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Enroute to the Saturn system:

Cassini's Millennium Flyby of Jupiter

NASA JPL image by Cassini of Io above Jupiter
Jupiter's moon Io floats above the planet's cloudtops in an image recorded by Cassini on January 1, 2001, two days after the probe's closest approach to Jupiter
Among the nine planets orbiting our star, Earth is the third out from the Sun. Jupiter is the fifth and Saturn is the sixth planet.

A spacecraft named Cassini was launched from Earth on October 15, 1997, to fly out to the giant planet Saturn where it arrived in 2004 to investigate that planet and its moons and rings. Along the way, on December 30, 2000, the Cassini spacecraft made a flyby of the planet Jupiter.

The Cassini spacecraft is composed of a Saturn orbiter and a Saturn moon Titan probe named Huygens. Together they make up one of the largest, heaviest and most complex interplanetary spacecraft ever built. It weighed 12,346 lbs. at launch, and is 22.3 ft. high and 13.1 ft. wide. Only the two Phobos probes sent to Mars by the former Soviet Union were heavier.

Meanwhile, the Galileo spacecraft had been exploring Jupiter since 1995. The arrival of Cassini was unusual in the history of space exploration with two robot craft – Cassini and Galileo on separate missions – actively observing a planet other than Earth at close range at the same time.

Cassini Jupiter Movie
Cassini recorded some 1,200 snapshots of Jupiter as it passed by Jupiter enroute to Saturn. NASA's Jet Propulsion Laboratory (JPL), which manages the Cassini program for NASA's Office of Space Science, made a kaleidoscopic movie from the Jupiter images. That video revealed unexpectedly persistent polar weather patterns on the giant gas planet.

NASA JPL image by Cassini of Io above Jupiter
Cassini portrait of Jupiter with the Great Red Spot atmospheric storm at bottom center right as wide as two Earths and over 300 years old, with peripheral winds of 300 mphs.

Jupiter videos  »
Cassini recorded the images in infrared light to cut through Jupiter's upper haze and show the clouds underneath in black and white. JPL's movie clip combined images recorded over a span of 70 days into a sequence less than a minute long. The movie showed that the small spots seen in the planet's atmosphere last a long time and move in organized patterns.

Long-lived storms and globe-circling belts of clouds are familiar features around Jupiter's midsection. They can be seen easily in still pictures. However, closer to the planet's poles, the still images show widespread mottling that appears chaotic.

There are thousands of storms on Jupiter the size of the biggest storms on Earth. Until now, however, astronomers didn't know much about the lifetime of the storms. The Cassini movie shows thousands of spots bumping into each other, but generally moving together within each band of latitude. The spots occasionally change bands or merge with each other, but usually they last for the entire 70 days. Each spot is an active storm in Jupiter's atmosphere.

The mystery is why the storms last so long. By comparison, storms on Earth last a week, then break up, and are replaced by other storms.

On the other hand, the better question might be why storms on Earth are short lived. Aftar all. astronomers claim Earth weather is the least predictable in the Solar System.

Cassini went into orbit around Saturn on July 1, 2004.

Revising Jupiter's Stripes
Cassini's pictures of Jupiter are flipping at least one long-standing notion upside down.

Stripes dominate Jupiter's appearance. Darker belts alternate with lighter zones. Astronomers had thought of the pale cloud zones as areas of upwelling atmosphere, just as many clouds on Earth form where air is rising. Therefore, thinking that what goes up must come down, astronomers saw the dark belts as areas where air generally descends.

However, Cassini pictures show that individual storm cells of upwelling bright-white clouds — too small to see from Earth — pop up almost without exception in the dark belts. Earlier probes from earth had given hints, but not the strong evidence provided by the Cassini images of 43 different storms.

The belts now seem to be areas of rising atmospheric motion on Jupiter. That implies the motion in the zones is sinking, according to NASA atmospheric scientists Goddard Institute for Space Studies in New York.

Lots of Pix
Cassini shot 26,000 images of Jupiter, its moons and its faint rings over a six-month period as the spacecraft passed by in 2001.

The range of illumination angles at which Cassini viewed Jupiter's main ring gave astronomers insight into the particles in the rings by the way they scatter sunlight. The particles appear to be irregularly shaped, not spheres. That means they might come from surfaces of one or more moons being eroded by micrometeoroid impacts. Spherical particles might have been melted droplets, not erosion.

Rings Metis and Adrastea
Cassini images showed the degree to which the orbits of two small moons near the ring, Metis and Adrastea, are inclined. That matches the vertical thickness of the ring. That means those moons probably are the sources of the ring particles.

One surprise in ultraviolet images of Jupiter's north polar region is a swirling dark oval of high-atmosphere haze the size of the planet's famous Great Red Spot. The newly-seen object gives astronomers information on how stratospheric circulation works. It shows the winds and the life cycle of clouds in the stratosphere.

Leopard Spots Race
Movies composed of infrared images reveal persistent bands of globe-circling winds extending north of the planet's conspicuous dark and light stripes. At high latitudes, the planet reminds astronomers of spots on a leopard. When seen in motion, all the spots at one latitude move in one direction and all the spots at adjacent latitudes move the opposite direction.

Atmospheric glow, volcanic plume
Other discoveries reported include atmospheric glows of the large moons Io and Europa during eclipses, a volcanic plume over Io's north polar region, and the irregular shape of a small outer moon, Himalia.

On to Saturn
Cassini will arrive in orbit around Saturn on July 1, 2004. Six months later, it will release its piggyback probe, Huygens, for descent through the atmosphere of the moon Titan.

Excited astronomers note that Cassini's results at Jupiter hint at spectacular findings that may await the interplanetary probe when it reaches Saturn.

Jet Propulsion Laboratory
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory (JPL), a division of Caltech at Pasadena, California, manages the Cassini program for NASA's Office of Space Science, Washington, D.C.

Learn More About the Jupiter System – the Planet, the Moons, the Rings
Jupiter Moons Oceans Volcanos Rings Radio
JIMO Galileo Cassini Pioneer Voyager Resources

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