Meteor Showers

Meteors, Meteor Showers (Shooting Stars)

Shooting stars is a non-Scientific but popular description of a Meteor.

The brief streak of light seen in a clear night sky, when a small particle of interplanetary dust, is called a meteoroid. Meteor showers have generally nothing to do with Meteorites. “Shooting stars” are dusty particles from comets, Whereas meteorites are metal, stony or a mixture of both and have mainly originated from the Asteroid belt.

On any clear night bright streaks can be seen in the sky from time to time. They appear suddenly and last for less than a second. These are meteors caused by specks of dust from comets burning up in the Earth’s atmosphere. A handful of so-called random meteors can be seen in an hour.

What is a meteor shower?

An increase in the number of meteors at a particular time of year is called a meteor shower.

Comets shed debris that “burns up in our atmosphere” most but not all meteor showers originate from Comets. As a comet orbits the Sun, they shed dirty ice, dusty debris streams behind the comet’s orbit. When Earth travels through this stream of debris, we will see a meteor shower. Depending on where the Earth and the debris stream meet, meteors appear to fall from a particular place in the sky. Meteor showers get their name by the constellation from which meteors appear to fall from, a spot in the sky astronomers call the radiant. For instance, the radiant for the Leonid meteor shower is located in the constellation Leo. But looking into the darkest part of the sky near the radiant is always the best bet.

What are shooting stars?

Shooting stars are names that people have used for many hundreds of years to describe meteors — intense streaks of light across the night sky caused by small bits of interplanetary rock and debris called meteoroids crashing and burning high in Earth’s upper atmosphere. Travelling at thousands of miles an hour, meteoroids quickly ignite in searing friction of the atmosphere, 30 to 80 miles above the ground. Almost all are destroyed in this process; the rare few that survive and hit the ground are known as meteorites.

When a meteor appears, it seems to “shoot” quickly across the sky, and its small size and intense brightness might make you think it is a star. If you’re lucky enough to spot a meteorite (a meteor that makes it all the way to the ground), and see where it hits, it’s easy to think you just saw a star “fall.” The general name Bolide refers to either an extraterrestrial object that collides with the Earth, or an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

Colours of the Meteorite

The colour emitted by the meteoroid as it burns up in the atmosphere, the visible light produced varies depending on the chemical composition. As the layers of the meteoroid are stripped off and ionised, the colour of the light emitted may change according to the layering of minerals.

How do we characterise them?

The term Meteor we characterise all the phenomena that occur when a body from space enters the Earth’s atmosphere, the body or its fragments are called Meteoroids or, if they land on the Earth’s surface they are called Meteorites.

How can I best view a meteor shower?

If you live near a bright city, drive away from the glow of city lights and toward the constellation from which the meteors will appear to radiate from. If you can see each star of the Ursa Minor, Little plough, your eyes have “dark adapted,” and your chosen site is probably dark enough. Under these conditions, you will see plenty of meteors.

What should I bring for meteor watching?

Take a comfortable chair if possible (a recliner is better for your neck!), insect repellent spray, food and drinks, blankets, plus a red-filtered torch for reading star maps, without ruining your night vision. You only need your eyes. So telescope and binoculars are not needed for this type of Astronomy. On these nights it is also great to have a look and see how many star constellation you can find!

The Science behind what is happening

The brief streak of light seen in a clear night sky, when a small particle of interplanetary dust, is called a meteoroid, burns it self up in the upper Earth’s atmosphere. The meteoroid is drawn into the upper layers of our atmosphere under the influence of the Earth’s gravitational pull.

As the Meteoroid collides with air molecules in the atmosphere at a tremendous speed, a large quantity of heat energy is produced which normally vaporises the particle completely.

This process is called Ablation. The vaporised atoms form the ablating meteoroid make further collisions with air molecules, and first excitation and then Ionization occurs as electrons and stripped from the atoms and molecules by the energy collisions.

After ten or so collisions the energy of an ablated atom has dropped below the level at which it can cause further excitation or Ionization.

The ablating meteoroid leaves behind it a trail of highly excited atoms that then de-excite to produce the streak of light. Which we call a visual meteor. The Ionization results in a long trail of positively-charged ions and negatively-charged free electrons both meteoric and atmospheric also being formed behind the meteoroid. This can scatter or reflect ground based waves transmitted from ground based radio equipment causing a radio meteor. The meteoroid starts to ablate at a height of about 70 miles (115km) It then dissipates its energy, distributes its ablated atoms and molecules in the atmosphere and produces the visual and radio meteors in the meteor region between about 40 and 70 miles (70 / 115km) above the earth’s surface.

A typical meteor reaches its maximum brightness and greatest electron density at an altitude of about 60 miles (95km).

Usually a visual meteor will persist for between one-tenth and eight-tenths of a second.

Most of the visible light from a meteor comes from the region immediately surrounding the ablating meteoroid. Sometimes a brighter meteor leaves a faintly glowing trail of luminosity along its path. This is called a wake if it lasts for less than a second and a persistent train if the duration is longer. Occasionally a meteor will display sudden bursts of light during its path; these are called Flares.

A fireball is described as a meteor that is brighter than any planet. A very nice sight for anyone to see.

Meteors may be seen at any time of the year and it is often quoted that on a clear, moonless night a casual naked-eye observer may see between six and eight of these so called sporadic meteors every hour. The eye will detect meteors down to about fifth visual magnitude, with the most probable magnitude being +2.5 and over three quarters of the meteors observed having magnitudes in the range 3.75 to 0.75. However, the number of sporadic meteors visible per hour varies not only during the course of a single night but also throughout the year.

In the early evening, an observer is on the trailing side of the earth as it moves along its orbit around the Sun at a velocity of about 18 miles (30km) per second.

This means that the meteors seen are catching up the Earth from behind, so the velocity with which they enter the atmosphere is reduced, and less energy is given out.

Later in the night, during the early morning hours, an observer is on the leading side of the Earth, and any meteors seen are meeting the Earth, and any meteors seen are meeting the Earth more or less head on! In this case the velocity of entry into the atmosphere will be correspondingly higher, and considerably more energy will be released. The consequence of the Earth’s motion is that more meteors render themselves visible to the naked eye in the early morning than the evening. Hence, a visual observer may see only six sporadic meteors per hour in the early evening, but this number rises steadily during the night, reaching a peak of 14 at about 4am. In addition to this diurnal variation in meteor rates they also vary on an annual basis. Sporadic meteor rates are generally rather higher in August and September, than they are in February and March, but they are short-term fluctuations superimposed on this annual cycle.

Meteoroids enter the atmosphere at a velocity somewhere between 6.9 and 45 miles per second (11.2 and 72 km/s). The lowest velocity is just the free-fall velocity of a particle hitting the Earth, (assuming it started with a zero speed relative to the Earth at an infinite distance away. The greatest velocity of an incoming meteoroid is obtained by summing its maximum velocity relative to the Sun at a distance of 1AU from the Sun-namely, 26 miles (42km) per second-with the Earth’s mean velocity around its orbit, which is 18 miles (30km) per second.
A large percentage of meteoroid influx to the Earth’s atmosphere (some 16,000 tonnes per year) is made up of particles in the size range that produces visual and radio meteors, with masses in the range from 10-6 to 10+6 grammes.

Although sporadic meteors occur continuously over the entire Earth’s surface (day and night), most people are aware that at certain times of the year meteors seem to be more numerous than normal. These are the occasions of the well-known meteor showers. Please note I have a PDF download for the shower dates and information on them.

They are caused as the Earth passes through a narrow region where the meteoroids are far more concentrated. This is called a Meteor stream and is due to the clouds of meteoric dust strewn along the orbit of a decaying periodic comet. Dust particles are pushed away from the cometary nucleus by gas pressure, when the nucleus is near perihelion. The ejected particles then have slightly different orbits to that of the parent comet. They form a dense cloud slightly ahead of and also slightly behind, the comet’s nucleus. This is the first stage of formation of a meteor stream, where the cloud of particles extends around only a very small fraction of the comet’s orbit. If the Earth’s orbit intersects that of the the parent comet, or passes near to it at a distance of not more than about 0.1au (usually near the ascending or descending node of the comet’s orbit), then a very strong meteor shower will be seen, but only very infrequently. The particles are distributed only across a narrow region in space, and although meteor rates may be exceptionally high, the resulting meteor “storm” will be of a very short duration- probably lasting only a few hours. Furthermore, sadly there may be many years, between the major storms when few, if any meteors are seem.

As a meteor stream gets older various processes cause the meteoroid debris to spread out and extend all around the orbit of the parent comet.

The time taken for this to occur is called the Loop Formation Time, and depending upon the size of the particle and orbit of the comet, may take anything from only a short-period comet, up to several thousand years in the case of a parent comet of long period.

The meteor stream will still be rather narrow, although it may be up to ten times broader at aphelion than it is at perihelion. The shower duration is still likely to be fairly short, but meteors will be visible every year, although the rates may not be particularly constant from one year to the next The older a meteor stream becomes the more broadening occurs, due to the collisions between the particles within the stream. This occurs mainly near perihelion, where the stream density is highest Planetary perturbations will also tend to cause broadening of the stream and in some cases particles may be lost from the stream altogether. They may also be some segregation of the different masses with in the stream, as the smaller dust particles slowly spiral in towards the Sun by virtue of a phenomenon called the Poynting-Robertson Effect. This effect is greatest for the very small dust grains and is negligible for the larger ones.

The oldest meteor streams will be the widest, with the result that the shower duration is a week or more. Meteor rates will tend to be reasonably constant from year to year. All the meteors of a particular shower will seem to come from one particular region of the sky, know as the radiant. Although all meteors within a stream are moving along parallel paths, it is an effect of perspective that makes them appear to radiate out in all directions from one point in the sky.

Imagine you are standing on a bridge with crosses over a long, straight stretch of motorway. The parallel lanes of traffic seem to radiate from a distant point near the horizon. It is essentially the same with meteors of a shower. The meteor showers are named according to the constellation which contains the radiant. The exception is the January shower, the Quadrantids, which come from an area of the now – rejected constellation Quadrans Muralis, near Bootes, the Herdsman. Nowadays most of the major annual meteor streams are associated with specific periodic comets. The two exceptions are the Quadrantids, where it is believed that the parent comet no longer exists, and the Geminids (December), where no parent comet has been found, but an Earth-crossing Asteroid 3200 Phaethon, appears to have an orbit that is identical with that of the Geminid meteor stream particles and is almost certainly associated with it.

Meteorites

Every day about 10 meteorites fall to Earth, although most land in remote areas or in the oceans and go unnoticed. Over 20,000 meteorites have been found, many long after their fall. Meteorites are composed of rock or iron or a mixture of both. Large meteorites are usually moving fast enough to dig a crater when they hit the Earth, but smaller ones are slowed by the atmosphere and drop harmlessly to the surface. Arizona has a very large meteor crater 0.75 miles wide (1.2km) in the desert from an impact about 50,000 years ago.

Asteroids or Minor Planets

These are small bodies of Iron and rock left over from the formation of the Solar System.

Most asteroids orbit the sun in a belt between Mars and Jupiter, but some stray from this group to cross the paths of the inner planets. Meteorites can be small fragments from an Asteroids.

Over a million asteroids are thought to exist although only about 20,000 have so far been discovered. Most asteroids orbit in the asteroid belt, but there are a group known as Trojan asteroids, that moves along Jupiter’s orbit. Members of another group names Apollo asteroids cross the earth’s orbit and therefore may collide with us!

Meteor Dates 2012

I have a PDF download for the shower dates and information on them:

MeteorDates.pdf [38.61units_k]

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