Encyclopedia of the Solar System 2nd ed

624 Encyclopedia of the Solar System

TABLE 1 Major Meteor Showers, Date of Shower Maximum, Radiant in Celestial Coordinates, Geocentric Speed

(km/s), Maximum Hourly Rate, Parent Objectsa

Radiantb

Name Date RA DEC Speed Rate Parent Objectc

Quadrantids Jan. 3 230 + 49 42 140

April Lyrids Apr. 22 271 + 34 48 10 Comet1861 I Thatcher

Eta Aquarids May 3 336 − 2 66 30 P/Halley

June Lyrids June 16 278 + 35 31 10

S. Delta Aquarids July 29 333 − 17 41 30

Alpha Capricornids July 30 307 − 10 23 30 P/Honda-Mrkos-Pajdusakova

S. Iota Aquarids Aug. 5 333 − 15 34 15

N. Delta Aquarids Aug. 12 339 − 542 20

Perseids Aug. 12 46 + 57 59 400 (1993) P/Swift-Tuttle

N. Iota Aquarids Aug. 20 327 − 631 15

Aurigids Sept. 1 84 + 42 66 30 Comet1911 II Kiess

Giacobinids Oct. 9 262 + 54 20 10 P/Giacobini-Zinner

Orionids Oct. 21 95 + 16 66 30 P/Halley

Taurids Nov. 3 51 + 14 27 10 P/Encke

Taurids Nov. 13 58 + 22 29 10 P/Encke

Leonids Nov. 17 152 + 22 71 3,000 (1966) P/Tempel-Tuttle

Geminids Dec. 14 112 + 33 34 70 Phaeton

Ursids Dec. 22 217 + 76 33 20 P/Tuttle

aAfter A. F. Cook (1973), In “Evolutionary and Physical Properties of Meteoroids” (C. L. Hemenway, P. M. Millman, and A. F. Cook, eds.), NASA SP-319,

183–191.

bRA, right ascension, and DEC, declination, in degrees.

cIf known, short-period comets are indicated by P/.

converted to heat, is sufficient to totally vaporize the me-
teoroid. During the deceleration of the meteoroid in the
atmosphere at about 100 km altitude, the meteoroid will
heat up and atoms from its outer surface will be ablated
until it is completely evaporated. A luminous train several
kilometers in length follows the meteoroid. It is this ionized
and luminous atmospheric gas and material from the me-
teoroid that is visible and that scatters radar signals. From
triangulation of the meteor train by ground stations (several
cameras or a radar station), the preatmospheric meteoroid
orbit is obtained with high accuracy.
During the atmospheric entry of objects larger than sev-
eral tens of kilograms or about 10 cm in diameter, a surface
layer of several centimeters in thickness will burn away,
and the object will be decelerated. That which reaches
Earth’s surface is called ameteorite. Meteorites of 1 kg
to several tons are sufficiently decelerated and fall on Earth
with the interior little altered by atmospheric entry. These
meteorites are the source of our earliest knowledge about
extraterrestrial material. [SeeMeteorites;Near-Earth
Objects;PlanetaryImpacts.]
Much of the ablated material from a meteor will con-
dense again into small droplets, which will cool down and
form cosmic spherules that subsequently rain down to
Earth. These cosmic spherules can be found and identi-

fied in abundance in deep-sea sediments and on the large

ice masses of Greenland, the Arctic, and Antarctica. An av-

erage of 40 tons of extraterrestrial material per day in the

form of fine dust falls onto the surface of Earth.

At certain times, meteor showers can be observed at a

rate that is a hundred (and more) times higher than the

average sporadic meteor rate (Table 1). Figure 4 shows

several meteors in a photograph of the night sky taken on

17 November 1966. The visible rate was about one meteor

per second. Because all of these meteoroids travel on par-

allel trajectories, to an observer they seem to arrive from a

common point in the sky (the radiant), which in this case

lies in the constellation Leo. Therefore, this meteor shower

is called the Leonid shower.

The explanation for the yearly occurrence of meteor

showers is that all meteoroids in one stream closely fol-

low a common elliptic orbit around the Sun but are spread

out all along the orbit. Each year when the Earth crosses

this orbit on the same day, some meteoroids of the stream

hit the atmosphere and cause the shower.

Many meteor streams have orbits similar to those of

known comets (cf. Table 1). It is a generally accepted view

that meteor streams are derived from comets. Millimeter-

to centimeter-sized particles that are emitted from comets

at low speeds (m/s) are not visible in the normal comet tail