The chemist can easily obtain materials for his research work from
reliable supply houses. The meteoriticist (as a scientist who studies
meteors and meteorites is known), is not this lucky. He must search for
the specimens he wishes to investigate wherever they may have landed on
the wide, wide earth. This “needle-in-a-haystack” problem could rarely
be solved if it were not for certain mathematical and instrumental aids
that swing the balance in favor of the meteorite hunter. When meteorites
are seen to fall, these aids can be brought into play only if certain
information is supplied by eyewitnesses of the falls. For this reason,
everyone ought to be acquainted with the facts about meteorite falls
that scientists will need to know in order to make finds, and should
understand how these facts must be reported in order to be of maximum
use to field parties.[5]
The problem of working out the path a fireball has followed in the sky
boils down to this. The investigating scientists must be able to fix the
position _in space_ of certain important points on the fireball’s path.
This idea of fixing points is not really difficult at all. Suppose, to
take an analogy from baseball, we have base runners on first and third.
These two players are intently watching their team’s clean-up hitter,
who is “crowding the plate.” Consequently their lines of sight intersect
at home plate and give a very good “fix on” its position, as navigators
say. This is the way a fix can be obtained in _two_ dimensions; that is,
essentially, in the plane of the earth’s surface.
[Illustration: A. A fix determined in two dimensions. The lines of
sight of the runners on first and third intersect at x.
B. A fix determined in three dimensions. The lines of sight of the
runners on the first and third intersect at x.]
Now, let us move into the _third_ dimension, since a fireball’s path
through the atmosphere lies in space, not in the “flat” plane of the
earth’s surface. Returning to our baseball diamond, let us suppose that
a helicopter with an enterprising photographer aboard hovers over the
centerfield bleachers so that he can take pictures of the record crowd.
While the umpire is dusting off home plate, the two runners on first and
third simultaneously sneak a look to see what the helicopter is doing.
Their lines of sight now intersect at the helicopter and fix its
position _in space_.
Similarly, the location of a fireball path in space is determined by the
fixing of certain points on the luminous streak seen in the sky. Instead
of using only two intersecting lines of sight (those of the runners on
first and third in our analogy), scientists investigating a meteorite
fall try to collect as many different lines of sight as possible from
people in the region above which the fireball streaked. The more
commonly determined points are those of the fireball’s appearance and
disappearance and those where “explosions” took place. These points are
generally located by use of the method we have described in some detail
above, the so-called _intersecting-lines-of-sight_ method.
The most important point on a fireball path is the point of
disappearance. The most valuable single piece of information you can
supply about a meteorite fall is as accurate an answer as possible to
the question: In what compass direction were you looking when you _last_
saw the fireball? This question has often been twisted around in
newspaper and radio accounts into the meaningless question: In what
direction was the fireball going when you saw it?
One person cannot give the answer to the second question because from a
single station it is impossible to determine the _true_ direction of
motion of an object seen in the sky. One person can report only an
_apparent_ direction of motion, which is of little or no value in
locating the last point on the luminous path, generally referred to as
the “end-point.” Therefore, though you cannot by yourself determine the
actual direction in which a fireball is _moving_, you can report the
direction in which you were _looking_ when you last saw the fireball,
that is, due south, southwest, northeast, etc.
[Illustration: O is an observer squinting along the top of a
ping-pong table. A ping-pong ball rolls along the top of the table
from B (beginning) to E (end). To the observer at O, however, the
ball would appear to start at B and end at E if it rolled along any
one of the dashed lines leading from OB to OE. By means of a similar
space-figure, it can be shown that a single observer at O cannot
determine the _true_ direction of motion of a luminous object in the
sky, like a meteor.]
Scientists are eager to obtain reliable reports on the compass direction
to the fireball’s point of disappearance from as many widely separated
eyewitnesses as possible. They then can plot the individual lines of
sight on a good map, marking exactly where these lines intersect. In
this way, the investigators can make reasonably accurate fixes of the
position of the point on the earth’s surface that is situated directly
below the end-point of the fireball path, as this end-point was seen in
the sky by each pair of eyewitnesses.
Instead of using the ordinary compass direction to a fireball’s point of
disappearance, you may prefer, as do astronomers, to use the azimuth.
What we have been calling a “compass direction” is one that is expressed
in terms of the cardinal points: north, south, east, west. An azimuth is
a direction stated in _degrees_. Rough azimuths can be taken with a
compass, but for accurate work, a graduated circle, like that on a
transit or theodolite, must be used. Astronomical azimuths begin at the
_south_ point and continue clockwise full circle to 360°. For example,
the lines of sight in the diagram, p. 87, could very well have been
given as astronomical azimuths. And, in the diagram, p. 91, the line of
sight C₁ could have had the precise designation 118° and C₂ that of
222°.
Every fix serves to guide field parties to areas that are to be
carefully searched for fallen meteorites. Extra-thorough searches are
made if the people living in a particular area reported that they heard
meteorite fragments hissing and whining on their way to earth or heard
the thumps of their impacts on the ground.
You will notice that so far we have been treating our problem as a
two-dimensional one. We have been working with _directions_ only and
have plotted out direction indicators on a map representing the plane of
the earth’s surface. Now, as we did in our baseball analogy, let us move
into the third dimension.
[Illustration: Diagram (not drawn to scale) showing plotted compass
directions to the last visible point on a fireball path. (The point
denoted by L in next diagram.) Black dots represent positions of
various observers. Each arrowed line is directed toward the last
visible point as it was estimated by the individual observer. The
oval area, which includes points of intersection of all observed
lines of sight taken in pairs, marks out region in which meteorites
have probably fallen.]
If, in addition to compass directions to the observed endpoint,
scientists can also obtain the apparent _elevation_, in degrees, of this
point as seen by the various eyewitnesses, then with the help of a
little trigonometry, they can fix the position _in space_ of the
end-point itself rather than the position of its _projection_ on the
surface of the earth.
This same procedure can be followed in fixing the space-position of any
well-observed point on the fireball path. It therefore becomes possible
when _both_ elevations and compass directions are reported for several
points on the fireball path to determine the flight-path or, as it is
technically called, the _trajectory_, of the falling meteorite through
the atmosphere. Trajectory determinations are of great scientific value.
You can estimate the compass directions and elevations to the important
points on a meteorite trajectory at the actual time of fall. Or you can
have the scientific field party make or check your measurement at some
later time by setting up a surveying instrument at the very point from
which you saw the fireball.
The accuracy of your measurements can be improved if you have been able
to “line up” the point, L, at which you saw the fireball disappear, with
some familiar object on the horizon, such as a church steeple, a tall
tree, a telephone pole, or a lightning rod on a farm building. You will
recall that an eleven-year old girl provided one of the field parties
from the Institute of Meteoritics with an excellent observation of the
point of disappearance of the Norton fireball. She was able to do this
because she remembered just where it went out of sight behind a familiar
landmark.
[Illustration: Method for locating a point on a fireball path. (In
this case the point of disappearance, L.)
O₁ First observer.
A₁ Apparent height of point of disappearance (50°).
C₁ Compass direction of point of disappearance (N 62° W).
O₂ Second observer.
A₂ Apparent height of point of disappearance (45°).
C₂ Compass direction of point of disappearance (N 42° E).]
If the fall occurs at night, you can help investigators greatly if you
are familiar enough with the brighter stars to use them as “skymarks.”
You simply note as quickly and sharply as you can just where the
fireball path was in reference to those prominent stars. This alert
observation of yours will at least be a great aid to investigators who
are searching for meteorites that may have fallen from the fireball;
and, moreover, there is no telling what else your quick eye might have
captured for science.
While looking through a window, Kayser, the Polish astronomer, saw a
fireball appear at Rigel and move to Sirius, where it disappeared. This
observation of his proved to be one of the most accurate and
_significant_ ever made of the fall of a meteorite. For it enabled the
German mathematician, Galle, to show that the Pultusk meteorite, which
produced the fireball Kayser saw, came into the Solar System from
interstellar space!
It is very essential to carefully notice and mark the exact spot from
which your observation was made so that you can return to it if
scientists wish to set up surveying instruments there.
The map and side view of the Norton County, Kansas, meteorite trajectory
show the practical results that the Institute obtained by use of the
intersecting-lines-of-sight method. The fireball accompanying the Norton
meteorite fall appeared at A. The first “explosion” took place at E₁,
the second at E₂, and the fireball disappeared at L.
If markers were dropped straight down to earth from each point along the
trajectory or flight-path of a meteorite through the atmosphere, the
line joining the points where the markers fell would be the
_earth-trace_ of this trajectory. The directions of sight to these
various points are indicated for people living in the towns along and
near the earth-trace of the Norton meteorite fall. The solid-line arrows
represent the direction of the point of disappearance; the dotted-line
arrows, the point of appearance; the dash-dot arrows, E₁; and the dashed
arrows, E₂. The probable area of fall is shown as an oval-shaped area,
the longer axis of which is identical with the direction of motion of
the meteorite.
[Illustration: Path of the Norton meteorite.]
The many fragments of all sizes recovered from the Norton fall were all
found within the bounds of this oval-shaped area, although unavoidable
errors of observation placed the center of the oval about 4 miles too
far to the north.
In addition to the questions about direction and elevation, there are a
few more that investigators of meteorite falls would like to have
observers answer.
At what time (determined as accurately as possible) did the fall
occur? Knowledge of this time is necessary if the path in which the
meteorite was moving about the sun is to be calculated by scientists.
Did you hear any sounds, either while you were watching the fireball
or after it disappeared? If you heard such sounds as the whining or
hissing of meteorite fragments flying through the air or the heavy
thumps of their impacts on the earth, then you were very close to
where the meteorite came down!
How many minutes and seconds (again determined as accurately as you
can) passed between the time when you saw the fireball vanish and the
instant when you first heard sounds from it? Such sound data permit
rough determination of the distance from the observer to the point
where the meteorite fell.
How long did the sounds set up by the meteorite last, and in what
direction did these sounds seem to die out?
If you or your neighbors find fragments that you suspect are pieces of
the meteorite, these specimens should be shown to the investigating
field parties at once—preferably undisturbed and in the very places
where they fell. In any event, the suspect masses should not be hammered
on and broken up! Even as late as 1958 in a country as science-conscious
as Germany, a beautiful stony meteorite, seen to fall and speedily found
by an alert group of children playing out of doors, was deliberately
broken up into 5 pieces in order that each of the children (aged 9 years
and up) might take home a “souvenir” of the event. Later, these pieces
had to be laboriously reassembled by scientists before any idea could be
gained of the original shape and surface features of the meteorite.
Even when thorough searches are made, not all the meteorite fragments in
the area of fall may be found for many months. But if the people living
in the region have been alerted and are on the lookout for unusual
specimens or signs of meteoritic impact (such as freshly made holes or
“craters” in the ground, shattered tree limbs, and so forth), the
chances of ultimately finding many or most of the fallen masses are
good.
As we have already mentioned, numerous fragments of the Norton meteorite
(including one weighing 130 pounds) were found within two to three
months after its fall on February 18, 1948. But the main mass was not
discovered until the following August, when a caterpillar tractor nearly
tipped over into the large impact funnel that this huge stone had made
in the earth. Fortunately, field searchers from the Institute had
already talked to one of the farmers using the tractor and had told him
that just such a “crater” might be found in the very area under
cultivation. Consequently, the crater was promptly reported.
In surveys concerned with the location and recovery of meteorites _not_
seen to fall, we find that sometimes meteorite fragments, particularly
the smaller ones, lie on the surface of the ground or at shallow depth.
Such fragments were probably too light to penetrate deep into the ground
or, in the years since their fall, the action of rain, wind, and frost
has uncovered them.
In such cases, a party of searchers generally spreads out in order to
get over as much ground as possible and each member of the group looks
for meteorite specimens without using instrumental aids. Visual searches
of this type have been very successful, for example, around the Canyon
Diablo crater, where almost the entire plain out to several miles from
the rim once was sprinkled with large and small fragments of meteoritic
nickel-iron. This type of meteorite hunt is of only limited
effectiveness because the specimens (or at least a part of each one)
must be visible to the searchers.
[Illustration: Collecting small surface specimens of meteorites with
portable detecting devices: a powerful alnico magnet mounted on a
light wooden sled, and a horseshoe magnet at the end of a cane. See
p. 98.]
To increase recoveries, searchers have employed, in addition to their
eyes, various types of permanent magnets, either mounted on the end of a
cane and used to probe the upper few inches of loose soil, or dragged
behind the searcher on a small, light sled. Meteorite hunters have also
used more powerful portable electromagnets to collect large amounts of
meteoritic material (both solid iron and iron-shale) not only from the
surface but also from shallow depths. Even the best of these simple
magnetic devices, however, are useless in the detection of really deeply
buried meteoritic material.
Meteorites do not merely fall upon the earth (as most astronomical
textbooks still insist), but usually penetrate into it—often quite
deeply. In fact, one of our mathematical investigations showed that
perhaps 100,000 times as much meteoritic nickel-iron is concentrated
below maximum plow-depth (approximately one foot) as lies above that
depth. Clearly, some form of instrument capable of detecting deeply
buried meteorites needed to be devised if this wealth of buried material
was not to be lost to science. This need was answered by the development
of special _meteorite detectors_.
Although meteorite detectors working on several different principles
have been constructed, we shall limit attention here to the simplest and
most field-worthy design. The essential principle on which it operates
is one familiar to any Boy or Girl Scout who has used a magnetic
compass. The first lesson Scoutmasters teach is not to read compass
directions from such an instrument when it is held near a mass of iron
of considerable size, such as an automobile. Such a large iron mass
alters or distorts the local magnetic field of the earth on which the
direction-finding ability of the ordinary compass depends. It is this
very characteristic, so bothersome to the user of a compass, that is the
principle on which meteorite detectors work. For if an electrically
driven meteorite detector capable of generating its own magnetic field
is carried over a deeply buried iron meteorite, the instrument’s
magnetic field will be distorted by the presence of the metal mass, just
as the local magnetic field of the earth was distorted by the metal of
the automobile.
[Illustration: A 146-pound iron, found by this girl without the use
of instruments although only a small corner of the meteorite was
visible above the surface of the ground.]
[Illustration: A commercially built meteorite detector in
operation.]
The operator of such a meteorite detector wears earphones and watches a
signal needle in plain sight on the top panel of the detector. Since the
phone and signal-needle circuits of the meteorite detector are _in
balance_ only when the magnetic field generated by the detector is
undistorted, the disturbing presence of a deeply buried meteorite is at
once revealed by a shrill note sounding in the earphones and
simultaneous motion of the signal needle. If, as in all buried treasure
stories, we use “X” to stand for the spot where the signals from the
detector are strongest, then the meteorite-hunter has only to dig deep
enough at “X” to recover the celestial treasure-trove he is after.