Eighteen years ago, the flagship-class planetary explorer Galileo was released in Earth orbit by

Space Shuttle Atlantis

to begin its six-year journey to Jupiter and its moons.

Despite delays and a primary equipment failure, Galileo

provided more information about the jovian system

than NASA’s

previous Pioneer or Voyager missions, which flew past but did not orbit the gas giant.

The spacecraft was

named after Galileo Galelei, the 17th century Italian astronomer

who first observed Jupiter via telescope. Development of the mission began

in 1977 at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Not only was Galileo’s launch postponed when the space shuttle fleet was grounded by the

January 1986 Challenger explosion, new safety rules imposed in the wake of the accident forced NASA planners to rethink how they would get to Jupiter. Originally the probe was to use a Centaur rocket stage to boost it out of Earth orbit, but after Challenger the cryogenic upper stage was deemed unsafe to launch in the shuttle’s cargo bay. Instead, NASA had to use the less-powerful Initial Upper Stage, which used a more stable propellant.

As a result,

JPL had to map out a new itinerary that included a series of

planetary gravity-assists-one from Venus and two from Earth. On its convoluted route Galileo made observations of Venus, Earth, the Moon and even two asteroids – Gaspra and Ida – before reaching the jovian


It was during these flybys that a

mechanical error was

discovered that

put the entire mission in jeopardy: the high-gain antenna that was to transmit Galileo’s vast data harvest back to Earth refused to unfurl upon command.

After failing to fix the balky antenna,

JPL engineers compensated for the loss by sending data through the emergency low-gain antenna, which transmitted at a much slower rate. Galileo’s

on-board computer system and Earth-based receivers were reconfigured with new software and data compression capabilities to maximize the data return at the lower transmission rates.

While still on

its way to Jupiter, Galileo captured

a direct view of the Comet Shoemaker-Levy slamming


Jupiter-the first direct observation of a comet hitting a planet-in July 1994.

On July 13, 1995, as it neared its destination, Galileo released a probe


entered Jupiter’s atmosphere Dec. 7 to make the first in-situ measurements of a gas giant. During its descent, the probe

sent back 58 minutes of data

on Jupiter’s chemical composition, temperature and pressure.

That same day, the Galileo mothership entered into its highly elliptical orbit about Jupiter.

Together the spacecraft

provided a greater understanding of Jupiter’s ammonia clouds, radiation belt and its enormous magnetosphere. The planet was found to contain a lot less water vapor than previous probe scans

indicated. Jupiter also was

found to have lightning storms that were

far less frequent

but far more powerful

than those on


Though Galileo’s primary mission was completed at the end of 1997, NASA extended its mission for another six years.

This extended mission focused on Jupiter’s 63 moons

– 23 of which the Galileo spacecraft discovered – mainly Io, Callisto, Ganymede and Europa.

The surface of Io was found to have changed substantially since it was last observed

by the Voyagers

due to

volcanic activity. In fact, based on Galileo’s observations, scientists declared Io to be

the most volcanically dynamic body in the solar system


Ganymede, larger than Mercury, was found to have its own magnetosphere – the only moon known to possess one.

Ganymede, along with

Callisto, also was


to possibly contain liquid seawater.

But neither Ganymede nor Callisto had the amount of water believed to lie

1,000 kilometers beneath Europa’s ice sheath. Europa

is thought to have more liquid seawater than Earth, and scientists believe the conditions necessary to support life may exist on the moon.

Galileo’s epic mission ended Sept. 21, 2003. With the spacecraft having

nearly depleted its storage of propellant and unable to reposition itself to send data back to Earth, JPL scientists sent it to burn up

in Jupiter’s atmosphere

rather than taking the chance of it

crashing onto a moon

like Europa

and possibly contaminating

it with Earth