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ROBOT EXPLORERS 71

Missions to Mars

The red planet Mars has had a special place in the human imagina-
tion as long as people have looked at the sky. About a century ago,
the astronomers Giovanni Schiaparelli and Percival Lowell thought
they saw the canals of an advanced civilization on the Martian sur-
face. Science fiction writers such as Arthur Clarke, Ray Bradbury,
and Kim Stanley Robinson have portrayed imaginative future histo-
ries of human exploration and the colonization of Mars.
As soon as people figured out how to launch spacecraft that could
overcome the Earth’s gravity, Mars (after the Moon) beckoned to be
explored. In the 1960s, many probes from the United States and the
Soviet Union failed to reach the red planet (or went silent), until in
1969, when the U.S. Mariner 6 and Mariner 7 spacecraft sent the
first stunning photos of the Martian landscape back to Earth.

Mariner 9

After earning her master’s degree in Aerospace Engineering at the
University of California in 1968, Shirley began a new job at JPL,
where she would spend most of her career. At first she worked on
a team that analyzed the complex trajectories, or paths, that space-
craft must take in order to navigate the solar system. This work is
difficult because, unlike cars or even airplanes, spacecraft cannot
take more or less direct paths in planning trips from, for example,
the Earth to Mars.
A trajectory not only has to be designed to bring spacecraft from
one moving planet to another, but it must also account for the gravi-
tational forces of the Sun, Moon, and other planets. Even the pres-
sure of the Sun’s light on the craft must be accounted for, because
even a small uncorrected acceleration over time can push the craft
off course and cause it to miss its target.
Because the amount of fuel is limited, some long-range space
missions use the gravity of one planet (such as Venus) to acceler-
ate the craft on its way to another planet (such as Mercury in the
case of Mariner 10). Since planets are not perfectly round, their
gravitational field is also irregular, and the exact angle and area