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A smashing end for Jupiter explorer$1.4 billion spacecraft collected data almost until its demise
By Richard Stenger
(CNN) -- A daring robotic explorer that circled Jupiter and its moons for eight years plunged into the scorching atmosphere of the giant planet Sunday -- a fiery end to one of the most productive space missions ever. 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 larger moons, considered some of the most promising sites to search for life beyond Earth. Its propellant running low and its electrical systems on the blink, Galileo nonetheless kept a handful of instruments on during the final hours, giving scientists a chance to squeeze some final observations about Jupiter's upper atmosphere from the $1.4 billion mission. "We're still collecting scientific data. I'm really surprised we haven't gone into emergency mode," Galileo project manager Claudia Alexander said an hour or so before impact. "This time last November, we were passing through this same zone, and we got clobbered." The craft went silent just before 4 p.m. EDT, having slipped behind the far side of Jupiter. Minutes later, it presumably screamed across the cloud tops on the night side of the planet, just south of the equator, speeding at more than 100,000 mph. The searing heat in the upper atmosphere, twice that of the surface of the sun, and the crushing pressure, which within minutes was more than 20 times that at Earth's sea level, likely vaporized the robot ship, according to astronomers. The sacrificial death ended an expedition that spanned 14 years and almost 3 billion miles. Since it left Earth in 1989, Galileo 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. In 1994, a year before it arrived in Jupiter orbit, 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. Dodging lava, finding oceansThe droid buzzed within dozens of miles of some Jupiter satellites, flying through volcanic plumes of sulfur on Io and detecting promising signs of hidden oceans underneath the surface of other planet-size moons.
"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." Although 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 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 a hellish alien 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." Trouble since the startJust getting to Jupiter in working order was a significant 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. 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 extensive technical improvisation.
"The engineers had to redesign the software, reprogram the tape recorder and had to get Galileo to do data compression," said Alexander, the seventh and final Galileo skipper 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 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 Ganymede, 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. Ganymede, the largest, is bigger than Mercury. Callisto, the outermost, has the oldest surface of any known planet or moon. Hints of water and lifeDespite their differences, the moons have 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," Belton said. "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 crash the craft. During the final descent, Galileo entered Jupiter's radiation belts. Earlier trips into the turbulent sea of highly charged particles subjected the craft to four times the radiation it was designed to withstand. The trips took a toll, damaging numerous electronic components, including the onboard camera. On its most recent foray in November, when it skimmed over a tiny inner moon called Amalthea, it weathered enough radiation to kill a human 1,000 times over and nearly lost its data recorder. Somehow mission technicians coaxed it back into service. "It was one of the most astounding recoveries that I think the mission made," said ex-mission manager Eliene Theilig, who returned to JPL mission control Sunday for a de facto funeral wake for the spacecraft. As hundreds of current and former Galileo scientists and engineers exchanged memories about their distant friend, some began looking to the future. Torrence Johnson, a mission alumnus and expert on outer solar system moons, described a Galileo successor on the drawing board. "All of the exciting discoveries have generated a lot of interest," Johnson said. "Right now we're laying plans to return to Jupiter with a new class of spacecraft." Powered by ion propulsion and a small nuclear reactor, the Jupiter Icy Moon Orbiter, or JIMO, would be able to go into orbit around individual moons. NASA hopes to launch the mission, which would be 100 times more powerful than Galileo, within a decade.
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All of the exciting discoveries have generated a lot of interest. Right now we're laying plans to return to Jupiter with a new class of spacecraft."
