CHICAGO - Small moons are acting like shepherds using their gravity to herd dust and boulders in Jupiter's faint rings, NASA scientists say.
The finding is one of several discoveries made from images captured in late February by the New Horizons probe in what NASA scientists called "a real-world fly-by" of Jupiter, 16 months into its mission to Pluto.
They also got the closest look yet at the "Little Red Spot," an Earth-sized swirling storm that scientists described Tuesday as a galactic rendition of Vincent Van Gogh's Starry Night painting, and dramatic views of volcanic eruptions on Jupiter's moon Io.
These postcards from Jupiter were taken by the piano-sized robotic probe's seven cameras as it came within 2.3-million kilometres of Jupiter on Feb. 28. About 70 per cent of the data has been relayed to Earth so far.
This image of Io, one of Jupiter's moons, shows the surface changes seen by New Horizons when compared with the surface of Io seen by the Galileo spacecraft in 1999. Small moons are apparently acting as shepherds using their gravity to herd dust and boulders in Jupiter's faint rings, NASA scientists say. (NASA/JHUAPL/Reuters)
The close encounter allowed New Horizons to make use of Jupiter's gravity to shave three years from its travel time to Pluto. It is expected to arrive in 2015.
The encounter with Jupiter "was a chance to practice ... before the rubber hits the road" with Pluto, Alan Stern, New Horizons principal investigator, told a televised news briefing.
Pictures of Jupiter's faint rings revealed the moons Metis and Adrastea herding rocks with their gravitational pull.
"The boulder-sized particles are definitely being controlled by these shepherding satellites," Jeff Moore of the NASA Ames Research Center in California told the briefing.
Also spied was what appeared to be remnants of an object that had recently collided with the rings. "Individual clusters of particles in the rings is a really new discovery," Moore said.
On Jupiter's volcanic moon Io, New Horizons revealed an umbrella-shaped plume rising 300 kilometres into space from the volcano Tvashtar.
"It's really exciting that it performed for us," said John Spencer of the Southwest Research Institute in Boulder, Colorado.
"Galileo was orbiting for six years and we never saw plumes like that," he said, referring to an earlier mission.
The probe also revealed a mystery volcano, so young that its lava has just reached Io's surface, but which has yet to emit enough gas to produce a plume.
"We're seeing the birth of a new volcano here," Mr. Spencer said.
In about eight years, the mission will produce the closest views of Pluto and the hundreds of other icy objects in the distant Kuiper belt region.
But for now, the NASA scientists are happy with the test drive.
"I think we're ready to rock 'n' roll." Mr. Stern said.
