Spacecraft gets close to Jupiter

Jupiter has very different weather to that on Earth

The Cassini spacecraft has made its closest approach to the planet Jupiter, capturing dramatic pictures of the massive storms that rage across the gas giant's "surface".

The probe, which is en route to Saturn, got within 9.7 million km (six million miles) of Jupiter on Saturday.

Camera equipment peered deep into the planet's cloud cover to reveal smaller storms being created and torn apart by larger ones.

Jupiter facts

Largest planet in the Solar System

2.5 times as massive as all the other planets

Almost entirely made of hydrogen and helium

Surrounded by 16 moons

Jupiter's famous Great Red Spot, visible with a telescope, was first seen 300 years ago and for many years was thought to be a thunderstorm stretching across an area three times as wide as the Earth.

Recent studies suggest instead that the Red Spot is a natural product of the complex laws which govern a spinning ball of gas the size of Jupiter. It is what researchers call a "strange attractor", a self-organising system of stable flow that is closely related to the chaotic turmoil around it.

Nevertheless, scientists hope the new data will help them reach a better understanding of the Earth's atmosphere. The main purpose of Cassini's approach to Jupiter was to give the 5,712kg (12,593lb) spacecraft a final gravitational push towards Saturn, where it is due to arrive on 1 July, 2004.

Weather puzzle

The new images from Jupiter suggest the massive storms draw their energy from absorbing smaller systems, said Andrew Ingersoll of the California Institute of Technology.

Smaller storms surround the Red Spot

"The weather is different on Jupiter," he told a news conference at Nasa's Jet Propulsion Laboratory. "We'd like to know why Jupiter's weather is so stable and Earth's is so transient."

Previous observations from the US space agency's Galileo spacecraft, which has been orbiting Jupiter since 1995, have suggested that smaller thunderstorms draw their power from below the cloudy "surface" of the hot, gassy planet.

Close-ups reveal lightning

Scientists are also using data from Galileo and the $3.4bn Cassini to study how the solar wind of particles speeding away from the Sun affects the huge magnetic region surrounding Jupiter.

William Kurth of the University of Iowa said: "For the first time we have the opportunity to have a 'weather station' in the solar wind."

Dr Carolyn Porco of the University of Arizona said the quality of the imagery coming back from the satellite was first class: "The camera has performed beyond our wildest imaginings - and that's saying something, because we've been imagining this for a decade now," she said.

Future task

One immediate finding was that the huge bubble of charged particles known as Jupiter's magnetosphere is changing size more rapidly than expected.

Cassini will spend four years studying Saturn

Dr Kurth said Cassini hit Jupiter's magnetic boundary on Thursday, much farther out than expected. It recorded a burst of radio waves similar to the sonic boom created by a jet breaking the sound barrier.

In recent months, Galileo has broken through Jupiter's magnetic boundary, but the bubble collapsed suddenly and unexpectedly back towards the planet, he said.

Cassini will end its flyby of Jupiter in March. When the spacecraft, launched in 1997, finally reaches Saturn, it will swing into orbit inside the outermost ring. It will send a European-built, parachute-equipped probe to the planet's largest moon, Titan.

Titan is thought to have an atmosphere much like Earth's, but with clouds, rain and weather patterns produced by methane gas.