Encyclopedia of Science & Technology 10e Vol. 11 (METE-NIT).pdf

Meteor

The luminous streak lasting seconds or fractions of a

second and seen at night when a solid, natural body

plunges into the Earth’s (or another planet’s) atmo-

sphere. The entering object is called a meteoroid

and, if any of it survives atmospheric passage, the

remainder is called a meteorite. Cosmic dust parti-

cles (with masses of micrograms) entering the atmo-

sphere and leaving very brief, faint trails are called

micrometeors, with the surviving pieces known as

micrometeorites. If the apparent brightness of a me-

teor exceeds that of the planet Venus as seen from

Earth, it is called a ﬁreball; and when a bright me-

teor is seen to explode, it is called a bolide. See ME-

TEORITE ; MICROMETEORITE .

Visual observation. Under normal, clear atmosphe-

ric conditions and dark skies (no moonlight or artiﬁ-

cial lights), an observer will see an average of ﬁve me-

teors per hour. The spatial distribution of meteoroid

orbits relative to the Sun, and the circumstances of

their intersections with the moving Earth are respon-

sible for pronounced variations in meteor rates.

As the Earth moves in its orbit, its velocity points

toward that part of the sky (sometimes called the

apex of meteor velocities) which is visible from

local midnight through morning to local noon, and

points away from that part of the sky which is vis-

ible from local noon through evening to local mid-

night. On average, an observer sees more meteors

during the early morning hours as the Earth sweeps

up objects in its path than in the early evening hours

when meteor-producing objects must catch up with

the Earth.

Physical characteristics. The heights of appearance

and disappearance of a meteor depend on meteoroid

initial velocity, angle of entry with respect to the

vertical, initial meteoroid mass, and meteoroid ma-

terial strength. The average meteor seen by the un-

aided eye starts with a meteoroid velocity of 18 mi/s

(30 km/s) and leaves a luminous trail from 67 to

50 mi (110 to 80 km) high. The fainter is the me-

teor (the smaller the meteoroid mass), the shorter is

the meteor length. The faster the meteoroid travels

just before hitting the atmosphere, the higher in the

atmosphere the meteor trail occurs.

In the end, most, if not all, of the meteoroid

material is vaporized, leaving a deposit of metallic

atoms (predominantly sodium, calcium, silicon, and

iron) in the upper atmosphere. This deposition is an

important mechanism in the wind-shear formation

of certain types of highly ionized, radio-reﬂecting,

upper-atmospheric phenomena called the sporadic

Elayers. See IONOSPHERE .

The meteor trails themselves are rapidly expand-

ing columns of atoms, ions, and electrons dislodged

from the meteoroid by collisions with air molecules,

and can be excited to temperatures of several thou-

sand degrees Celsius. For a time after trail formation,

the free electrons are dense enough to reﬂect radio

wavesinthe very high frequency range, and there-

fore can be used to transmit radio messages that are

brief (0.1–15 s) but high in information content (on

account of their large bandwidth) for up to 1300 mi

(2200 km). Since meteor-reﬂected signals are not as

subject to ionospheric and other disturbing inﬂu-

ences as are other means of radio communications,

there has been interest in using meteors for certain

military and commercial purposes. See RADIO-WAVE

PROPAGATION .

Under the right circumstances, particularly with

high-power ultrahigh-frequency (UHF) radars, the

ionization right around and moving with the mete-

oroid itself is seen. This is known as the head echo,

and a determination of its velocity is the most ac-

curate way to determine radar meteor speeds. If the

radar beam is very narrow, such as that at the Arecibo

National Observatory, the radiant and the orbit

around the Sun can also be accurately determined.

See RADAR ; RADAR ASTRONOMY .

Some meteors show relatively long-lasting glows

along their paths. The glows are called meteor trains.

Meteor — Myzostomida

Meteor

Novice observers often confuse this glow with the

brief meteor wake that is sometimes seen just behind

the luminous gases surrounding the meteoroid itself.

These trains are subject to winds in the upper atmo-

sphere, and for many years their apparent motions

(twisting and drifting) were the only probes of wind

velocities in the mesosphere, or middle atmosphere

(42–72 mi or 70–120 km high). The motions of me-

teor trains and trails detected through the Doppler ef-

fect on reﬂected radar signals have been extensively

studied, and global mesospheric wind patterns have

been derived. See DOPPLER EFFECT ; MESOSPHERE .

Meteoroid orbits and velocities. The Earth moves

around the Sun with an average speed of 18 mi/s

(30 km/s). According to the laws of celestial me-

chanics, if a meteoroid comes from beyond the solar

system, its velocity at the Earth’s distance from the

Sun must be greater than 26 mi/s (42 km/s). If

such a meteoroid hits the Earth head-on, indica-

tions of preatmospheric speeds in excess of 45 mi/s

(72 km/s) would be observed. The fact that the vast

majority of observed meteoroids have orbits with

Earth-approaching velocities of less than 45 mi/s in-

dicates that most of these are comet and asteroid

fragments, and are therefore long-term members of

the solar system. Early velocity measurements were

quite crude, and the large errors led to the belief

that a sizable fraction of meteoroids actually had

speeds great enough to escape the solar system or

come from the outside. Once these errors were re-

duced, the number of hypervelocity orbits decreased

dramatically, with doubts expressed that any mete-

oroids could come from beyond the solar system.

However, in the 1980s and 1990s, a combination of

spacecraft and high-power radar observations indi-

cated that hypervelocity micrometeoids do indeed

exist with seeming interstellar dust connections. An

even more curious connection is the discovery of

similarly sized particles with peculiar isotopic abun-

dances within meteorites of presumed asteroidal ori-

gin. These micrometeorites appear to predate the

solar system with an apparent origin in early super-

novae, novae, or supergiant stars. See COSMOCHEM-

ISTRY ; INTERSTELLAR MATTER .

The slowest velocity of Earth-meteoroid encoun-

ter occurs when the meteoroid has to catch up with

the Earth. The gravitational attraction of the Earth

keeps such an encounter from producing a zero

atmospheric velocity. The velocity that an object will

achieve by falling toward the Earth from an inﬁnite

distance and in the absence of the Sun’s gravitational

attraction is 7 mi/s (11 km/s). High-power ultrahigh-

frequency radars detect a considerable number of

micrometeors with velocities well below the Earth-

attraction limit. These are either artiﬁcial satellite

debris or natural debris captured by the Earth from

a large, co-moving interplanetary dust zone that has

been discovered in the Earth’s orbit. See CELESTIAL

MECHANICS ; ESCAPE VELOCITY ; INTERPLANETARY

MATTER ; ORBITAL MOTION .

Radiants and showers. A combination of the meteo-

roid’s and Earth’s velocities of travel around the Sun

makes the meteor itself seem to originate from a spe-

ciﬁc direction in the sky called the radiant. If there

are numerous meteoroids in nearly the same orbit

(sometimes incorrectly called meteor streams), the

Earth sweeps them up at speciﬁc times of the year

and a so-called meteor shower is observed. Meteor

showers are named after the constellation or single

star in the sky from which they appear to radiate.

While shower meteoroids are really moving nearly

parallel through space and result in nearly parallel

meteor trails, the effects of perspective make the me-

teors appear to diverge from the radiant (see illus-

tration ). If the meteor shower is particularly long-

lasting, the radiant will appear to drift slowly in posi-

tion from night to night as the relative directions of

the Earth-meteoroid velocities change. Radiants are

not geometric points, but areas in the sky that can

be several degrees in diameter. Hence, quoted radi-

ant positions are averages over such areas and often

differ somewhat from one compilation to another.

Meteors that cannot be shown to be associated with

a known shower are termed sporadic meteors.

While all meteor showers show both day-to-day

and year-to-year variations, some are more reliable

than others, and these are called annual or major

showers ( Table 1 ). There are over 100 minor show-

ers, some of which are of special interest ( Table 2 ).

Many minor showers have hourly rates so low that

they are often not noticed except by experienced

observers or when there is an unusual burst of ac-

tivity. Some showers have been discovered by radio

or radar methods to occur only during the hours of

daylight, with no visible counterpart seen at night.

Most of the radiants listed refer to geocentric radi-

ants where the effects of the Earth’s gravitational

Ursa Major

Ursa Minor

Polaris

Cassiopeia

Pleiades

head of Leo

Orion

Belt

Leonid meteor shower observed at the Modra Astronomical Observatory in Slovakia

through a ﬁsheye lens. The exposure on November 16–17, 1998, lasted 4 h. About 150

bolides brighter than magnitude − 2 can be seen; the brightest ﬁreball is about magnitude

− 8. Zenithal hourly rate of the shower estimated from visual observations was about 400.

Positions of some well-known constellations and features are indicated. Meteors appear

to radiate from the head of Leo. ( Modra Astronomical Observatory )

Meteor

TABLE 1. Major meteor showers

Approx. radiant

coordinates, degrees ∗

Meteoroid

Approx.

orbital speed

date of

Right

ascension Declination mi/s

Suggested

Shower

Duration

maximum

km/s

Strength †

parent body

Notes

Quadrantids

Jan. 1–6

Jan. 3

230

extinct (?) nucleus

of comet

C1490 Y1 (?)

Sharp maximum

Lyrids

Apr. 20–23

Apr. 22

271

M-W

1861 I

Good in 1982

Puppids

Apr. 16–25

Apr. 23

110

− 45

—

W-M

Grigg-Skjellerup

Highly variable

η Aquarids

Apr. 21–May 12

May 4

336

− 2

Halley

Second peak May 6

Arietids

May 29–June 19

June 7

—

Daytime shower

Perseids

June 1–17

June 7

+ 23

—

Daytime shower

Taurids

June 24–July 6

June 29

+ 19

Encke

Daytime shower

Aquarids

July 21–Aug. 25

July 30

333

−

—

Primary radiant

S. o Aquarids

July 15–Aug. 25

Aug. 6

333

− 15

—

Primary radiant

Perseids

July 23–Aug. 23

Aug. 12

1862 III

Best-known shower

Orionids

Oct. 2–Nov. 7

Oct. 21

+ 16

Halley

Trains common

S. Taurids

Sept. 15–Nov. 26 Nov. 3

+ 14

Encke

Known fireball producer

Leonids

Nov. 14–20

Nov. 17

152

W-S

1866 I

Many peaks seen 1996

to 2003

Puppids-Velids Nov. 27–Jan.

Dec. 9

135

−

—

2102 Tantalus?

Many radiants in region

Geminids

Dec. 4–16

Dec. 13

112

+ 32

3200 Phaethon

Many bright meteors

∗

When the stream velocity is unknown or poorly known, the radiant coordinates are those of the apparent radiant rather than the geocentric radiant.

† Estimate of relative meteor hourly rate for visual observers: S

strong (sometimes above 30 per hour at peak); M

moderate (10 to 30 per hour at peak); W

weak (5 to 10 per hour at

peak).

attraction have been removed. Radiant positions that

have not been corrected for the Earth’s attraction

(usually because the meteoroid velocities are un-

known) are called apparent radiants.

Stationary meteors. Occasionally, a meteor is seen

coming directly toward an observer and shows a

bright point of light rather than a trail. Such me-

teors are called, somewhat inaccurately, stationary

meteors. The positions of these head-on meteors de-

ﬁne the radiant precisely and are quite important

to observe, particularly during showers. However,

short glints of light from Earth-orbiting spacecraft

and space debris are sometimes mistaken for sta-

tionary meteors. It has been suggested that a certain

TABLE 2. Minor meteor showers ∗

Approx. radiant

coordinates, degrees †

Meteoroid

Approx.

orbital speed

date of

Right

ascension Declination

Suggested

Shower

Duration

maximum

mi/s

km/s

parent body

Notes

Coma Berenicids

Dec. 12–Jan. 23

Jan. 17

186

+ 20

1913 I

Uncertain radiant position

α Centaurids

Jan. 28–Feb. 23

Feb. 8

209

− 59

—

Colors in bright meteors

Leonids

Feb. 5–Mar. 19

Feb. 26

159

—

Slow, bright meteors

Virginids

Feb. 3–Apr. 15

Mar. 13?

186

+ 00

—

Other radiants in region

δ Normids

Feb. 25–Mar. 22

Mar. 14

245

− 49

—

Sharp maximum

Pavonids

Mar. 11–Apr. 16

Apr. 6

305

−

—

Grigg-Mellish

Rich in bright meteors

σ Leonids

Mar. 21–May 13

Apr. 17

195

− 05

—

Slow, bright meteors

α Scorpids

Apr. 11–May 12

May 3

240

− 22

—

Other radiants in region

Herculids

May 19–June 14

June 3

228

—

Very slow meteors

Ophiuchids

May 19–July

June 10

270

− 23

—

One of many in region

Corvids

June 25–30

June 26

192

−

—

Very low speed

June Draconids

June 5–July 19?

June 28

219

+ 49

Pons-Winnecke

Maximum only 1916

Capricornids

July–Aug.

July 8

311

− 15

—

May be multiple

Piscis-Australids

July 15–Aug. 20

July 31

340

−

—

Poorly known

α Capricornids

July 15–Aug. 25

Aug. 2

307

− 10

1948 XII (1948n)

Bright meteors

N. α Aquarids

July 14–Aug. 25

Aug. 12

327

− 06

—

Secondary radiant

Cygnids

Aug. 9–Oct. 6

Aug. 18

286

—

Bursts of activity

N. Aquarids

July 15–Sept. 20

Aug. 20

327

− 06

—

Secondary radiant

S. Piscids

Aug. 31–Nov. 2

Sept. 20

—

Primary radiant

Andromedids

Sept. 25–Nov. 12 Oct. 3

+ 34

—

“Annual” version

October Draconids Oct. 10

Oct. 10

262

+ 54

Giacobini-Zinner Can be spectacular

N. Piscids

Sept. 25–Oct. 19

Oct. 12

—

Secondary radiant

Leo Minorids

Oct. 22 –24

Oct. 24

162

+ 37

1739

Probable comet association

Pegasids

Oct. 29–Nov. 12

Nov. 12

335

+ 21

1819 IV

Probable comet association

Andromedids

Nov. 25

Biela

Once only, 1885

Phoenicids

Dec. 5

− 50

—

Once only, 1956

Ursids

Dec. 17–24

Dec. 22

217

+ 76

—

Good in 1986

Peak strength for visual observers usually less than 5 per hour. Only showers of special interest are listed here.

† When the stream velocity is unknown or poorly known, the radiant coordinates are those of the apparent radiant rather than the geocentric radiant.

2003 EH 1

∗

Meteor

number of visible light pulses from cosmic x-ray and

gamma-ray sources are also mistaken for stationary

meteors. Such cosmic-ray events are, however, far

too brief to be seen with conventional meteor equip-

ment or by the unaided eye. See GAMMA-RAY ASTRON-

OMY ; X-RAY ASTRONOMY .

Bright meteors. Extremely bright meteors rivaling

even the full moon (often bolides with associated

sonic phenomena) are usually not associated with

the major showers. Instead, they have meteoroid or-

bits that are more characteristic of those minor plan-

ets and short-period comets that are in highly eccen-

tric orbits in the inner solar system. If the observed

luminous-trail end points of these events are less than

12 mi (20 km) high in the atmosphere, there is a good

chance that recognizable fragments (meteorites) will

fall to Earth and perhaps be recovered. Most ﬁre-

balls that drop meteorites occur in the afternoon or

early evening, when velocities are low. For example,

both a meteorite fall on October 9, 1992, which de-

posited a stone in Peekskill, New York, and one on

June 14, 1994, which scattered numerous fragments

over Quebec province, east of Montreal, appeared

in the early evening. Thus most types of meteorites

are believed to be samples of minor planets and pos-

sibly certain short-period comets. This connection

with minor planets is veriﬁed by the fact that the re-

ﬂecting properties of many minor planets resemble

reﬂections from known meteoritic materials.

It is usually very difﬁcult to detect even the bright-

est meteors in the atmospheres of other planets, al-

though there have been several instances of space-

craft detection. An exception occurred July 16–22,

1994, when over 20 fragments of Comet Shoemaker-

Levy 9 crashed into Jupiter in one of the most

spectacular astronomical events ever observed. Even

though the impacts happened only on the night side

of Jupiter, several of their ﬂashes were seen by re-

ﬂection from the Jovian satellites. Several spacecraft

were able, because of their offset position from the

Earth, to view a larger portion of the Jovian night

side than was visible from Earth and hence saw

the impacts directly. Numerous observatories with

large telescopes, including the Hubble Space Tele-

scope ,watched as hot plumes of material rotated into

view at Jupiter’s edge. Most impacts left mysterious

dark clouds in Jupiter’s atmosphere, several of which

were visible for months even in small telescopes. Sim-

ilar impacts on the Earth are thought to be respon-

sible for the extinction of the dinosaurs 65 million

years ago, and several other bioextinctions. See AS-

TEROID ; COMET ; JUPITER .

Origins of shower meteors. Anumber of meteor

showers have been observed to be in orbits that

are similar to those traveled by known comets. Thus

an association between shower meteors and comets

has gradually become a ﬁrmly entrenched concept

(Tables 1 and 2). There are numerous theoretical

scenarios where vaporization of the more volatile

cometary ices ejects small solid particles from the

surface of the nucleus. A fair proportion of these

fragments, particularly the smaller dust-sized ones,

escape and take up their own orbits as meteoroids.

Cometary nuclei have been known to split into two

or more pieces and, when this occurs, it is likely that

particles larger than dust size are released as well.

Gravitational attractions of the major planets and

the disturbing effect of solar radiation pressure on

individual particles tend to spread meteoroids out

from the parent object position. Thus “young” show-

ers are those that last only brieﬂy (some as short as

an hour or less), while “old” showers may show a

few meteors per night but last a month or more.

Many showers have a nonuniform structure along

their orbits with highest meteoroid densities near

the parent body. Lacking orbital synchronism with

the Earth’s position, these showers do not have an

annual appearance at a reliable level. Instead, they

show a tendency for strong showings to be sepa-

rated by intervals roughly equal to the meteoroid or-

bital periods. The concentration of particles in these

orbiting clumps can be relatively high, giving rise

to brief deluges called meteor storms, where equiv-

alent rates of thousands of meteors per hour have

been noted for times that are at most an hour or so

long. These numbers, however, give a false impres-

sion of the actual number density of meteoroids in

space. With relative velocities on the order of tens

of miles per second and a collecting area for each

observer of a few hundreds of miles in diameter, the

average separation between individual meteoroids is

still a few thousand miles. Away from these maxima,

however, the meteoroid number densities and hence

the observed meteor hourly rates are quite low.

There are some instances where the Earth crosses

the meteoroid stream twice per year, giving rise to

two separate meteor showers. For example, Comet

Halley gives rise to the May Aquarids and the

October Orionids, while Comet Encke gives rise to

the June Taurids and the November Taurids. See HAL-

LEY’S COMET .

While the parent comet idea nicely explains many

features of meteor showers, there are problems with

this simple picture. First, certain minor planets re-

semble what might be termed extinct comet nuclei.

Some of these have been observed at times with faint

atmospheres, a main characteristic of comets. These

identiﬁcations have been strengthened by the space-

craft observation of the properties of the nucleus of

Comet Halley. Second, a perfectly respectable minor

planet, 3200 Phaethon, which was discovered by the

Infrared Astronomical Satellite (IRAS) , has an orbit

nearly the same as the prominent Geminid annual

shower visible in December. A minor planet, 2003

EH 1 (designated for now as a minor planet, but

possibly the extinct nucleus of an ancient comet),

has been suggested as the parent of the Quadrantid

stream (Table 1). Evidence has also been found for

several other minor planet connections with a small

number of minor meteor showers. There is convinc-

ing evidence that a major source of micrometeoroids

with orbits that go out through the asteroid belt is a

relatively small number of asteroidal parent objects.

There is also accumulating evidence that much of

what is termed the sporadic meteor background is re-

ally a superposition of millions of ancient meteoroid

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