A colourized image of Jupiter, the largest planet in the solar system. (NASA/Jet Propulsion Laboratory)

Long before Galileo pointed a telescope in its direction in 1610, ancient astronomers were impressed enough with Jupiter to name it after the chief of the Roman gods.

Thought to be a wandering star, Jupiter stood out as one of the brightest objects in the sky — only the Sun, the Moon and Venus are brighter. But when Galileo discovered its four largest moons, Jupiter revealed itself for what it really is: a giant among planets.

Jupiter, the fifth planet from the Sun, is more than twice as massive as all the other planets in the solar system combined and over 300 times more massive than Earth. It also stands out because of its enormous magnetic field, countless moons and the swirling storms on its edge, including the iconic Great Red Spot.

The U.S. space agency's New Horizons spacecraft, which launched in January 2006 on a mission to Pluto, has flown past Jupiter, using the planet's gravity to propel itself toward its destination on the fringe of the solar system.

En route, New Horizons is collecting and sending back data of Jupiter and has already sent back the first close-up images of the planet and its moons since NASA's Galileo spacecraft completed its eight-year orbit and plummeted into the planet in 2003.

Of particular interest to scientists are Jupiter's moons, particularly the volcanic Io and the ice-covered Europa, which could have liquid water. But the researchers at NASA and the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., will also be looking at Jupiter's stormy atmosphere and its huge magnetic field, which stretches past the orbit of Saturn.

New Horizons promises to provide the best pictures yet of Jupiter, said Victoria Hipkin, the program scientist for planetary exploration at the Canadian Space Agency.

"When you think of the advances of digital cameras on Earth in the last 10 years, that's the difference between the equipment on Galileo and New Horizons," said Hipkin.

Jupiter 101

Jupiter is, on average, 778 million kilometres from the Sun.

An image of Jupiter taken Jan. 8, 2007, with the New Horizons long-range imager while the spacecraft was about 81 million kilometres from the planet shows the volcanic moon Io to the right. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Its years are long and its days are short. Jupiter takes about 12 Earth years to orbit the Sun but less than 10 hours to rotate once.

Of course, no human could experience life on Jupiter long enough to count the hours, not even a few seconds. The gas giant is mostly hydrogen and helium, with traces of methane, ammonia, water and minerals.

Jupiter's mass means the planet exerts enormous gravity, which generates heat from the compression of gases toward its core. The swirling belts and spots seen from space are only the outermost layer of the planet's turbulent storms.

Astronomers speculate Jupiter has a rocky core wrapped in a layer of hydrogen so dense that it is liquid and ionized enough to conduct enormous electrical currents and produce the planet's magnetic field.

What is at Jupiter's core can only be guessed at using indirect data, as every probe sent down to the planet has been destroyed by the intense heat and gravity. Jupiter was first visited by Pioneer 10 in 1973. The U.S. subsequently sent other spacecraft to Jupiter: Pioneer 11, Voyager 1 and 2, Ulysses and Galileo. NASA's Cassini spacecraft also studied the planet on its way to Saturn.

A mini-solar system

Jupiter is far too cool and small to be considered a star and doesn't produce energy by nuclear fusion, which is what happens in the Sun. Nevertheless, the compression of gases toward the planet's core generates more heat and energy than it receives from the Sun.

The four large moons Galileo discovered 400 years ago with his telescope. From left to right and in order of proximity to Jupiter, they are the volcanic Io, icy Europa and the crater-marked Ganymede and Callisto. (NASA/Jet Propulsion Laboratory)

The energy it produces has also helped power its magnetic field, which traps high levels of energetic particles to form a radiation belt similar to but approximately 10 times stronger than Earth's Van Allen radiation belt.

The planet's gravity has also helped it collect more than its share of satellites. Like Saturn, Jupiter is orbited by a ring of debris. However, the ring is much fainter and smaller than Saturn's and doesn't contain traces of ice.

Jupiter has at least 63 moons, with astronomers predicting many more have yet to be discovered. The four largest moons — the ones spotted 400 years earlier by Galileo — are Io, Europa, Ganymede and Callisto.

Of all the moons in the solar system, Callisto is the most heavily cratered, while Ganymede is the largest and has old features that suggest a volcanic past. Io is the most volcanically active body — planet or moon — in the solar system.

Europa has particularly drawn the interest of astronomers, who hope one day to pierce its icy exterior and examine what they believe is a liquid ocean underneath.

Images from Voyager and Galileo suggest Europa is covered in ice and perhaps even liquid water, though recent estimates based on crater depths suggest the solid ice on the exterior is at least 19 kilometres thick.

Europa and water

The notion of an ocean of water underneath the ice on Europa excites scientists, who plan to keep a close eye as the New Horizon spacecraft gives them another chance to look at it.

Scientists always pay special attention to planets and moons that show signs of liquid water and a stable heat source, both considered important preconditions for the development of life on Earth.

Hipkin of the Canadian Space Agency said water's unique properties as a bipolar molecule make it susceptible to a large range of interactions with other elements and chemical compounds. Water's role as a solvent is key to the biochemical processes needed to build life, she said.

"I've going to have my eye on Europa, for sure. The idea there might be an ocean of water is very exciting, and whenever you take a closer look at something, you're sure to learn something," said Hipkin.

"Look at Saturn's moon Enceladus, where Cassini discovered what look like geysers. It's these totally unexpected discoveries that you hope for, and that build momentum for future research."

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