Cassini snaps pics of Jupiter lightning, elusive moon

larger   These two images taken of Jupiter's day and night sides by Cassini show storms visible on the day side as the sources of visible lightning on the night side.

By Richard Stenger
CNN.com Writer

(CNN) -- A NASA probe passing by Jupiter has beamed back impressive new images of the gas giant, including a nighttime lightning storm, the satellite Io and another more elusive moon named Himalia.

The Cassini spacecraft took most of the pictures in early January shortly after swooping by Jupiter. The $3.4 billion robot ship used the planet's gravity to gain a boost on its journey to Saturn.

The first pair of images focuse on a region of the planet where lightning storms take place. The right half shows it during the day; the left half at night. The storms, seen in the night image as bright spots in two locations, correspond in the day image with two groups of high-altitude clouds.

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The gap between the storms slightly widens from image to image because the winds moving the two systems travel at different speeds, according to NASA astronomers.

The moon Io seems almost to touch the jovian atmosphere in the second image, captured on January 1, two days after Cassini's closest approach to Jupiter. But the perspective is an illusion, NASA scientists said.

larger   The satellite Io, with Jupiter as a backdrop

Some 217,000 miles (350,000 km) separates the two celestial bodies, a distance roughly twice the diameter of Jupiter, the largest planet in the solar system. Io, the most volcanically active body in the solar system, is as large as Earth's moon.

Himalia, the brightest of Jupiter's outer moons, was captured and resolved for the first time in several Cassini images. The spacecraft was 2.8 million miles (4.4 million km) from the moon when it took the one to the right.

The inset shows the satellite image magnified 10 times and a graphic of its projected shape. Astronomers speculate that the small moon was captured it into its present orbit and that it has an irregular shape. The side facing the camera in this image is roughly 99 miles (160 km) from top to bottom.

Cassini will continue to make observations and measurements of the Jupiter system though March, some in coordination with another NASA spacecraft, Galileo, which has orbited Jupiter for five years. Cassini should reach Saturn in 2004.

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Cassini-Huygens Mission to Saturn and Titan

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