A small spacecraft en route to Pluto has flown past Jupiter, picking up enough speed from the giant planet's gravity field to shave three years off what would have been a 12-year voyage.

In exchange, the New Horizons spacecraft is taking a lingering and long-awaited look at Jupiter, which was the focus of the now-defunct, eight-year Galileo mission. Of particular interest is Europa, a large Jovian moon that shows strong evidence of a subterranean, salty ocean.

Antenna problems on Galileo prevented scientists from getting many high-resolution images of Europa, which sports a smooth, flat, icy surface. Scientists hope infrared sensors aboard New Horizons will reveal upwellings on the surface from the hidden sea below.

"I'm hopeful that we will get some real clues about the surface of that ocean," Bill McKinnon, a planetary scientist with Washington University in St Louis said before the encounter.

The prospect of a liquid ocean increases the chances that life may exist on the moon. Similar habitats on Earth, such as under the Antarctic and Arctic ice sheets, have been found to be teeming with life, said Jere Lipps, a marine biologist at the University of California at Berkeley.

New Horizons' closest approach to Jupiter occurred at 0543 GMT, when it passed 2.3 million kilometres from the planet. The studies of Jupiter and its four largest moons began several weeks ago and are scheduled to continue through June.

In addition to the Europa studies and observations of three sister moons, including Io, which has active volcanoes, New Horizons is scheduled to make a long trek down the length of Jupiter's magnetic tail, which extends for tens of millions of kilometres beyond the planet.

The tail stems from Jupiter's vast and dynamic magnetic field, which is buffeted and shaped by the high-speed river of charged particles constantly flowing from the sun.

"This is an unprecedented opportunity," said New Horizons principal investigator Alan Stern, with the Southwest Research Institute in Boulder, Colorado. "It's never been done on a giant planet before."

Other highlights of New Horizon's Jupiter tour include a first-time close-up look at a storm known as the Little Red Spot, which formed after Galileo's demise in 1995.

After relaying to ground control stations its data from Jupiter, New Horizons will be put into hibernation for the remaining eight years it will take to reach Pluto. The spacecraft, which is powered by the slow decay of radioactive plutonium pellets, was launched in January 2006.

The encounter with Jupiter boosted the probe's speed by 14,000 kph, so that it is now streaming toward Pluto at 84,000 kph.

The spacecraft is moving far too fast to stop and drop into orbit once it reaches Pluto in 2015. Rather, it will study the small planet-like world and its main moon Charon on the fly before heading out to study other icy bodies in the Kuiper Belt region.

-Reuters