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and a second one toward Uranus
and Neptune. The plan required
a new long-range spacecraft and
costs grew steadily. Then in 1971,
NASA canceled the Grand Tour as
it needed cash to fund the Space
Shuttle program.
The exploration of the outer
planets was handed back to the
Mariner program. The mission was
named Mariner Jupiter–Saturn, or
MJS77 for short—77 referred to the
launch year. To reduce costs, Pluto
was removed from the tour
itinerary. Instead, one craft was
to visit Jupiter, Saturn, and finally
Saturn’s huge moon Titan. Titan
was considered more intriguing
than distant Pluto. It was larger
than Mercury, and thought at the
time to be the largest moon in the
solar system. It was also the only
moon known to have its own
atmosphere. This change meant
that the mission would be
budgeted as an exploration of the
two gas giants, not a Grand Tour.
However, the spacecraft, code-
named JST, was to have a back-up,
JSX. Its mission would also include
Jupiter and Saturn if JST failed.
The X represented an unknown
quantity. If required, JSX would
go to Titan, but if JST achieved
its mission, then JSX would be
sent to Uranus and Neptune.Mission profile
In 1974, mission design manager
Charles Kohlhase began to make
a master plan for the MJS77
mission. He had to consider every
aspect, from the spacecrafts’
design, size, and launch system
to the many variables they would
encounter along their routes—the
radiation levels, light conditions,
and contingencies for altering the
missions. It took Kohlhase and his
team eight months to eventually
settle on two trajectories that metEXPLORING THE SOLAR SYSTEM
all the criteria and would take the
spacecraft as close to as many
points of interest as possible.
Neither Kohlhase nor anyone
else working on MSJ77 liked
the name. As the launch date
approached, a competition for a
new name was organized. Nomad
and Pilgrim made the shortlist,
but by the time the two identical
spacecraft were ready, they were
known as Voyager 1 and Voyager 2.
At 1,590 lb (720 kg), the two
Voyager spacecrafts were nearly
50 percent heavier than any
previous flyby craft. About 220 lb
(100 kg) of that was scientific
equipment, comprising two
cameras, magnetic field sensors,
spectrometers that would analyze
light and other radiation to show
which chemicals were present in
atmospheres, and particle detectors
for investigating cosmic rays.
In addition, the radio system
could be used for a variety of
experiments, such as probing
atmospheres and Saturn’s rings.
The spacecrafts’ trajectories wereVoyager 1 captured a 100-mile-
(150-km-) high eruption on Jupiter’s
moon Io. Strongly affected by Jupiter’s
gravity, Io is the most volcanically
active place in the solar system.
Jupiter’s moon Europa was
photographed by Voyager 2. It is
covered in a thick crust of ice, which
has been fractured and filled in with
material from the moon’s interior.