Spotlight: Surprising Jupiter Busy Galileo spacecraft showed jovian system is full of surprises
September 17, 2003
After orbiting Jupiter
34 times and surviving four times the amount of radiation it was design
to withstand, the resilient Galileo spacecraft is finally at the very
end of its 14-year mission. To avoid even the most remote possibility
of colliding with a pristine moon in the jovian system, the out-of-fuel
spacecraft will dive into Jupiter on Sunday, Sept. 21, 2003.
Since its launch in
1989, the sturdy spacecraft traveled more than 4.6 billion kilometers
(almost 2.8 billion miles), about the equivalent of seven times the distance
between Earth and Jupiter. Despite communication problems and a temperamental
tape recorder, Galileo returned 30 gigabytes of data, including 14,000
pictures.
This wealth of information
drastically expanded our understanding of the solar system's biggest planet
and its moons. The mission was possible because it drew its power from
two long-lasting radioisotope thermoelectric generators provided by the
Department of Energy.
Asteroids
Unveiled
The exciting
list of discoveries started even before Galileo was able to get a close
glimpse of Jupiter. As it crossed the asteroid belt in October 1991, Galileo
snapped images of Gaspra, returning the first ever close-up image of an
asteroid. Less then a year later, the spacecraft got up close and personal
with yet another asteroid, Ida. Images from Ida revealed the asteroid has
its own little "moon," Dactyl, the first known moon of an asteroid.
Location,
Location
In 1994 the
spacecraft was in the right place at the right time and made the only
direct observation of a comet impacting a planet. It took images of fragments
of comet Shoemaker-Levy 9 crashing into Jupiter. Images of the impact,
which was not visible from Earth, helped scientists better understand
this type of event.
At Jupiter
Galileo began its tour of the jovian system in December 1995. Carefully
designed orbits allowed the spacecraft to observe Jupiter's atmosphere,
revealing numerous large thunderstorms many times larger than those on
Earth, with lightning strikes up to 1,000 times more powerful than terrestrial
lightning. Data collected by the descent probe made the first in-place
studies of the planet's clouds and winds, and it furthered scientists'
understanding of how Jupiter evolved. The probe also made measurements
designed to assess the degree of evolution of Jupiter compared to the
Sun.
As the first spacecraft
in long-term residence in jovian orbit, Galileo also successfully studied
the global structure and dynamics of Jupiter's magnetic field. Galileo
also determined that Jupiter's ring system is formed by dust kicked up
as interplanetary meteoroids smash into the planet's four small inner
moons. Data also showed that Jupiter's outermost ring is actually made
up of two rings, one embedded within another.
Moons' Wonders
Galileo extensively investigated the geologic diversity of Jupiter's four
largest moons: Ganymede, Callisto, Io and Europa. Stunning images revealed
the contrasting and changing surfaces of these moons.
Io has extensive volcanic
activity, which is continually modifying the surface. The heat and the
frequency of eruption can be 100 times more than that of Earth, something
reminiscent of Earth's early days. The similarities make Io an ideal laboratory
for the study of what Earth was like more than 3 billion years ago.
The moon Europa, Galileo
unveiled, could be hiding a salty ocean up to 100 kilometers (62 miles)
deep underneath its frozen surface. Images also reveal ice "rafts"
the size of cities that have broken and drifted apart to create a scalloped
and broken surface. There are also indications of volcanic ice flows,
with liquid water flowing across the surface. These discoveries are particularly
intriguing since liquid water is a key ingredient in the process that
may lead to the formation of life.
The biggest discovery
surrounding Ganymede was the presence of a magnetic field, the first moon
of any planet known to have one. Images of this moon featured a faulted
and fractured surface that demonstrated high tectonic activity. Like Europa
and Io, Ganymede has a metallic core. Galileo magnetic data also provided
evidence that Ganymede might have a liquid-saltwater layer as well.
Galileo determined
that, while Callisto doesn't have a metallic core, its surface shows evidence
of extensive erosion. Data collected raise the question of whether Callisto's
surface may also hide an ocean.
Last Dance
Galileo's own discovery of a likely ocean hidden under Europa's surface
raises the possibility of life there and concern about protecting it.
For that reason, in its final victory lap the Galileo spacecraft will
dive into the atmosphere of the gaseous planet and disintegrate. Predictably,
some of the spacecraft findings raised intriguing questions that will
have to be answered by future mission. But Galileo Galilei, the first
modern astronomer, would be immensely proud of the discoveries made by
the spacecraft that carries his name.
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