The Astronomy Book

322

the Prop-M ever carried out its
mission, but it was programmed
to operate without input from
Earth. A radio signal between
the moon and Earth travels in less
than 2 seconds, but a signal to
or from Mars takes between 3
and 21 minutes to arrive, varying
with the planet’s distance from
Earth. For a Martian rover to be
a successful explorer, it needed
to work autonomously.

Bounce down
In 1976, NASA’s two Viking landers
sent back the first pictures of Mars.
Following this success, many more
rovers were planned, but most of
these projects never reached their
destination, succumbing to what the
press dubbed the “Martian Curse.”
NASA eventually had a success
with its 1997 Mars Pathfinder
mission. In July of that year, the
Pathfinder spacecraft entered the
Martian atmosphere. Slowed first
by the friction of a heat shield and
then by a large parachute, the
spacecraft jettisoned its outer
shielding, and the lander inside
was lowered on a 65-ft (20-m)
tether. As it neared the surface,

huge protective airbags inflated

around the lander, and retrorockets

on the spacecraft holding the tether

fired to slow the speed of descent.

The tether was then cut, and the

lander bounced across the Martian

surface until it rolled to a stop.

Fortunately, once the airbags had

deflated, the lander was the right

side up. The three upper sides or

“petals” of the tetrahedral lander

folded outward, revealing the

24-lb (11-kg) rover.

During development, the

rover was called MFEX, short

for Microrover Flight Experiment.

However, it was known to the

public as Sojourner, meaning

“traveler” and chosen for its link

to Sojourner Truth, a 19th-century

US abolitionist and rights activist.

Rolling on Mars

Sojourner was the first rover to

take a tour of the Martian surface.

However, the Pathfinder mission

was really a test for the innovative

landing system and the technology

that would power larger rovers in

the future. The minuscule vehicle

EXPLORING MARS

traveled just 300 ft (100 m) during

its 83-day mission, and never

ventured farther than 40 ft (12 m)

from the lander. Now named the

Carl Sagan Memorial Station, the

lander was used to relay data from

the rover back to Earth. Most of the

rover’s power came from small solar

panels on the top. One of the goals

of the mission was to see how

these panels stood up to extreme

temperatures and what power

could be generated in the faint

Martian sunlight.

The rover’s activities were

run from NASA’s Jet Propulsion

Laboratory (JPL) in California, and

JPL has remained the lead agency

in developing Martian rovers.

With the time delays inherent in

communicating with Mars, it is

not possible to drive a rover in real

time, so every leg of a journey must

be preprogrammed. To achieve

this, cameras on the lander were

used to create a virtual model

of the surface around Sojourner.

Human controllers could view the

area in 3-D from any angle before

mapping a route for the rover.

Spirit and Opportunity

Despite its limitations in terms of

size and power, Sojourner’s mission

was a great success, and NASA

We landed in a nice flat spot.

Beautiful, really beautiful.

Adam Steltzner

Lead landing engineer, Curiosity

During its 83 days of operation,

the tiny Sojourner rover explored

around 2,691 sq ft (250 sq m)

of the planet’s surface and

recorded 550 images.

Whatever the reason you’re on

Mars is, I’m glad you’re there.

And I wish I was with you.

Carl Sagan

in a message for future explorers