Astronomy - USA (2020-08)

46 ASTRONOMY • AUGUST 2020

You must be less than 120 miles (200 kilometers)

from a meteor to see it.

Meteors become visible at an average height of

55 miles (90 km). Shower meteors burn up before

they reach an altitude of 50 miles (80 km).

No shower meteor has survived its flight through
the atmosphere and been recovered as a meteorite.

They’re just too small.

The typical bright meteor is produced by a particle
no larger than a pea with a mass less than 1 gram.

The meteors you’ll see during a shower are even

smaller. Their size is roughly that of a grain of sand.

Astronomers estimate that the average total mass

of meteoritic material entering Earth’s atmosphere

is between 100 and 1,000 tons (91,000 and

910,000 kilograms) per day.

The typical rate for meteors bright enough to

see with unaided eyes on a “non-shower” night is

approximately six per hour. Astronomers call these

random streaks sporadic meteors.

A meteoroid from the Perseid shower enters the

atmosphere at an average speed of 133,000 mph

(214,000 km/h).

FAST FACTS ABOUT METEORS

it’s large enough to survive

the fiery encounter and land

on Earth, it then becomes a

meteorite.

Almost all meteor showers

originate with comets. As a

comet orbits the Sun, the heat

from our star sublimates (turns

solids directly into gases) the

comet’s ice, which also releases

any trapped dust particles.

This action leaves a debris

trail. If our planet crosses that

trail, which it would do around

the same date each year, the

particles will enter our atmo-

sphere and a meteor shower

will occur. One notable excep-

tion to the comet rule is the

Geminids, a meteor shower

active in December. Its par-

ticles came from the asteroid

3200 Phaethon.

Perseid meteors are par-

ticles from Comet 109P/Swift-

Tuttle. It was discovered in

July 1862 by American

astronomers Lewis Swift of

Marathon, New York, and

Horace Tuttle, at Harvard

College Observatory in

Cambridge, Massachusetts.

When they found it, it glowed

at magnitude 7.5, but, by two

months later, it had bright-

ened to magnitude 2, the

same brightness as Polaris

(Alpha [α] Ursae Minoris). It

orbits the Sun every 133 years

and 102.3 days. It reappeared

in 1992, reaching perihelion

(closest approach to the Sun)

on December 11.

Comets are named for their

discoverers. Asteroids are

named by their discoverers.

A meteor shower, however, is

named for the constellation

that contains its radiant,

the point on the sky from

which all the meteors seem

to originate. Another way to

think about this is that if you

traced the meteor trails from

a single shower backward,

they would all intersect at one

point, the radiant. The posi-

tion of the radiant is impor-

tant to meteor observers.

The higher the radiant is in

the sky, the more meteors

you’ll see.

One meteor shower that

seems to contradict the

named-for-the-constellation

rule is the Quadrantids,

which are active in January.

In actuality, this name does

follow the rule, although the

constellation for which it was

named — Quadrans Muralis

— is no longer a recognized

group. When the constella-

tions were formalized in 1928,

Quadrans Muralis ceased to

exist and became part of the

current constellation Boötes.

Currently, the International

Astronomical Union’s Meteor

Data Center lists 821 meteor

This Perseid was bright enough to be seen through moderately thick monsoon clouds. It burned up above the Himalayas
near Shishapangma, Tibet, on August 12, 2015. The Teapot asterism of Sagittarius is visible at center. JEFF DAI

ABOVE: This meteor, which exhibited

a small flare-up, appeared over

Hongcun, a UNESCO World Heritage

Site in Anhui Province, China, on

August 12, 2018. JEFF DAI