Passing NASA Probe Examines Jupiter's Moons, Storms

Visions of a volcanic plume spewing out of Jupiter's moon Io and a swirling storm are among the first images returned by a NASA probe as it approached an early-morning swing past the gas giant Wednesday.

NASA's New Horizons spacecraft sent home the new look at Jupiter's "Little Red Spot" and the planet's volcanic moon Io as it closed in on the gas giant during a planetary flyby that was due to reach its closest approach at about 12:45 a.m. EST (0545 GMT) Wednesday.

"This is the best image of a large volcanic plume on Io since the Voyager flybys in 1979," John Spencer, deputy leader of the New Horizons Jupiter Encounter Science Team at Colorado's Southwest Research Institute (SwRI), said in a written statement released Tuesday.

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The flyby is a major milestone for New Horizons' flight and allows mission scientists to collect new Jupiter observations in a dress rehearsal for the probe's planned Pluto encounter in 2015.

At its closest pass, New Horizons was expected to swing within 1.7 million miles (2.3 million kilometers) of Jupiter before shooting onward on a course through the gas giant's long magnetotail — the non-Sunward side of the Jovian magnetic field.

It is during that time that astronomers hope to uncover new secrets of the interactions between Jupiter's magnetosphere, the Sun's solar wind and the gas giant's aurora displays.

"This is really the payback time," SwRI's David McComas, principal investigator for New Horizon's Solar Wind Around at Pluto (SWAP) instrument, told before Wednesday's planned flyby. "The big show, it probably isn't just one day or two ... we'll be inside the magnetosphere of Jupiter for many weeks."

Launched in January 2006, New Horizons is expected to grab a 9,000-mile per hour (14,484 kph) speed boost from its pass through Jupiter's strong gravity field.

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The extra speed, mission managers have said, will cut three years on New Horizon's long trek for its flyby of Pluto and its three moons Charon, Nix and Hydra.

The Applied Physics Laboratory at Johns Hopkins University in Maryland is overseeing the mission for NASA.

A clutch of Jovian moons

As a sort of flyby appetizer, the fresh look at Io by New Horizons has provided the clearest view to date of the Jovian moon's Tvashtar volcano, mission managers said in a written statement.

The probe used its Long Range Reconnaissance Imager (LORRI) to photograph Io from a range of about 2.5 million miles (four million kilometers) on Feb. 26, 2007 as it neared Jupiter.

Other images from the camera in the last two days include a new portrait of Jupiter's moons Ganymede and Europa, and the immense "Little Red Spot," a swirling storm about half the size to the planet's "Great Red Spot."

First discovered in 1999 in ground observations and by the Galileo probe, the Tvashtar volcano is one of Io's most active features and can be seen spouting a dust plume more than 150 miles (241 kilometers) high to form an umbrella-like shape in the New Horizons view.

Io's Tvashtar volcano can be seen in another New Horizons image, a three-millisecond exposure aimed at resolving surface features, where it appears to be a dark spot in the 11 o'clock position surrounded by a dark ring.

The eruption's volcanic fallout covers a region the size of Texas, New Horizons officials said.

The new Io images of the plume resolve features as small as 12 miles (20 kilometers) in size, which is about 12 times sharper than those taken by the Hubble Space Telescope and three times sharper than views snapped by the Cassini spacecraft, which flew past Jupiter in late 2000 on its way to its current orbit around Saturn, New Horizons officials said.

"If the Tvashtar plume remains active, the images we take later in the encounter should be even better," Spencer said.

New Horizons fresh Jovian system images are just some of the more than 700 science observations planned during the probe's Jupiter flyby.

The bulk of those images, however, will remain locked inside the spacecraft's computer for later downlink to Earth in March and April, mission managers have said.

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