"Shooting stars", falling stars" or meteors, call them what you like. These
pinpoints of light that streak across the night sky are tiny bits of rock from
space. They enter our atmosphere at speeds up to 71 km/s (~158,000 miles mph).
They glow because friction with air molecules heats them to incandescence. Most are smaller
than a grain of rice. They burn up in a second or two at altitudes of around 80
km, high in the ionosphere. An especially bright meteor is called a fireball or bolide.
Figure 1: A composite image of meteors from the Geminid meteor shower of 2007
produced by astronomer Erno Berkó. Over
four nights, he captured 123 meteors in 113 photographs, then composited them into
this single spectacular image. This image clearly shows that the
meteors are streaming from a point (know as a "radiant") near the constellation Gemini. Image
copyright by Erno Berkó.
There are two kinds of meteors – sporadic meteors and shower meteors.
Sporadics originate from random bits of solar system dust that orbit the Sun.
Their chance encounters with Earth are unpredictable. While they do slightly
cluster in various parts of the sky, their occurrence is sporadic –
hence the name. Sporadics are the ones most people see while gazing
into the night sky. Naked-eye rates for sporadic meteors seldom exceed five
per hour. As far as we know, all meteors that reach the ground - meteorites - come from sporadics."
Figure 2: This is a composite infrared image of fragments from Comet 73P/Schwassman-Wachmann 3
captured by the Spitzer Space Telescope. The diagonal line in this image is a dust trail that
marks the path of the comet through space. Fragments of the comet appear as bright spots
within the dust trail. The bright streaks extending to the left of the comet's fragments
are "tails" produced by the solar wind (the sun is to the right of this image).
Shower meteors come from the dust released by comets as they travel through our solar system. The dust spreads out
along the comet's orbit and forms an elliptical trail of debris that passes around the sun and crosses the orbits
of the planets. Meteor showers occur when the Earth passes through this
trail of debris during its yearly orbit around the sun. The following year,
Earth passes through that same debris trail again on about the same date.
This is why meteor showers are predictable annual events. (See Figures 2 and 3.)
Some meteor showers last only a few hours, others last for several days. The
duration depends on how wide the dust trail is; some are narrow,
others are wider. Sunlight and particles from the solar wind, a
stream of hot, fast ions that is continuously blowing outward from
the sun, can push the dust away from the comet's orbit. The smaller
the particle, the more it can be moved. As a result, the dust trail
can broaden and when it does, it takes the Earth longer to pass through it.
(See Figure 2.)
Only rarely is a meteor large enough to survive its fiery passage through the
atmosphere and reach the ground. These are called meteorites. No shower meteor
is known to have ever reached the ground, which means that comet dust is in the form of very
Major Meteor Showers
January 1 to 5
Asteroid 2003 EH1
April 15 to 28
April 19 to May 28
May 22 to July 2
Marsden sungrazer comets
July 12 to August 19
Kracht/Machholz sungrazer comets
July 17 to August 24
October 2 to November 7
December 7 to 17
Minor planet 3200 Phaethon
December 17 to 26
Figure 3: A simplified diagram of the solar system showing the concentric orbits of the
planets and the elliptical orbit of Halley's comet. Note how the orbit of the comet crosses Earth's orbit.
The "Radiant" of a Meteorite Shower
All of the meteors in a meteor shower come from the same direction in space. From the ground,
they appear to radiate from single location in the sky, called
the radiant. It's like driving your car through a tunnel: some parts of the
tunnel pass on your left, or right, over head or beneath the car. In this
case the "radiant" would be "straight ahead." Meteor showers are named for
the constellation from which they appear to radiate.
For example, the "Geminids" appear to originate in the constellation Gemini.
(See Figure 1.)
How Many Showers, How Many Meteors?
There are hundreds of meteor showers and new ones are being discovered each
year. Some of the major meteor showers are listed in the table above.
Meteors produce hot trails of ionized gas behind them. Some of these trails
may be visible in the night sky for several minutes after the meteor passes.
This gas reflects
radar waves and as a result the meteors can also be detected during the day.
Recently Dr. Peter Brown and his collaborators at the University of Western
Ontario used ground based radars to identify 13 new meteor showers.
At its peak, a good meteor shower might produce a hundred meteors per
hour, the so-called zenith hourly rate, or ZHR. Occasionally a meteor
storm takes place, where the ZHR exceeds 1000 meteors per hour. The
Leonid meteor storm of 2002 was a terrific display with well over 3000
meteors per hour for about half an hour.
How Do Comets Produce Meteor Showers?
Figure 4: A NASA image of Comet Hale-Bopp, showing its two tails.
Comets are small bodies composed primarily of ice with a little bit of
sand or gravel. A typical comet's nucleus is a few miles across.
It spends most of its time in a lazy, elliptical orbit in the outer solar
system where its nucleus is cold and largely inactive. For example,
Halley's comet has a period of 76 years and at its furthest
point from the sun is beyond the orbit of Neptune. Here the surface
temperature of the comet is about 47 degrees above absolute zero (-375 F).
But during the comet's passage near the Sun, its surface heats up, some of the ice
evaporates and dust is released. Each comet has two tails, one composed of
dust, the other of gas. Both tails stretch away from the nucleus and point
more or less away from the Sun. This is because very hot particles coming
from the Sun (solar wind) push the tails outward, regardless of the
direction the nucleus is moving.
The dust streams may look uniform but they usually consist of several
individual streams, like strands of a rope. Each strand was produced by a
different passage of the comet through the inner solar system. The
elliptical stream of particles also shifts very slightly from year to
year owing to Jupiter's gravitational field. As a result, the number of meteors
can vary from one annual shower to the next as the Earth passes through different
parts of the dust stream. By about 2099 the orbit of comet Tempel-Tuttle (source
of the Leonid meteors) will no longer intersect the Earth's orbit. The result? No
more Leonid meteor shower.
Comets are the origin of most meteor showers but a few come from asteroids.
These may well be very old comets. After enough passages through the warm,
inner solar system, the ice has been completely evaporated, leaving a loose
assemblage of dust particles held together by their own feeble gravity.
These so-called "rubble pile" asteroids may be the remains of former comets.
Nowadays, some meteors are actually bits of man-made space debris.
These tend to be things like paint chips and spent rocket hardware.
The meteors they produce can sometimes be identified as man-made because
they travel much more slowly across the sky than natural meteors.
Figure 5: Simplified diagram of Earth approaching a comet's dust trail. In this
diagram you are looking down onto the Earth's North Pole. Note how the morning side
of Earth will plow into the dust but the evening side will be somewhat shielded.
This is why there are often more visible meteors after midnight - you are then
on the side of the Earth that is plowing into the dust.
How to Observe a Meteor Shower
First you need to find out when the meteor shower is (See the table above).
Next you want to find a place with a clear view of the entire sky. Dark areas
well away from any city lights are best. Avoid places where vehicle headlights
will momentarily dazzle you. The best approach is to recline in a lawn chair
or on the ground with a pillow so you are comfortably looking up. Knowledge of
the constellation where the radiant lies may be useful but not necessary: meteors
can appear anywhere in the sky. Then relax and gaze into the heavens.
Binoculars are not necessary to see the meteors but may be helpful in seeing
the vapor trail after an especially bright meteor. Other useful equipment is
insect repellant in summer. A flashlight can be useful but be sure it has a
red filter to avoid losing your dark adaptation when using it.
In general, we can see more meteors after midnight. Here is why. The earth
is rotating as it moves through the dust trail of a comet. In the evening
we are on the side of the earth that is shielded from the dust trail,
but in the morning we are on the side of the earth that has rotating towards
the dust trail. It's like driving through the rain: you always get
more rain on the windshield than on the rear window.
(See Figure 5.)
Moonless nights are best because
the moon brightens the sky. With a full moon, the eye cannot become completely
dark-adapted. Full adaptation takes about 20 minutes.
Maybe you are reading this article because you are getting ready to watch a
meteor shower. I hope that you have fun and enjoy the experience. If
you would like to see a meteor shower get your calendar out and mark it for one of the
showers listed in the table above. Now that you know how meteor showers
work you will not want to miss it.
David K. Lynch, PhD, is an astronomer and planetary scientist living in
Topanga, CA. When not hanging around the San Andreas fault or using the large
telescopes on Mauna Kea, he plays fiddle, collects rattlesnakes, gives
public lectures on rainbows and writes books (Color and Light in
Nature, Cambridge University Press) and essays. Dr. Lynch's latest book
is the Field Guide to the San Andreas Fault. The book contains twelve
one-day driving trips along different parts of the fault, and includes
mile-by-mile road logs and GPS coordinates for hundreds of fault
features. As it happens, Dave's house was destroyed in 1994 by the
magnitude 6.7 Northridge earthquake.
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