got some folding camp chairs? Bring em. Leave the binoculars at home, however, because you won't need them.A flashlight with a red filter over it is handy to read star charts or maps in the dark without killing your night vision and is also nice for not stumbling over everything in the darkness on your way back to the car. Photo by GD Star Ratingloading...