_Name of | _Dates of |_Date of| _Parent comet_ | _Remarks_
stream_ | occurrence_ |maximum_| |
---------------+-------------+--------+------------------+------------
Quadrantids |Jan. 1–4 |Jan. 3 |1861 I |Observed
| | | | longer than
| | | | 100 years.
| | | |
Lyrids |April 19–23 |April 21| |Observed
| | | | longer than
η Aquarids |May 2–5 |May 4 |Halley (1835 III) | 2500 years.
| | | |
δ Aquarids |July 14-Aug. |July 30 | |
| 19 | | |
| | | |
ι Aquarids |July 16-Aug. |July 30 | |
| 25 | | |
| | | |
Perseids |July 29-Aug. |Aug. 12 |1862 III |Observed
| 17 | | | more than
| | | | 1200 years.
| | | |
α Capricornids |Aug. 1–21 |Aug. 17 |1948 n |
| | | |
Cygnids |Aug. 9–22 |Aug. 17 | |
| | | |
Taurids |Sep. 15-Dec. |Nov. 12 |Encke (1957 c) |
| 2 | | |
| | | |
Draconids |Oct. 9–10 |Oct. 10 |Giacobini-Zinner |13-year
| | | (1946 V) | period;
| | | | great
| | | | showers in
| | | | 1933, 1946;
| | | | none in
| | | | 1959.
| | | |
Orionids |Oct. 18–26 |Oct. 22 |Halley (1835 III) |
| | | |
Leonids |Nov. 14–20 |Nov. 17 |Temple-Tuttle |Observed
| | | (1866 I) | since
| | | | A.D. 902.
| | | |
Geminids |Dec. 7–15 |Dec. 14 | |
| | | |
Ursids |Dec. 17–24 |Dec. 22 |Temple (1939 X) |
The close approach of a comet sometimes causes a fantastic shower
of “shooting stars,” and hundreds or even thousands may be counted
in a single night. At the approach of the debris of Comet Biela on
November 27, 1885, some 75,000 meteors were visible from a single
place during a period of an hour. Irregularly occurring or sporadic
meteors not associated with a known comet also occur and pelt the earth
unexpectedly.
_The Green Fireballs_
On the evening of September 18, 1954, a group of astronomers and their
wives from the observatory at Sacramento Peak, New Mexico, were having
a picnic at the White Sands National Monument, near Alamogordo. In this
great desert of pure white gypsum the air is extremely hot during the
daytime but cools to a pleasant warmth after sunset. Supper finished,
the picnickers had taken off shoes and stockings to wade in the soft
warm sand. By 8:30 it was dark and some of the astronomers had already
left but others (including Dr. Menzel) had lingered to watch the stars,
which stand out sharply in the clear skies over the desert.
Suddenly, far to the north, appeared an enormous green fireball. Of
blinding brilliance, it was moving slowly and majestically from east to
west in a substantially horizontal path about seven degrees above the
horizon, leaving behind a luminous trail that persisted for at least
fifteen minutes. At about the same time thousands of other persons on
the ground in New Mexico and Colorado, as well as the crews of several
planes in flight, were observing the fireball. It passed over a crowded
football stadium in Santa Fe, interfered with radio and TV transmission
as it appeared over Albuquerque, and over Denver turned night into day.
A United Airlines pilot at about 15,000 feet near Laramie, Wyoming,
saw the blue-green ball crossing his course and for some ten minutes
observed the luminous cloud it left behind[V-6]. At almost the same
instant, the fireball was sighted in the Bay of San Francisco, 1000
miles away. One publication cited this meteor as two separate UFOs,
one flying over San Francisco, the other over New Mexico and the
Southwest[V-2].
When telephone calls swamped the newspaper offices, reporters
interviewed Dr. Lincoln La Paz of the Institute of Meteoritics at the
University of New Mexico. Although he had not observed this particular
specimen, he had seen similar green fireballs a few years earlier and
he commented that this was no ordinary meteor but something unusual. A
new wave of UFO excitement began to sweep the country. Were mysterious
machines from outer space again patrolling New Mexico?
The astronomers who had admired the fireball at White Sands were
amazed at the public reaction. As professionals who had spent their
lives in observing and analyzing astronomical phenomena, they agreed
that the object had been unusual in its slow movement, its color, and
its brilliance. But an unusual meteor is still only a meteor, not a
spaceship, and they easily recognized it as a green fireball of the
type that had appeared over the Southwest a few years earlier.
The first epidemic of green fireballs had begun in early December
1948, and for nearly two months the brilliantly burning objects had
appeared almost every night in the skies over New Mexico[V-7, p. 71].
Their apparent collision course startled plane crews in the air, and
their steady, seemingly purposeful motion frightened observers on the
ground. The fireballs showed a family resemblance in their bright-green
color, their great size and brilliance, their level flight path, their
noiseless disappearance, and their failure to leave material fragments
on the ground.
New Mexico was a particularly sensitive area, studded with military
bases and research installations carrying out vital work in ballistics,
guided missiles, atomic energy, and space science in general. Since the
unusual meteors seemed to be concentrating on New Mexico, Air Force
Intelligence had to face the question: Were the fireballs natural
astronomical phenomena or were they experimental guided missiles from
another country, perhaps Russia?
After consulting Dr. La Paz and hearing his evaluation of the evidence,
the Air Force felt growing concern. Perhaps unconsciously influenced by
the general hysteria of the past year, Dr. La Paz concluded that the
objects were not meteors but must be “something unusual” because they
differed from “normal” meteors in their color, trajectory, velocity,
size, brilliance, and apparent lack of fragments.
With very little knowledge of meteors and great faith in machines from
outer space, saucer enthusiasts reasoned that since the fireballs were
not normal meteors they must be artificial objects. Since they were
artificial, they must be under intelligent control. Since they were
intelligently controlled, they must be unmanned missiles or manned
vehicles launched from an alien spaceship hovering hundreds of miles
above the earth whose purpose might or might not be destructive, or
they might be merely ranging devices sent as a warning to earthmen.
The Air Force was not particularly worried about interplanetary
visitors, but it was concerned with the possibility that the fireballs
were man-made vehicles, a potential danger to the country. One
scientist had suggested that the Russians might have constructed a
guided missile whose nose cone, the final stage in a multistage rocket,
was made of ice and various other chemicals. In re-entering the earth’s
atmosphere, such a cone would burn up; the vaporizing ices would
account for the green color observed, for the silent disappearance of
the object, and for the lack of material traces on the ground. Whatever
the true explanation, members of the Air Defense Command could not
afford to guess; they had to know.
In mid-February 1949 they assembled at Los Alamos a conference of
military and intelligence officers, physicists, and astronomers, to
discuss the problem of the green fireballs. After two days of studying
the evidence, most of the members agreed that the fireballs were
meteors of an unusual type and, as natural phenomena, not a threat to
national security. To take care of the extremely remote chance that
this conclusion might be wrong, the conference turned over the problem
to the scientists at Air Force Cambridge Research Center which, in
the late summer, organized Project Twinkle to equip and establish
three cinetheodolite stations in New Mexico. Fitted with a diffraction
grating to split the spectrum into its component colors (and thus
identify the chemical elements present), the cameras were to photograph
and record the altitude, size, speed, and spectrum of the luminous
objects.
Since the green fireballs, meanwhile, had all but vanished from the
skies, enthusiasm for the research project diminished. Only one camera
(designed by Dr. Menzel) was ever put into operation and it never found
anything to photograph. After two months of futile searching, the Air
Force finally abandoned Project Twinkle as a waste of time.
In the years following, green fireballs occasionally appeared. An
astronomer observed one over Lafayette, Colorado, at 7:45 P.M. on June
4, 1950. One soared over the New England states and eastern Canada
on November 2, 1950, and a year later, on November 2, 1951, a plane
crew over Texas sighted another which was dramatically publicized in
_Life_ magazine, and described in another publication as a missile that
ejected flaming balls. Few other fireballs made the headlines until the
one of September 18, 1954, but even that caused only brief excitement
and the Air Force expressed no alarm.
_Meteors in the Records_
The American Meteor Society, whose members specialize in the study
of meteors and meteorites, for years have collected reports of such
phenomena. From a large enough number of good descriptions of a given
meteor, astronomers can analyze the data mathematically and determine
the meteor’s radiant--the point in the heavens from which it seems to
come. The meteor is then identified by its radiant and given an AMS
number. For several years the data were published in _Meteoritics_, a
journal issued jointly by the Meteoritical Society and the Institute of
Meteoritics of the University of New Mexico. Dr. Charles P. Olivier,
president of the American Meteor Society, was a contributing editor.
The records in _Meteoritics_ for the years 1950 to 1955 list dozens
of fireballs, many of them green, that were somehow overlooked by
saucer enthusiasts. On August 11, 1950, during the maximum of the
Perseid shower, a blue-green fireball (AMS 2336) apparently oval- or
cigar-shaped appeared over Washington, Oregon, and Idaho at 7:30 P.M.
and was reported by more than 100 witnesses. So brilliant that it
showed a noticeable disk, it flew in a horizontal path, silently broke
into three pieces, and disappeared[V-8, p. 379].
September 20, the same year, was a big day for meteors. At 1:35 A.M.
a giant fireball (AMS 2326) roared over southeastern Illinois from
north to south, leaving a luminous train visible in five states and
illuminating the sky and countryside from St. Louis to Louisville and
from Memphis to Knoxville. The final detonation, over western Kentucky,
was heard over an area 1000 miles square and shook buildings from
Paducah to Memphis. Fragments showered farms over a twenty-five-mile
area, struck five buildings, and penetrated one roof. About fifty
pounds of meteorites dropped in Murray, Calloway County, Kentucky, and
are now in the Smithsonian Institution in Washington. That same night
about 10:45 P.M., fireballs were reported by plane crews flying over
a six-state area--Idaho, Wyoming, Utah, Colorado, Arizona, and New
Mexico[V-9, p. 115]. Similar fireballs that vanished without trace
were reported on September 28, 1953 (AMS 2331); October 4, 1953 (AMS
2330); May 15, 1954; and October 27, 1954 (AMS 2337).
The green fireballs still appear now and then, as they always have.
None of them has yet changed into a spaceship.
_Fallacies about Meteors_
Most flying-saucer enthusiasts still refuse to believe that the green
fireballs were natural phenomena. Misinterpreting or distorting the
statements made by professional astronomers, they cite the unusual
nature of these meteors as proof that they were not meteors at all but
machines from another world. Advocates of this belief need more than
a refresher course in logic; they also need to learn some facts about
meteors.
The space-vehicle interpretation rests on a series of mistaken beliefs
and illogical conclusions about the nature and behavior of meteors.
These false premises may be summarized as follows:
1. _Color._ Meteors do not contain copper; since the peculiar shade of
green shown by the green fireballs could come only from copper, the
fireballs were not meteors but spacecraft.
2. _Speed and trajectory._ Meteors do not travel at a slow rate of
speed and do not follow a horizontal path; since the green fireballs
did both, they were not meteors but spacecraft.
3. _Size and brilliance._ Meteors do not show such great size or
brilliance as did the green fireballs, which were therefore not meteors
but spacecraft.
4. _Sound._ Meteors produce a loud noise; since the green fireballs
moved silently, they were not meteors but spacecraft.
5. _Fragments._ Meteors deposit material fragments on the earth which
can be located if the investigator maps the flight path and makes a
search; since the green fireballs left no fragments, they were not
meteors but spacecraft.
In the pages that follow we shall attempt to correct each of these
mistaken ideas in turn, to present the actual facts known to
astronomers, and to show clearly that the green fireballs were not
spacecraft, but meteors.
_Facts about Meteors_
1. _Color._ Copper-green meteors are not a new phenomenon. This
unusual shade of green is only one of the many possible colors that
meteors may display--white, green, blue, yellow, orange, red, and all
shades in between. Descriptions received by the Meteoritical Society
include adjectives such as bright-green, copper-green, blue-green,
fiery white, green-white, orange, blue, yellowish, silver, red-orange.
Perceptions of color vary greatly among different observers, so that
several witnesses may choose different words for the color of the same
object. The most common adjective used is “brilliant”; an observer who
has only a few seconds to look at the object often has real difficulty
in deciding just what color accompanied the brilliance. Very common
phrases are blue-green, greenish-white, orange-yellow, orange-red,
greenish-yellow, yellow-green.
Both the chemical structure and the velocity of the meteoric body help
determine its apparent color. As the burning object plunges through the
atmosphere and vaporizes, the chemical elements produce their typical
colors. At higher velocities, atmospheric friction heats the body to
higher temperatures and whitens the color; as the body slows down and
becomes less hot, it is apt to appear redder.
In a few instances astronomers have been able to photograph the
color spectrum of a meteor in flight, to analyze the spectral lines
and determine exactly what elements were present[V-10]. As a rule,
however, the chemical content must be found from a laboratory analysis
of recovered meteorites. Some meteors do contain traces of copper,
and free nodules of pure copper have been found in several meteorites
[V-5, p. 81]. Magnesium occurs in fairly high percentages in most
meteorites and the amount is unusually high in green meteors[V-11].
It produces a color almost identical with that from copper. Seeing
the green of a vaporizing meteor, no observer could tell whether the
color came from copper or from magnesium unless he could photograph the
spectrum or make a chemical analysis of the meteorite.
The color displayed by the New Mexico fireballs may have come from
copper, but more probably from magnesium. Another possible source
is frozen nitrogen. Laboratory experiments relating to problems
of satellite re-entry[V-12] have shown that when frozen nitrogen
vaporizes, it emits a brilliant green glow whose wave length is
almost identical with that of the New Mexico fireballs, as judged
from the paintings made by witnesses. One of the prevailing theories
suggests that meteors of this type may be icy “cometoids”--cometary
debris, chunks of ice, and frozen gases (including nitrogen) at very
low temperatures. When they enter the earth’s atmosphere and are
slowed down to speeds of several hundred miles an hour, they become
heated and vaporize, and the surface alternately melts and refreezes;
the vaporizing nitrogen would produce the green color seen in the
fireballs. Such a process would account for the color, the short
lifetime, and the lack of fragments of the New Mexico meteors.
To summarize: Meteors _can_ exhibit the particular green color shown
by the New Mexico fireballs. It can result from copper, magnesium, or
frozen nitrogen, which can normally occur in meteors.
2. _Speed and trajectory._ Meteors vary widely in their velocities
and flight paths. They plunge from space into the earth’s atmosphere
at speeds estimated to range from seven to forty-five miles a second
relative to the earth--from 25,000 to more than 150,000 miles per hour.
Members of a particular meteor stream usually show a characteristic
velocity. The Perseids, for example, travel at high speed, some
thirty-six miles a second, while the Geminids saunter in at a mere
twenty-one miles a second. Most of these “falling stars” become
visible to us when they have descended to around sixty or seventy miles
above the earth. Flashing down in a steep path, they usually burn
up and vanish by the time they have fallen to around fifty or forty
miles. The larger the meteor’s body, the longer its life and the lower
its point of disappearance. Most meteors maintain a straight course
as they descend toward earth. A typical path is that photographed by
Smithsonian astronomers in New Mexico on the night of November 23, 1960
(see Plate IIIa). Some fireballs have been reported to change course
after exploding. More probably, the witness is actually observing the
shifting pattern of the smoke cloud left by the meteor. The Puerto Rico
fireball of January 12, 1947, left an erratic trail of this type, which
was photographed ten to twenty minutes after the meteor had disappeared
(see Plate IIIb).
The original entrance velocity, angle of entry, size, and chemical
structure all influence the shape of a meteor’s path and its time of
survival. The apparent angle of descent as seen by the observer depends
on the distance and the direction the object is moving relative to the
observer. When the meteor travels parallel to the observer’s line of
sight, it seems much slower than when it passes the line of sight at
right angles. The greater the distance between the observer and the
meteor, the slower its apparent motion[V-13].
Some meteors move very slowly; traveling at an almost leisurely rate,
they soar through the sky on a long, level path almost parallel with
the earth. The slow fireballs in the great meteor procession of 1913
maintained a horizontal course over a distance of several thousand
miles, from western Canada to Brazil[V-14].
Astronomical records show that green meteors are usually slow. Some
230 persons reported to the American Meteor Society that on November
28, 1953, at 6:30 P.M., a fireball moved slowly through the sky from
Massachusetts to Pennsylvania. Described as blue-white-green, changing
to orange-yellow-red, it was huge, disk-shaped, and vanished silently
without depositing fragments [V-1, p. 273]. On May 15, 1954, at 11:22
P.M., more than 100 persons observed (and reported) a slow-moving
fireball, blue-green changing to red, of luminosity so great that it
woke sleeping people. Toward the end of its course it seemed to stop,
spiraled a couple of times, and then simply vanished without leaving
fragments [V-8, p. 336].
To summarize: Meteors _can_ travel at low velocities and in apparently
horizontal paths.
3. _Size and brilliance._ Giant meteors of great luminosity have been
recorded throughout history. Some fireballs have been visible to
observers throughout an area of thousands of square miles. Typical
descriptions are: dazzling, like an airplane falling in flames, bigger
than the full moon, of blinding brilliance, so bright it turned night
into day, like the headlight of a locomotive, as big as the setting sun
but three times as brilliant.
The luminosity does not depend on the actual size of the meteoric body.
A fragment no larger than a pinhead can create a brilliant flash as
it vanishes. A spectacular fireball that lights up the country over
hundreds of miles may have a small body that burns up completely miles
above the earth. A larger body can survive longer, so that it continues
to flare for several seconds or more. The larger, long-lasting
fireballs may explode into smaller fragments and cascades of sparks. In
exploding, they can produce a luminous cloud of particles that remains
visible for fifteen or twenty minutes and then peppers the ground
with meteorites that fall like hail or buckshot. A giant fireball can
deposit chunks of metal weighing a ton or more like those found in
Mexico, or can leave a truly enormous body that penetrates the ground
and carves out a great crater like those in Arizona and Texas.
To summarize: Huge fireballs of great brilliance are not new.
4. _Sound._ Some meteors produce noise; others do not. Most meteors
silently vaporize high above the earth. When one does reach the
ground, it may strike with no noise but the faint thud of its impact.
Shooting through the air, it sometimes makes weak noises that have been
described as rumbling, crackling, rustling, whistling, or hissing.
Meteors sometimes explode with one or more crashing detonations that
rattle or even break windows. The noise has been described as like a
heavy clap of thunder, the explosion of a volcano, or a whir as if a
million bumblebees had been disturbed. The noise from the explosion of
the Siberian meteor in 1908 was heard over a distance of 600 miles, and
the shock registered as an earthquake in England.
Many meteors, like the Pennsylvania fireball of January 29, 1952, (AMS
2328) are completely silent. This blue-green object, so large that it
showed a definite disk, was reported to the American Meteor Society by
more than 400 witnesses from Maine to Virginia and from New York to
Ohio; none of the observers heard any noise [V-1, p. 264].
To summarize: Some meteors end with a bang, but most of them don’t even
whimper.
5. _Fragments._ Most meteors burn up high in the atmosphere. A few, if
they are large enough in size (at least ten to twenty pounds) and tough
enough in structure, survive to reach the earth as stony or metallic
fragments. Marked differences characterize the various meteor streams.
The Taurids (maximum November 12) are relatively rigid structures,
unusually tough, and show little tendency to break up in their flight.
The many Taurid fireballs show that fairly large bodies have survived.
The Geminids (maximum December 14) are of average strength but appear
to be very dense, while the Draconids (October 10) are featherlike and
fragile, with low density. Some of the most brilliant fireballs may be
structures of ice and frozen gases which quickly vaporize on reaching
the earth and hence leave no detectable fragments. The fiery object
that struck Siberia in 1908 may have been such an “icy cometoid”;
although it devastated an area of hundreds of square miles and uprooted
or knocked down some eighty million trees, it apparently left no
physical trace[V-15].
If some of the physical body does survive to reach the earth’s surface,
finding it is still a problem. Recovery is rare even when the fall
occurs in daylight over well-populated country and the flight path
can be charted from the accounts of reliable witnesses. When the
fall occurs at night, recovery is even rarer[V-5]. After dark, even
experienced observers find it difficult to judge true directions and
distances, and they may plot a place of fall that is many miles from
the actual point of impact. Meteoriticists know that there is small
chance of finding meteorites that fall at night except in regions where
most of the land is under cultivation. In the fifty years between 1898
and 1948, of forty-eight recoveries from observed meteorite falls in
the United States, only seven were made from falls occurring after 8
P.M.[V-5].
Recovery depends on many factors: the number of persons who saw the
event, the accuracy of their estimates of distance and direction, the
size of the meteorites, the patience of the searchers, the time and
money available for the search, and, most important of all, just plain
luck.