After nearly 20 years of planning, construction, delays, and a
dramatically altered flight plan, it's payback time for the nearly
50 researchers involved in the Galileo mission to Jupiter.
Shortly before 3 p.m. PST on Thursday, 746 pounds of human
ingenuity will slam into Jupiter's atmosphere. For 75 minutes, this
probe will relay information about the atmosphere to its mother
ship high above. Then, as it plunges ever deeper into the planet,
climbing temperatures and pressure will vaporize the probe until it
becomes part of the planet itself.
Once the probe finishes its work, the Galileo orbiter will begin
a two-year, 11-orbit study of Jupiter, the solar system's largest
planet, and its moons. It will get close-up looks at Ganymede,
Europa, and Callisto, and longer-range looks at Io and several
Galileo represents "the most complex spacecraft we've ever sent
to the outer planets," says Jay Bergstralh, Galileo program
scientist at the National Aeronautics and Space Administration's
headquarters in Washington, D.C. It also is the second to the last
of its generation of flagship planetary missions. The final
spacecraft in this grouping is the Cassini mission to Saturn,
scheduled for launch next October.
As the critical day approaches, the scientists involved are
mindful of the $1.4 billion mission's stakes. Given the technical
hitches that already have occurred on Galileo, and mindful of the
loss of the Mars Observer just as it was to enter orbit around the
Red Planet in August 1993, they remain wary. "The big thing that
one fears is failure," concedes Alvin Seiff, the principal
investigator on the probe's atmospheric structure instrument.
"We've had good success but we're not home yet. We stand to learn
so much that it sometimes seems worthwhile taking a risk."
The Galileo project's origin dates back to 1976, when a
committee led by James Van Allen proposed the probe-orbiter
combination. Scientists' curiosity had been piqued by the Pioneer
10 and 11 missions that returned the first close-up images of
Jupiter. Meanwhile, the Voyager project had begun, which would send
two spacecraft past Jupiter on their way to the ends of the solar
system. Observations from the two would raise even more questions
about the planet and its moons that only a long-term dedicated
mission could help answer. Once approved, work began on the project
in 1977. Galileo was scheduled for launch from the space shuttle in
The project's complexity soon became apparent. "There were well
over 100 major design changes," Dr. Bergstralh says, centered
around the question of "what we were going to use to launch."
The mission plan originally called for a direct flight to
Jupiter, which would have taken about 2-1/2 years. Designers
settled on a liquid-fuel upper stage that would ignite after the
craft was released from the shuttle's cargo bay. Launch was reset
for March 1986. But in January that year, the shuttle Challenger
exploded 70 seconds into its launch, killing the seven astronauts
on board. The fleet was grounded indefinitely, and the liquid-fuel
upper stage on Galileo was determined to be too dangerous to carry
aboard a shuttle. Galileo planners were forced to use a solid-fuel
upper stage that was too weak for a direct flight. They realized,
however, that what the upper stage couldn't supply, gravity could.
By boosting the spacecraft's energy with the help of a Venus flyby
and two Earth flybys, Jupiter was within reach - albeit in six
years and 2.3 billion miles.
During that extra time en route, Galileo reaped scientific
dividends. In a fitting parallel with its namesake, the Italian
astronomer who discovered moons orbiting Jupiter and so altered
humanity's notions about its place in the universe, the spacecraft
discovered a tiny asteroid, Dactyl, orbiting a larger asteroid,
Ida. That has been Galileo's "most spectacular discovery" to date,
Bergstralh says. …