Close-up on Jupiter's moons

Amalthea: the feature Ida is top left

The Galileo spacecraft has braved Jupiter's lethal radiation belts to capture the best ever images of three of the planet's innermost moons - Thebe, Amalthea and Metis.

The images were dribbled back to Earth via a single, slow antenna but reveal details as small as two kilometres in size.

Galileo had circled the planet at 630,000 km above the giant planet's surface, but last summer dived down to just 330,000 km. The risky manoeuvre took the spacecraft into the heart of Jupiter's magnetic fields and charged particle radiation belts, in order to fly close by the volcanic moon Io.

The risk was judged worthwhile as Galileo, launched in 1989, is nearing the end of its long mission.

Bright streak

The images show two views of Amalthea, a 250-km (155-mile) long, irregularly shaped moon that has a bright streak on it nicknamed "Ida".

Jupiter's inner moons: From top, Thebe, Amalthea and Metis

The images suggested that Ida, which is 50-km (31-miles) long, could have been kicked up by a meteoroid impact or could mark the crest of a local ridge, according to Damon Simonelli of Cornell University.

Other members of the Galileo Imaging Science Team come from the US National Optical Astronomy Observatories, Arizona and Nasa's Jet Propulsion Laboratory, California.

The images of Amalthea also reveal a large meteoroid impact crater about 40 km (25 miles) across. The moon Thebe has also suffered an asteroid collision, leaving a similar-sized crater.

Slow work

The researchers said they were pleased to get any images at all. Galileo's main, high-gain antenna was lost from use shortly after launch.

The scientists were therefore forced to use a less powerful antenna to get the images, reducing data transfer rates from about 100,000 bits per second to just 40 bits per second.

A two-part strategy was developed to retrieve the images stored on the spacecraft's digital tape recorder. These data are replaced during each of the spacecraft's subsequent orbits and this gives little time for downloading.

So an initial, highly compressed image was transferred immediately after each image of the moons was captured. From this the researchers could determine what portion of each image contained the moons and then this part was relayed in full-resolution, as the spacecraft sped back toward Jupiter for its next orbit.