A resilient robot ship that has explored Jupiter and its moons for eight years will dive into the crushing atmosphere of the giant planet Sunday, a spectacular finale to one of the most productive deep-space missions ever.

Jupiter's moon, Io, the solar system's most volcanically active body, is shown in a July 1999 image taken by NASA's Galileo spacecraft. The unmanned spacecraft is on track to conclude its 14-year, US$1.5 billion exploration of Jupiter and its moons on Sunday, Sept. 21, 2003, with a streaking suicide plunge into Jupiter's atmosphere. [AP Photo/NASA]

Galileo, its propellant running low and its electrical systems on the blink, will nonetheless keep a handful of its 10 instruments on during the final hours, giving scientists a chance to squeeze some final data from the US$1.4 billion mission.

NASA charted the collision course to prevent Galileo, a heap of metal, plutonium and gadgets the size of a sport utility vehicle, from striking Jupiter's largest moons, considered some of the most promising sites to search for life beyond Earth.

"Galileo is one of the most successful outer worlds missions that the Earth has ever launched," said Colleen Hartman, NASA's director of solar system exploration. "This spacecraft has given us some unbelievable discoveries."

Since its launch in 1989, the droid has managed to do quite a bit with a computer brain comparable to that of an Apple II.

It snapped the first picture of an asteroid with a moon. And in 1994, a year before it began orbiting Jupiter, it witnessed the biggest explosion ever recorded on a planetary body, a comet that struck Jupiter with hundreds of times the explosive force of the world's nuclear arsenals.

Galileo dived within dozens of miles over many of Jupiter's satellites, flying through volcanic plumes of sulfur on Io and detecting promising signs of hidden oceans underneath the surface of other planet-size moons.

While seemingly obscure, such finds demonstrate how closely connected the planets really are. Without Jupiter's penchant for eating comets, life on Earth may not have survived.

Astronomers speculate that the largest planet in the solar system swallowed swarms of comets in the solar system's infancy, preventing them from colliding with catastrophic results with smaller planets in the inner solar system.

And while Io's tortured, boiling surface seems like an alien hellish world, the volcanism there could resemble conditions on our planet in its youth.

"The discoveries made on Io, talking about sulfur volcanism, were totally different [than what we expected]," said Michael Belton, a Galileo imaging team scientist from Tucson, Arizona. The surprisingly hot temperatures suggest "it could be like on Earth billions of years ago."

A remarkable journey

Just getting to Jupiter in working order was a major technical achievement. Years of launch postponements forced mission engineers to detour the interplanetary trip from a straight shot to a spiral trajectory, sending Galileo on a series of gravitational slingshot boosts by Venus and Earth again before it headed into deep space.

This is an artist's rendition, released by NASA, of the Galileo Probe as it approaches Jupiter. [AP Photo/NASA]

The delays proved costly. Tightly wrapped in storage much longer than planned during the interplanetary cruise, the main antenna failed to deploy. To beam back data, NASA had to rely instead on a much smaller secondary one and do some serious technical improvisation.

"The engineers had to redesign the software, reprogram the tape recorder and had to get Galileo to do data compression," said Claudia Alexander, the Galileo project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

"It was a complete stretch to get it to do that, which saved the mission."

The workaround allowed Galileo to perform most of its intended science but changed the focus enough to provide some serendipitous rewards.

"That loss forced us to concentrate on the satellites," Alexander said. "And from my perspective, the satellites are where the action is. That's where the big surprises came."

Besides Io, Galileo closely inspected Ganymeade, Callisto and Europa. The probe's camera beamed back thousands of pictures of the moons, each a study in eccentricity.

Io, the innermost of the four, located in Jupiter's powerful radiation belts, is a burning, yellowish sphere with dark and light volcanic splotches. Europa, the next one out, boasts a frozen surface dotted with city-size chunks of ice and crisscrossed by mysterious dark red bands.

Ganymeade, the largest, is bigger than Mercury. Callisto, the outermost, has the oldest surface of any known planet or moon.

Extraterrestrial oceans

Despite differences, the moons have some striking similarities, Galileo found. All most likely have thin atmospheres. Three are thought to hide vast stores of liquid or slushy oceans, stoking speculation that they could harbor some hardy form of primitive life.

One in particular displays the most convincing signs of a hidden ocean -- warm and constantly replenished with material from the icy surface.

"Europa is the star of the show," said Belton, the imaging team scientist. "By proving that there is indeed a liquid, briny ocean, [Galileo] transformed it from a mere moon to a prime candidate for extraterrestrial life."

To prevent the chance, however small, of any surviving terrestrial germs on Galileo from contaminating Europa or its sibling satellites, NASA decided to make sure the craft, which has little fuel left to control its flight, would not hit the moons.

As it makes its final approach, Galileo will first enter the turbulent, highly radioactive sea of charged particles surrounding Jupiter.

Having done so on numerous occasions before, the spacecraft has endured four times the radiation it was designed to withstand. The trips into the radiation belts have come at considerable cost.

Some instruments are damaged, its camera shuttered and its onboard memory slightly corrupted. During its most recent foray into the belts in November, Galileo weathered enough radiation in one day to kill a human 1,000 times over, according to NASA.

The instruments that work likely will switch to safe mode when Galileo goes inside the danger zone one final time. But NASA scientists said they hope to obtain a few more hours of data before Galileo goes silent, which it invariably will do when it slips behind Jupiter shortly before 4 p.m. EDT Sunday.

Soon afterward it will plunge into the nightside of the planet, just south of the equator, speeding at more than 100,000 mph into the cloud tops.

The searing heat, twice that of the surface of the sun, and the dense pressure, which within minutes will be more than 20 times that at Earth's sea level, should vaporize the robot ship, according to astronomers.

The sacrificial death will end an expedition that spanned 14 years and almost 3 billion miles.

"It's been a fabulous mission for planetary science, and it is hard to see it come to an end," said Alexander.

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