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
October 29, 1991.
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.
the impact from the last major fragment of comet Shoemaker-Levy
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.
clouds 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.
||Europa's icy surface
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
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.
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.
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.
For more information and high resolution images see:
NASA's Jet Propulsion Laboratory