Thursday, March 6, 2003

University of Hawaii

UH finds 7 more
Jupiter moons

2 Hawaii astronomers share
in the discovery of Koko Head-size
rocks in oddball orbits

By Rod Thompson

HILO >> Two University of Hawaii astronomers and their associates have discovered seven new moons circling Jupiter, bringing the total to 47.

The new, unnamed satellites are small, from 1 1/4 miles in diameter, about the size of Koko Head, to twice that diameter.

Astronomer Scott Shepherd, who discovered them along with colleague David Jewitt, sent a one-page announcement to the International Astronomical Union in Cambridge, Mass., where their findings were made known Tuesday.

"It will tell us, hopefully, something about the way that Jupiter was formed," Jewitt said. "The reason we're doing this is to find out how the satellites were captured."

The discoveries fit into the existing picture of Jupiter's moons, which fall into five groups.

The big four -- Io, Europa, Ganymede and Callisto, all about the size of Earth's moon -- were discovered by Galileo in 1610.

All of Jupiter's other moons are much smaller, ranging in sizes roughly comparable to the various Hawaiian Islands or smaller. One group circles Jupiter inside the orbits of the "Galilean" moons, and three groups circle outside those four.

Two of the newly discovered moons fit into an outside group that circles Jupiter in the same direction that the planet rotates, but at odd angles to the planet's equator, Shepherd said. They are about 27 times farther from Jupiter than Earth's moon is from Earth.

The remaining five discoveries are farther out, about 45 to 72 times as far from Jupiter as Earth's moon is from Earth. This group also orbits at odd angles and "backward," in the opposite direction from Jupiter's rotation.

A Web site by Jewitt and Shepherd says the backward orbits are evidence that these satellites once circled the sun like miniplanets but were captured by Jupiter.

"The trouble is, we don't know how that capture was done," Jewitt said. But he said he is optimistic the recent discoveries could lead to further understanding on how such an orbital capture was possible.

It would be nearly impossible for Jupiter to capture satellites now, but it may once have had a much bigger atmosphere that slowed objects as they neared the planet, the Web site says.

The current discoveries were made Feb. 4 through Tuesday using Japan's Subaru telescope, the Canada-France-Hawaii telescope and the University of Hawaii 88-inch telescope, all on Mauna Kea, Shepherd's announcement said.

He and Jewitt were assisted by astronomers Jan Kleyna, Yanga Fernandez and Henry Hsieh.

The latest set of discoveries follows the discoveries by Shepherd and Jewitt of 11 satellites around Jupiter in 2000 and another 11 in 2001.

Several months ago, yet another moon was discovered by experts from the University of Hawaii Institute for Astronomy.

The Associated Press contributed to this report.

UH Institute for Astronomy

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