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Potato-shaped moon riddled with holes

By Richard Stenger

View of Amalthea from earlier flyby
View of Amalthea from earlier flyby

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(CNN) -- A daring flyby by a NASA probe revealed that an oddly shaped moon orbiting Jupiter is full of empty gaps and as light as ice, the space agency said this week.

The tiny satellite, called Amalthea, is probably a loose collection of rocky rubble with empty gaps taking up more space than the boulders, according to mission scientists.

The newly released data comes from Galileo, which last month flew within 100 miles (160 kilometers) of the reddish moon, giving an unprecedented glimpse into the physical qualities of the potato-shaped planetoid.

"The density is unexpectedly low," said John Anderson of NASA's Jet Propulsion Laboratory in Pasadena, California, which manages Galileo.

"It's probably boulder-sized or larger pieces just touching each other, not pressing hard together."

The overall density compares with that of frozen water, but planetary scientists are convinced that the oblong-shaped object, which is about 170 miles (270 kilometers) in length and half that in width, is not a space iceberg.

"Nothing in the Jupiter system would suggest a composition that's mainly ice," Anderson said.

Galileo has mapped and studied many of Jupiter's 39 known moons, in particular four the size of small planets -- Io, Europa, Ganymede and Callisto.

The November 5 flight was the riskiest of Galileo's seven-year stay in the Jovian system, bringing it three times closer to Jupiter than ever before.

Drawing of $1.4 billion Galileo craft
Drawing of $1.4 billion Galileo craft

The descent took the $1.4 billion craft deep into the largest planet's radiation belts, which unleash punishing levels of energy that in the past have temporarily knocked out onboard instruments.

Galileo has survived four times the radiation it was designed to withstand, but the bus-sized craft is showing signs of wear and tear.

Its camera has been shuttered for months. Its tape playback data has repeatedly stuck. Its fuel supply is nearly exhausted.

Overall, it has proved remarkably resilient, taking more than 10,000 images of molten volcanoes on Io, strange ice groves on Europa and a mysterious bright spot on Amalthea.

The most recent flyby helped position Galileo for one final mission, a fatal dive into the crushing atmosphere of Jupiter in September.

The plunge will ensure that Galileo does not strike and contaminate Europa, thought to contain an underground saltwater ocean that might harbor microbial life.

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