In answer to an exam question, a freshman astronomy student wrote:
A _meteor_ is the flash of light
Made by a falling _meteorite_
As it rushes through the air in flight—
I hope to gosh this answer’s right!
Doggerel or not, the student’s definition correctly stated the true
distinction between the two terms, and the teacher marked his off-beat
answer correct.
Defined in more scientific terms, a meteor is the streak of light
(usually of brief duration) that accompanies the flight of a particle of
matter from outer space through our atmosphere. This particle may be as
small as a tiny dust grain or as large as one of the minor planets which
are called asteroids. Fortunately for the inhabitants of the earth, most
of the meteor-forming masses encountered by our globe are of the
“small-fry” variety!
As the rapidly moving particle plunges earthward through denser and
denser layers of atmosphere, the air molecules offer ever-increasing
resistance to its passage. This resistance heats up the meteorite body
until it glows. Technically speaking, it becomes incandescent. _The
meteor is this incandescence._ We see it as a darting point. Or as a
ball of white, orange, bluish, or reddish light. But the _material
object_ that produced this light is the _meteorite_. The distinction
between these two terms—meteor and meteorite—we must emphasize again and
again because people continue to use them incorrectly, as, for instance,
when they keep saying “meteor crater” instead of “meteorite crater.”
The majority of the meteors we observe represent the heat-induced
“evaporation” of exceedingly small fragments of cosmic matter. The
smallest meteor-forming bodies reach the surface of the earth only as
the finest of dust particles or as microscopic droplets of solidified
meteorite melt.
These residues descend slowly through the atmosphere and may be carried
for great distances. Afterwards, they may be found scattered so widely
and uniformly on the ground that their presence in any given locality
cannot be accounted for by the fall of any specific meteorite. This is a
fact that, for example, one school of modern Russian meteoriticists
overlooked when they were dealing with tiny granules of meteoritic dust
that had been recently found at Podkamennaya Tunguska. These scientists
tried to identify the tiny granules with the meteorite that had fallen
there, June 30, 1908. But the members of the latest (1958) Russian
expedition to that region about the impact point of 1908 clearly
recognize the widespread character of meteoritic dust. So they reject
the theory that such dust found in the Podkamennaya Tunguska area is
specifically connected with the meteorite that fell there a half century
ago.
If sizable chunks of meteoritic material enter the atmosphere, they may
produce exceptionally large and brilliant meteors. A spectacular meteor
is generally known as a “fireball” if it is as bright as Venus or
Jupiter. It receives the French term _bolide_ if, in addition to showing
great brilliance, its flight is accompanied by detonations like the
alarming sounds heard at the time of the Ussuri and Norton meteorite
falls.
[Illustration: COURTESY OF UNIVERSITY OF NEW MEXICO PRESS
A bright Giacobinid meteor, photographed from a B-29 during the
shower of October 9, 1946. See p. 115.]
The term “shooting star,” which is often applied to meteors, in
newspapers and magazine articles, is a misnomer. A meteor is _not_ a
distant sun (that is, a star) in rapid motion, for the whole path of the
meteor lies close at hand within a restricted zone of the earth’s
atmosphere.
The word “meteor” comes from the Greek word _meteōra_, which once
applied to any natural occurrence _in the atmosphere_—for example,
rainbows, halos, auroras, and so forth. Nowadays, the word “meteor” is
used in a much more specialized sense than it was by the ancient Greeks.
We have a specialized word, _meteoritics_, for the study of meteors and
meteorites. No one should confuse meteoritics with _meteorology_, which
is the science of things _other_ than meteors and meteorites, in the
atmosphere—for example, clouds, storms, air currents.
The region in which meteoric phenomena take place was long the subject
of controversy. Some persons felt that meteors were nearby, like
lightning. Others said that they moved at the distances of the remote
fixed stars. This controversy on the whereabouts of meteors became
heated, although it could have been settled quickly by a simple
experiment you can try out for yourself.
Hold a pencil against the tip of your nose and look at it first with
your right eye closed and then with your left eye closed. Repeat this
experiment with the pencil held at arm’s length. In the first case, the
pencil will seem to shift position very greatly; in the second, although
the same base line (the distance between your eyes) is used, the pencil
will seem to shift position only slightly.
Such an apparent shift in position is called a _parallactic
displacement_, or, simply, _parallax_. The notion of parallax is of the
greatest importance in most branches of astronomy, and it leads (with
proper instruments and a little mathematics) to exact determinations of
the distances of remote objects.
For our purpose, we need not go into all the interesting but complicated
details. Our experiment with the pencil shows that if a meteor was close
by, like a blinding bolt of lightning, then, as seen by a pair of
observers separated by only a few blocks, the meteor would show a large
parallax. But if this meteor was as far away as the stars, it would show
no parallax at all, no matter how widely the pair of observers were
separated on the earth.
There were many clever scientists among the Greeks, and it is quite
possible that a pair of them actually tried out this simple parallax
experiment on the meteors and so were able to prove that these beautiful
light effects occurred in the high but not too distant layers of the
atmosphere. The earliest calculations of meteor heights that are so far
known, however, were made in Bologna, Italy, in 1719 and 1745—long after
the heyday of Greek science.
The meteor heights found by the Italians were quite low in the
atmosphere, probably for two reasons. First, the visual (unaided-eye)
observations they had to use were made by eyewitnesses stationed so
close together that accurate fixes were impossible. Secondly, these
visual observations must have related only to the very brightest and
therefore lowest portions of the luminous paths of the meteors through
the atmosphere.
In 1798, two German students operating from carefully chosen and widely
separated stations began the systematic observation of meteors for
parallax. They found that the height of appearance of most meteors lay
between 48 and 60 miles above the earth’s surface. It is now known that
most meteors, as observed with the naked eye, appear at about 70 miles
and disappear at about 50 miles above the surface of the earth. These
figures, obtained from visual work, still stand in spite of the
development of such modern techniques as photographic and radar
recording of meteor paths.
Rarely, meteors may appear at heights of 150 or more miles and fireballs
may penetrate to within a few miles of the earth. The average meteors,
however, appear and disappear within a well-defined, high-altitude zone
in the atmosphere. Fortunately, this atmospheric zone serves us as an
effective shield against the constant bombardment of the smaller and
much more numerous particles from outer space.
In earlier times, scientists thought that the particles becoming visible
as meteors must be tiny dense masses of iron or stone like the material
composing the recovered meteorites. Most modern investigators, however,
believe that the typical meteor-forming particles may be small loosely
bound-together “dust-balls”; that is, fluffy clusters of matter held
together by frozen cosmic vapors, generally referred to simply as
“ices.” In any event, these masses are usually very small, ranging
perhaps from the size of a pinhead to that of a marble.
Because we cannot collect the tiny masses that are seen only as meteors,
it is impossible to determine their composition by ordinary laboratory
methods. The best we can do is to observe and record carefully the light
these masses give off when they become incandescent in their plunge
through the atmosphere.
We can examine this meteor light by using the spectroscope and
spectrograph. Through these specially designed instruments we can make
the meteor light reveal the chemical elements present in the
incandescent masses. Each such element sends out light rays as
characteristic of its nature as fingerprints are of the individual who
made them. Photographs taken of these characteristic light rays are
called _spectrograms_, and what might be termed the “fingerprints of
light” recorded on these spectrograms are known as _spectra_—which is
the plural of the word _spectrum_. If the source of light is a meteor,
the photograph shows a meteor spectrum.
From a study of a considerable number of good quality meteor spectra,
scientists have found that the principal elements in the masses
responsible for meteors are iron, calcium, manganese, magnesium,
chromium, silicon, nickel, aluminum, and sodium.
As we have already noted, the resistance encountered by meteor-forming
particles as they dash through our atmosphere is so great that they
become incandescent and vaporize. These small bodies must therefore be
in very rapid motion.
Before we attempt to find out the nature of the paths in space followed
by meteorites, we must take into account the fact that these bodies are
observed from a station—the earth—which is itself in rapid motion. You
may have noticed that on a still day, when rain drops fall vertically
downward, the streaks they leave on the windows of a swiftly moving car
are not vertical but almost horizontal. Obviously, it would be wrong to
say the rain drops are falling from left to right or from right to left
when they are actually falling almost straight down, and it is only the
forward motion of the car that makes them leave horizontal streaks.
[Illustration: Diagram showing meteorite moving along a “closed”
(elliptic) orbit, e, which intersects the earth’s orbit, E. Held by
the gravitational attraction of the sun, the meteorite is a
permanent member of the Solar System.]
Similarly, neither the apparent speed nor the apparent direction of
motion of a meteorite with respect to the moving earth is significant.
The important factor is the meteorite’s velocity _with respect to the
sun_ at the time the meteorite is picked up by the earth.
[Illustration: Diagram showing meteorite moving across the earth’s
orbit, E, along an “open” (hyperbolic) orbit, h. The meteorite is
traveling at such high velocity that it will pass right through the
Solar System and back out into space unless it should chance to
collide with the earth or another planet. The sun, however, in any
case is able to change materially the direction of motion of the
transient visitor to our Solar System.]
This factor enables us to determine in which of two possible kinds of
path the meteorite was moving _before_ it was “fielded,” as we might say
in baseball, by the earth. This factor tells us whether the meteorite
was moving about the sun in a relatively short, closed, oval-shaped path
or, instead, was following an indefinitely long, open path which began
in the depths of space and would have returned there if the collision
with the earth had not prevented.
Either type of path is technically called an _orbit_. The closed orbits
are what the mathematicians term _ellipses_; the open orbits,
_hyperbolas_.
To scientists, the nature of the orbits followed by meteorites is most
important, especially in efforts to determine the mode and place of
origin of these bodies. To rocket engineers and astronauts, it also
matters a good deal whether the meteorites encountered on flights
through space are traveling sedately along closed orbits about the sun
or are zipping swiftly along open orbits.
The greater the speed of these cosmic “hot-rods,” the more dangerous
they are to space travelers. For example, a mere grain of nickel-iron
moving at 40 miles per second is quite as lethal as a .50-caliber
machine-gun slug, which, relatively speaking, is traveling at only a
snail’s pace.
As our earth moves along its orbit about the sun, meteoritic bodies can
run into it from any direction. The direction from which they do
approach strongly influences the speed of these bodies as they plunge
through the earth’s atmosphere. A meteorite moving slowly about the sun
in the same direction as the earth and chancing to catch up with our
globe more or less from behind will have an observed speed of only a few
miles a second. For example, the speed calculated from Harvard
meteor-photographs of one such not-too-spectacular “rear-end” collision
amounted to no more than 7.3 miles per second, just about the speed a
rocket must acquire to escape from the apron strings of Mother Earth.
[Illustration: Meteor shower. Earth and particle-swarm passing
through the intersection of their orbits at nearly the same moment.]
In contrast to such a “rear-end” collision, the speed observed would be
far greater if the meteorite happened to collide exactly “head-on” with
the earth. For, in this case, the orbital speed of our planet would be
_added_ to that of the meteorite about the sun. As an example, suppose
that at the earth’s average distance from the center of our Solar
System, the speed of a meteorite with respect to the sun were 32.23
miles per second. (This speed was actually found for the mass that
produced one of the first meteors photographed simultaneously by the
Harvard stations at Cambridge and Oak Ridge, Massachusetts.) Then if
such a meteorite ran “head-on” into the earth, the speed observed for it
in the atmosphere would be over 51 miles per second. And mathematics
would show that the orbit of this meteorite with respect to the sun was
a wide open hyperbola.
If the orbit of the earth and the orbit of a swarm of particles of
cosmic matter intersect, and if the earth and the swarm pass through
this intersection in space at nearly the same moment, multitudes of
meteors appear. We then say that a _meteor shower_ takes place. The
position of the point at which the particle-swarm crosses the earth’s
orbit about the sun fixes the date of the meteor shower.
Because the particles that make a meteor shower are moving through space
along parallel paths as they come into the earth’s atmosphere, the
meteors all seem to shoot out from a single small area in the sky. You
may have seen something like this in the case of the sunrise or sunset
effect known as “the sun drawing water.” In this more familiar
phenomenon, the sun’s disk is the area from which shafts of sunlight
radiate out in a beautiful, if somewhat irregular, fan-like pattern. The
area from which the meteors of a given shower seem to come is the
_radiant_ of that shower.
Meteor showers are named for the constellation in which their radiant
lies. The suffix “-id” (Greek for “daughters of”), or some modification
of this suffix, is added to the name of the constellation from which the
meteors seem to radiate. The Orionid radiant, for example, is in Orion,
the Hunter; the Leonid radiant is in Leo, the Lion; and the Lyrid
radiant is in Lyra, the Harp. Exceptions to this rule do occur, however.
Astronomers may refer to a shower sometimes appearing on the night of
October 9 as the “Giacobinid” shower in honor of the comet
Giacobini-Zinner, which is associated with this particle-swarm.
[Illustration: Radiant of a meteor shower. Generally not a point but
a small area, here intentionally exaggerated in size. Solid arrows
represent plotted paths of observed meteors. By extending these
paths backwards, observer can determine the radiant.]
In the course of each year, the earth passes through a number of
particle-swarms of varying densities. Some of the resulting meteor
showers, like the Leonids and Giacobinids, are very feeble in most
years, but sometimes produce spectacular displays.
The more important recognized meteor showers are:
NAME OF SHOWER DATE OF MAXIMUM
Quadrantids January 1-3
Lyrids April 21
Eta-Aquarids May 4-6
Perseids August 10-14
Giacobinids (Nu-Draconids) October 9
Orionids October 20-23
Leonids November 16-17
Geminids December 12-13
Certain daytime streams are also known to be active during June and
July. These daytime showers are, of course, invisible in the glare of
sunlight, but they can be picked up by radar devices like those used in
World War II to spot enemy airplanes.
Some meteor showers have been splendid enough to make a place for
themselves in the historical record. Examples are the Leonid returns of
1833 and 1866, and the Giacobinid showers of 1933 and 1946. During these
displays, meteors fell in a veritable fiery snowstorm, several hundred
meteors sometimes appearing within a minute.
Not every annual return of a meteor shower is spectacular, however,
since conditions may not be favorable each year for a brilliant display.
After all, both parties to a traffic collision at an intersection must
try to pass through the intersection at the same time. Our earth, like a
well-managed train, always goes through the intersection on schedule,
but the particles responsible for meteor showers are much more erratic.
They may be early or late—or they may not show up at all. Of the meteor
showers seen annually, the Perseids are the most dependable. The Leonids
put on their best shows at intervals of 33 years (1799-1800, 1832-33,
1866, etc.). The Giacobinids at intervals of 6½ years (1933, strong;
1939-40, poor; 1946, magnificent).
If you plan to observe a meteor shower, here are some suggestions. You
will need:
Acquaintance with the stars, both faint and bright, in the region
containing the radiant of the shower.
Comfortable reclining lawn-chair.
Warm clothing (including blankets) for winter showers or summer ones
at high elevations.
A patient family that will not only approve of your observing but will
help you get up to watch after midnight, when most showers are at
their best.
A corner of your back yard (or sun roof) where you can shade your eyes
from street lights and other illumination.
Timepiece, preferably with radiant dial.
Sit back and watch Nature put on her show. Any records you make may have
some scientific value even if you note only these two things: Hourly
number of meteors seen. Condition of the sky (clear, hazy, cloudy, etc.)
during each hour of your watch.[6] At present, we know of only one
instance in which it seems probable that a meteorite came to earth
during a meteor shower. The Mazapil, Mexico, iron meteorite fell at 9:00
p.m. on November 27, 1885, during a return of the now very weak Bielid
meteor shower. Scientists still cannot decide whether or not a mere
coincidence was involved in this case.
As we have already mentioned, most of the cosmic particles rushing into
our atmosphere evaporate and do not reach the earth at all except as the
tiny congealed droplets and spherules of their own melt. Some cosmic
particles, the _micro-meteorites_, are so tiny that they “stall” rather
than fall down. These minute objects do not melt or disintegrate and so
preserve their original cosmic form unchanged. Scientists have developed
various methods for the collection of both of these types of material in
order that at least rough estimates of their rate of accumulation on the
earth can be made.
One of the simplest methods of collecting this so-called “meteoritic
dust” is to expose a sticky glycerine-coated glass microscope slide for
at least a 24-hour period in a protected spot well away from locations
where any industrial contamination is in the air. At the end of the
period of exposure, the “catch” on the slide is examined
microscopically, and the individual trapped particles are counted and
classified. Meteoritic dust is also carried down to the ground by rain,
snow, and hail and can therefore be obtained by filtering rainwater or
melted glacier-ice, snow, and hail.
Such collection efforts have been plagued by the difficulty of
identifying the particles. How can a collector be sure that the dust he
has trapped, even though magnetic and possibly even in part metallic,
does not come from some smelter or other industrial plant? Because of
such uncertainties, the current estimates of the annual deposit of
meteoritic dust for the world range from approximately 20 tons to
several million tons. We need improved collection and identification
techniques if we are to obtain trustworthy figures.
Recent analyses of rainfall records indicate that the infall of
meteoritic dust produces at least one interesting weather-effect. These
analyses show that rainfall peaks often occur some 30 days after the
appearance of important meteor showers. Apparently, as meteoritic dust
particles from the meteor showers filter down through the cloud systems
in the lower layers of the atmosphere, the individual particles serve as
centers about which atmospheric moisture condenses to form raindrops.
The time lag of approximately a month is considered to be due to the
very slow rate of fall of such tiny particles. It looks very much as if
Mother Nature had beaten man to the idea of “seeding” the clouds to
produce rainfall!