The Astronomy Book

267

on this otherwise relatively quiet
planet is the tilt of its axis, which
is roughly 90°. As a result, Uranus
does not spin as it orbits, but
“rolls” around the sun.
The final port of call was
Neptune, reached in August 1989.
This deep-blue planet was found
to have the strongest winds in
the solar system, up to 1,500 mph
(2,400 km/h)—nine times stronger
than anything experienced on
Earth. The Voyagers’ mission
controllers were able to abandon
caution as the planetary mission
drew to an end. Without regard
for the safety of its final trajectory,
Voyager 2 was redirected to fly
past Neptune’s moon Triton. The
images of the huge ice moon showed
geysers blasting fountains of slush
from the surface.

Continuing mission
The Voyager program continues
and the two craft are still in touch
with NASA. As of 2016, Voyager 1
was 12.5 billion miles (20 billion
km) and Voyager 2 was 10 billion
miles (16 billion km) away. Six

times a year, the craft spin around

to measure the cosmic rays around

them. This data shows that the

craft are approaching the edge of

the heliosphere, the region of space

that is influenced by the sun. Soon

they will enter interstellar space

and measure the cosmic wind from

ancient stellar explosions.

In 2025, the two spacecraft will

power down and go quiet forever,

but their mission may still not yet

be complete. A committee chaired

by Carl Sagan selected content for

a gold-plated phonograph record

(its analog groove would be easier

to read than a digital format). They

included greetings from the world,

the sounds and sights of Earth,

and even human brain waves.

The record is a calling card from

humankind to an alien civilization.

The Voyagers are not heading for

any star systems; the closest they

will get is when Voyager 1 passes

1.6 light-years from a star in 40,000

years’ time. In all likelihood, they

THE TRIUMPH OF TECHNOLOGY

The golden records carried by the
Voyager spacecraft included a selection
of music, greetings in 55 different
languages, and images of humans,
animals, and plants.

will never be found by intelligent

life, but the golden records are a

symbol of the hope with which the

two interplanetary spacecraft were

sent on their way. ■

The spacecraft will be

encountered and the record

played only if there are

advanced spacefaring

civilizations in interstellar

space. But the launching

of this “bottle” into the

cosmic “ocean” says

something very hopeful

about life on this planet.

Carl Sagan

By 2005, the Voyagers had

reached the termination shock,

where the solar wind slows

and becomes turbulent as it

mixes with the interstellar

medium (matter in the

space between star

systems), entering

the heliosheath

region. By 2016,

they were nearing

the heliopause,

where the solar

wind is stopped

by the interstellar

medium.

Earth

Voyager 1

Heliosheath

Solar wind

He

lio

pa

use Te

rmi

nat

ion

s

ho

ck

Voyager 2