Maps of Tethys on Saturn,
taken by the Voyager spacecraft.
Planetary astronomers generally categorize the Solar System as consisting
of outer planets and inner planets. While many spacecraft have been sent
to the inner planets-Mercury, Venus, and particularly Mars, far fewer have
been sent to the outer planets-the gas giants Jupiter, Saturn, Uranus, and
Neptune, and the small distant rock known as Pluto. In fact, only American
spacecraft, with some European participation, have visited the outer
planets. The reason is that these planets are so distant that they require
spacecraft that can operate many years while travelling to reach them.
in the late 1960s, scientists from the National Aeronautics and Space
Administration (NASA) and engineers at California Institute of
Technology's Jet Propulsion Laboratory (JPL), which operates under
contract to NASA, began planning to send the first space probes past Mars
to Jupiter and beyond. In 1971 they launched the first of these space
probes, a craft named Pioneer 10, which became the first spacecraft to
reach the outer planets, flying past Jupiter in December 1973. During its
pass by Jupiter, it achieved enough velocity to become the first
spacecraft to leave the solar system. Its twin, Pioneer 11, flew past
Jupiter a year later, in December 1974. Both spacecraft returned colour
photographs of the complex cloud structures of the Jovian atmosphere,
measurements of the magnetic fields around the planets, and a few blurry
images of several of Jupiter's larger moons. But in order to reach Jupiter
at all, the two little space probes had to be so small and had to travel
so fast that they could not enter orbit around the planet and sped on past
it, making it extremely difficult for astronomers to obtain much
information from the data.
11 left Jupiter behind to encounter Saturn five years later in September
1979. It provided new information on the ringed giant's magnetic and
charged-particle environment. But the spacecraft's images of Saturn were
less impressive than those of Jupiter, and it did not return images of
complex cloud patterns like those found on Jupiter.
Image of Saturn collected by
Pioneer 11 in September 1979.
planetary astronomers got another chance beginning in January 1979, when a
more capable spacecraft named Voyager 1 approached Jupiter. Only 17 hours
before its closest approach, the spacecraft turned its cameras to the side
for an 11-minute-long exposure. When the image came back, it startled
virtually everyone on the project team by revealing that Jupiter had a
thin ring, something that they thought impossible due to the planet's
1 also took pictures of a number of Jupiter's moons. One of the moons it
photographed was Europa. The blurry images of Europa revealed that it was
crisscrossed with lines that the scientists could not explain. An even
bigger surprise came as the spacecraft approached a second moon, Io,
covered with splotchy patches of black, white, orange, and yellow sulphur.
On March 8, as one of Voyager 1's navigators, Linda Morabito, was looking
at an image of the edge of Io to evaluate the spacecraft's trajectory, she
discovered a small volcanic plume rising from its surface. Soon other
scientists found other volcanic eruptions, and Io proved to be the most
geologically active body in the Solar System.
2 reached Jupiter four months after its sister craft. It revealed
significant changes in the weather patterns on the gas giant. It took
better pictures of the volcanoes on Io and also higher quality images of
Europa that showed its very smooth, intricately cracked ice surface. There
were no well-defined impact craters like on most other moons, and the
surface features implied that something strange was going on under
1 and 2 then sped on toward Saturn. Voyager 1 reached Saturn in the fall
of 1980. One of its more fascinating discoveries was that the winds at
Saturn's equator reached speeds of 1,800 kilometres per hour, four times
faster than on Jupiter. On October 6, a Voyager team member noted dark,
spoke-like features extending radially across one of the planet's rings,
which they could not explain. Voyager 1 also photographed Saturn's moons,
including Titan, which has a thick atmosphere of its own, and Mimas, which
has a giant impact crater that makes it look like the Death Star from the
movie Star Wars. Voyager 2 encountered Saturn 10 months later and also
returned vivid photographs. Both spacecraft discovered new moons around
Jupiter and Saturn that are invisible from Earth.
A heliocentric (sun-centered) view
of the Voyager trajectories.
their beautiful pictures were what made them famous, Voyagers 1 and 2 were
also equipped with other scientific instruments for detecting magnetic
fields, charged particles, and other invisible phenomena. While Voyager
1's pictures of Saturn's rings were extremely useful, scientists wanted to
get a better idea of how thick the rings were, so they sent the spacecraft
zooming up, past the rings. This allowed them to measure the spacecraft's
transmissions as they travelled through the rings and thereby determine the
rings' thickness. But this manoeuvre also meant that Voyager 1 left the
ecliptic, or the plane that the planets travel in around the sun, and it
could not continue on toward any other planets. Voyager 1's cameras were
shut off in December 1980 as the spacecraft sped “up” out of the Solar
System (i.e., “north” according to the Earth's orbit around the sun).
2 continued even farther out in the Solar System. In November 1985 it
started providing regular data on Uranus, a bluish ball that previously
had appeared as only a tiny dot in the most powerful ground-based
telescopes. It returned information about the planet's wispy rings and its
moons, each radically different from the other. But the featureless
Uranian atmosphere disappointed most of the scientists. In late August
1989, Voyager 2 swept through the Neptune system, providing a spectacular
end to the spacecraft's career. Neptune was a deeper blue than Uranus and
also had complex but subtle features in its atmosphere. Surprisingly, it
also had a feature that scientists named the Great Dark Spot that looked
similar to the Great Red Spot on Jupiter. (Both are massive hurricane-like
1 and 2 rank as the most spectacularly successful robotic spacecraft ever
built. They returned a tremendous amount of data about planets that were
previously nothing more than smudges on photographic film. But they both
sped past their targets. NASA scientists wanted more and continuous data
that they knew could be achieved only by orbiting a spacecraft around the
in the 1970s, JPL scientists began designing a new spacecraft that would
actually orbit Jupiter. NASA named the probe Galileo, and it was scheduled
for a 1986 launch aboard the Space Shuttle until the Challenger accident
grounded it for several years. When it was finally launched, in October
1989, its delicate high-power communications antenna failed to properly
unfurl, and the spacecraft could send data back to Earth at a much lower
rate. This forced JPL scientists to completely revise the mission plan and
dramatically scale back their experiments.
December 1995 Galileo entered orbit around Jupiter. Five months before it
arrived, a small atmospheric probe separated from Galileo and entered the
Jovian atmosphere. After using a heat shield to brake, it popped open a
parachute and descended through the clouds of Jupiter's upper atmosphere
for 58 minutes until the pressure became so great that it squashed the
Double ridges, dark spots, and
smooth icy plains on Europa, one of Jupiterďs moons.
Photo taken by the Galileo spacecraft, November 1998.
Meanwhile, after Galileo entered orbit, the spacecraft began the first
phase of its revised exploration plan, which focused upon Jupiter itself.
The primary phase ended in December 1997 and a two-year Galileo Europa
Mission (GEM) started as the spacecraft turned its attention to Jupiter's
largest and most interesting moons, particularly Io and Europa. This
mission was extended again into the 21st century. Successive flybys of Io
and Europa revealed that Io's surface is constantly being remade by
volcanic forces, and provided strong evidence that underneath Europa's
cracked icy surface is a vast saltwater ocean, an important prerequisite
for life. This conclusion has placed Europa near the top of the list of
places that planetary scientists wish to study in much greater detail. In
addition, because of the possibility of life on Europa, NASA scientists
decided to crash Galileo into Jupiter to prevent it from possibly crashing
on Europa and contaminating it with bacteria.
NASA launched the Cassini spacecraft to Saturn atop a powerful Titan IV
rocket. Cassini is a big spacecraft by current standards and was designed
based upon some of the mistakes learned from Galileo. For instance, it has
a large, rigid communications antenna rather than the extendable type that
failed on Galileo. Cassini also includes a small European Space Agency
probe named Huygens for descending into the moon Titan's atmosphere.
Cassini is scheduled to reach Saturn in July 2004 and will orbit it for
years, studying Saturn, its amazingly complex rings, and its moons.
late 1990s, NASA officials proposed two other spacecraft for visiting the
outer planets. These were the Europa Orbiter and Pluto-Kuiper Express.
Both missions were cancelled in 2002, but will probably re-emerge in the
future in highly modified form.