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22 Space

WHERE DO COMETS COME FROM?

Comets seem to be bits left over from the

formation of the Solar System. Some are

in a belt beyond Neptune. Millions more

form a giant spherical swarm, called the

Oort Cloud, nearly a light year from the

Sun. When a comet travels in from the

edge of the Solar System and is warmed

by the Sun, the gas and dust it gives off

make it look much brighter.

A comet is a small, icy lump that

travels in towards the Sun from the

outer reaches of the Solar System.

As it warms up, it develops a

shining head and two tails.

HOW OFTEN HAS HALLEY'S COMET BEEN SEEN?

English astronomer Edmond Halley was the first to

realize that the comet he saw in 1682 was a regular

visitor to Earth’s skies, returning every 76 years or so.

Historical records show that Halley’s Comet was

spotted as long ago as 240 BCE. It made its last

visit in 1986 and is due to return in 2061.

ARE WE SENDING ANY MESSAGES INTO SPACE?

In 1974, the Arecibo radio telescope beamed a radio message to the stars in digital

code, describing life on Earth. Pictures and sound recordings have also been carried

into space by the probes Pioneer 10 and 11 , and Voyager 1 and 2.

HOW CAN LIFE EXIST ON OTHER PLANETS?

Life could be widespread on distant planets because

life elements, such as carbon, hydrogen, and oxygen,

are common in space. A living planet also needs

energy (warmth and light) and water. In. SETI

projects, astronomers are searching for intelligent life.

SETI stands for the Search for Extraterrestrial Intelligence.

Astronomers tune into radio waves from outer space, looking for

coded signals that might come from other intelligent beings.

Extraterrestrial life means life

outside the Earth, or alien life. No

life has been found on other planets

in the Solar System, but it might

exist on planets around other stars.

FIND OUT MORE. Interplanetary Spacecraft 29 • Dwarf Planets 21 • Solar System 14

EXTRATERRESTRIAL LIFE

Extraterrestrial Life

FIND OUT MORE. Astronomy 11 • Life on Earth 70 • Observatories 12 • Space Travel 30

SETI

4 ARECIBO DISH

The giant radio telescope at

Arecibo in Puerto Rico scans

distant star systems for signs of

intelligent life. The main reflector

dish is 305 m (1,000 ft) across

and is set in a natural bowl in

the mountains.

MARTIAN LIFE? 3

Microscopic tubes

found inside this

Martian meteorite

could be fossils of

bacteria-like life.

The meteorite hit

Earth 13,000 years ago.

SIGNS OF INTELLIGENCE 3

The radio signals caused by stars,

planets, and other bodies look

like fuzzy noise on a computer

screen. If astronomers spot a

pattern in the signals, it could be

proof of extraterrestrial life.

1 HEAD AND TAILS

Inside the gas cloud that makes

up a comet’s coma is a tiny, solid

nucleus of snow and dust, a few

kilometres across.

1 ... AND IN 1986

From Earth, Halley’s Comet was

faint but visible with binoculars.

Space probe Giotto flew close to

the comet to take measurements.

1 HALLEY’S COMET IN 1066 ...

The return of Halley’s Comet in

1066 is recorded on the Bayeux

Tapestry, which shows the

Norman conquest of Britain.

A test signal shows up as

a thin, solid line. If aliens

contact us, a line like this

might be our first evidence

Gas and dust streaming

from the comet form

two separate tails that

can stretch for hundreds

of millions of kilometres

Nucleus and

coma make up

the comet’s head

Coma thins out

behind head

COMETS

Comets

ET life

comets

ET life