Meteors and Meteorstreams

Meteors - everyone remembers gladly the former times, when one was sufficient romantic, in order to believe that one would have a free wish while seeing a shooting star. If you have already a full list of wishes, find here, on the one hand, the times, at which you can process this list faster. On the other hand we additionally offer, the physics of these brief, "atmospheric" appearances. The sky comes rarely so close to us as by means of meteors...

What is a meteor?

Figure 1: 5 meteors of the Leonids, 18. November 1999. © Ch. Bosshard

Meteors are called in the vernacular also shooting stars. In former times meteors were regarded as finery wastes of the stars.

Meteors are the optical phenomena, which develops, if particles enter the terrestrial atmosphere and start ionizing the molecules of the high atmosphere. The glow comes from the recombination of the ionized gases.

The earth is met continuously by material from space. Scientists estimate the mass caught by the earth to thousands of tons per day.

The major part of this matter arrives at the ground as cosmic dust. These are particles of fractions of a millimeter. Their mass/surface relation is such small that they get through the atmosphere without prejudice and reach the ground as micrometeorites.

Figure 2: In November 1799 this meteor shower was observed at full moon off the coast of Florida by Andrew Ellicott. He wrote: In every instant the meteors were as numerous as the stars. The storm of the Leonids of 1799 was a key event with the discovery of the 33 years lasting period of the Leonids. This illustration appeared 1872 in the book The Midnight Sky by Edward Dunkin.

How the light phenomenon originates

Against the wide-spread opinion the origin of this light phenomenon is not the glowing of the dust particle produced by the friction with the air molecules. Rather than this, the kinetic energy which is in the particle is used partly to wrest the electrons of the atoms of surrounding air particles. The atoms are ionized. During the reunification of atomic nuclei and electrons light is radiated (recombination light).

In front of the meteoroid a shock wave from a mixture of air and evaporated matter, which are strongly heated up and thus ionized, forms. This starts at a height of approximately 100 km. From this a brightly lit gas ball develops, which can often have a diameter of several hundred meters. The causing body, the very much smaller meteoroid is melted at the surface by friction heat and loses more than half of its original mass. The material is strongly heated up and evaporates partly, so that the color spectra of the elements get visible (line spectra). Hence, for some meteors, colours are recognizable. The spectrum covers the colors from red over orange-yellow to blue or greenish. Most frequently, beside white colored fireballs are sighted as greenish gas balls. This may be due to the fact, that the human eye is most sensitive in the green spectral region. Remains of the material, which did not evaporate, can be observed still for a long time as a smoke trail, i.e. for several minutes.

Where do the particles come from?

1. Sporadic Meteors

Figure 3: Movement of the earth through a swarm of interplanetary particles. Since in the morning the observer is positioned toward the direction of the movement of the earth, apparently more meteors occur during this time of day.

One can call it leftovers of the formation of the solar system.

Some pieces remained with the formation of the planets, or were blown off from an existing planetary body in an early phase of the solar system. Most sporadic meteors come from the belt of asteroids, between Mars and Jupiter, where pieces of rock from few centimeters to some hundred kilometers in diameters fall around the sun (meteoroid). If they are diverted from their original course, they can fly toward the inner solar system.

The pieces of rock follow a course, which is determined by the force of gravity of the sun and the planets. If a piece of rock intersects the course of the earth, it becomes a meteor.

Most bodies are only the size of dust grains and are completely distroyed during the entrance into the atmosphere.
Larger pieces can get through and fall, at the end of a hot journey through the terrestrial atmosphere, to the ground as meteorites.

You can observe sporadic meteors each night. Dailyperiodic fluctuations of occurance are related to the rotation of the earth (time of day) and the direction of motion of the earth on its course around the sun (cf. figure 3). If the observer is pointing toward the earth movement (midnight to noon), more collisions with meteoroids are taken place than with a site on the side turned away from the yearly motion of the earth (noon to midnight). This explains the higher frequency of meteors in the early morning hours.

2. Meteors from Meteor Streams

The origin of meteor streams is connected closely to the existence of comets. Comets leave small particles, which move then along the comet orbit, and slowly distribute in the orbital plane of the originating comet. The distribution of the dust particles resembles a pipe. If a comet orbit intersects those of the earth, then the emergence of a meteor stream is probable.

Figure 4: Representation of the radiant of meteors. The earth meets the meteor stream of the Leonids. For the viewer all meteors seem to emerge from one point (radiant). (figure not to scale). Globe: The Living Earth

By accurate observations and calculations, scientists succeeded to bring some showers in connection with certain comets.

Thus the earth moves each year on 12 August through the inheritance of the comet Swift-Tuttle. The developing meteor stream is called Perseids. At maximum, up to 100 meteors per hour can be observed. The density of the particles originating from comets amounts to only 0.3g/cm³. This means that the particles have a very fine structure, which survives the journey through the atmosphere only for fractions of seconds.

The designation of the meteor stream is derived from the constellation, from which the meteors apparently emerge. In the case of the Perseids it is the constellation Perseus. This point is called radiant (cf. figure 4). The diagram shows the radiant of the meteor stream of the Leonids, in the constellation Leo.

Although the particles move parallel in space, it makes the impression to us, as if they appear from only one point in the sky. This perception hangs together with the perspective effect to the observer.

Since we move around the sun with the earth, the radiant does not remain stationary within a constellation. The accurate point of the radiant is connected thus with the date of observation. CalSky calculates this dependancy and lists the current position of the radiant in the daily calendars.

The corridor on which the particles move, has a diameter of some thousand kilometers. The concentration of the particles varies with each meteor stream, thus the different meteor rates can be explained. This is the amount of meteors to be observed during one hour.

Some of the showers have a sharply defined maximum of the rate, which takes only few minutes or hours. Others are very active over several days. For example, the Perseids can be observed at higher rates some days before and after the actual maximum.

Meteor vocabulary

Meteoroid

A meteoroid is a very tiny cosmic particle with a diameter from some meters to fewer than 0.1 mm, which revolves around the sun.

Meteor

If particles entering the earth atmosphere possess the size of a grain of sand or a pinhead (approximately one gram), they produce a meteor. They light up at a mean geocentric velocities of 30 to 70 km/s between 300 and 100 km height. Depending upon the starting size they draw a more or less long course and extinguish at an altitude of approximately 80 to 30 kilometers. A typical meteor glows not longer than one second. Larger and brighter Meteore produces traces of light, which are several hundreds of kilometers long.

Fireball / bolide

The brightest meteors emits as much light that they can throw shadows. They are called fireballs and can also be seen at broad daylight. They are accompanied occasionally by a noise similar to thunder; this is the result from a sonic boom. Sometimes fireballs explode also in flight. They are then called bolide.

Meteorite

Most meteors are completely destroyed during the flight through the atmosphere. Nevertheless, some of them succeed to reach the surface. These are then called meteorites .

Radiant

Apparent point of origin of meteors from a meteor stream. Moves in the order of days relative to the stars.

Zenithal Hour Rate ZHR

Number of meteors theoretically seen by an observer with radiant in his zenith and a certain limiting magnitude. This number varies with the position of earth within the stream and shows a daily variation. This figure can be calculated from actual observations, and hence a comparision among various observers can be made.