The Astronomy Book

263

closest planet to Earth at that time
at 35 million miles (56 million km),
but Neptune was 2.5 billion miles
(4 billion km) away, and a journey
to it would take several years.

Planetary slingshots
A Grand Tour spacecraft would
have to change course several times
in order to fly past all its planets.
Flandro’s plan had to use gravity
assists to fling the craft from
planet to planet. Also known as a
gravitational slingshot or swing-by,
a gravity assist had first been used
by the Soviet Luna 3, which had
swung around the far side of the
moon in 1959, photographing as
it went. It had never been used to
guide spacecraft as far from Earth
as the outer planets. The planned
slingshot required the craft to
approach the planet head-on,
traveling in the opposite direction

from the planet’s orbital motion.

The planet’s gravity would speed

up the craft as it made a loop

around the planet. It would then

slow down again as it headed off

into space, having done an about

turn. If the motion of the planet

were ignored, the craft’s escape

speed would be more or less equal

to its approach speed. However,

taking the motion of the planet into

account, the craft would leave the

See also: Life on other planets 228–35 ■ The nebular hypothesis 250–51 ■ Exoplanets 288–95 ■
Understanding comets 306–11 ■ Studying Pluto 314–17

THE TRIUMPH OF TECHNOLOGY

It was a chance that came

around once in every 176 years

and we prepared for it. Out of

that emerged the greatest

mission of planetary

exploration to this day.

Charles Kohlhase

The best way to learn

about planets is to send

robotic spacecraft

to them.

A Grand Tour could send probes to

study them throughout this period.

All the outer planets

will be close together

for a short period.

The Voyager program makes a

Grand Tour of the giant planets.

An artist’s impression shows
Voyager 1 in space. This craft and
its twin, Voyager 2, communicate
with Earth via radio waves transmitted
and received by a 12-ft (3.7-m) dish.

planet having added approximately

twice the speed of the planet to

its own velocity. A slingshot would

not only redirect the craft, but also

accelerate it on to its next target.

Taking a Grand Tour

In 1968, NASA set up the Outer

Planets Working Group. It proposed

the Planetary Grand Tour mission,

which would send one spacecraft

to visit Jupiter, Saturn, and Pluto, ❯❯