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Jupiter is closest to Earth June 2, awaits Galileo space probe visit

ANN ARBOR—Jupiter, called " King of the Planets," dominates Michigan's evening sky all summer. It is closest to the Earth on June 2, when our home planet is almost exactly between it and the sun. On that date, Jupiter is 400 million miles from Earth, and sunlight reflected from its icy clouds takes more than 30 minutes to get here, according to University of Michigan astronomer Richard Teske.

Jupiter, the brightest " star" in the southeastern sky after dark in June, holds a well-deserved place in the solar system's book of records, Teske said. The largest of the sun's family of nine planets, it weighs more than all the others put together" even though it is composed mostly of the lightest chemical elements, hydrogen and helium.

"Jupiter is the fastest rotating planet, turning once around on its axis in just under 10 hours. Because it spins at 28,000 miles an hour at the equator, Jupiter exhibits a pronounced equatorial bulge," Teske said. " Someone looking at it through a small telescope quickly notices its oblate shape. With persistence, an observer can watch Jupiter rotate once around during a single 10-hour night of viewing. "

Small telescopes reveal striped cloud patterns on Jupiter's perpetually veiled face. The clouds wrap completely around it as bright and dark bands paralleling the equator, stretched out by forces produced through the planet's speeding rotation. Unlike the familiar clouds of Earth's sky, Jupiter's are made up of crystals of frozen chemical ices floating in the hydrogen-helium air, Teske explained.The planet owes its brilliance to their high reflectivity of sunlight as well as to its great size.

"There is no evidence of a solid surface beneath Jupiter's banded clouds," Teske said. " Planetary scientists believe an astronaut descending through the clouds would encounter thicker and denser hydrogen-helium air, until several hundred miles down when he or she would enter a mushy layer that gradually blends into a planet-wide liquid hydrogen ocean. "

Conditions in Jupiter's cloudy upper atmosphere have been pieced together by scientists from observations made by four spacecraft that flew by Jupiter between 1973 and 1979. Air pressure near the cloud tops is about the same as at the top of Earth's clouds, but it is a lot colder on Jupiter; the air temperature there falls to 250 degrees below zero F.

A few months from now, scientists will begin an accurate, on-site survey of Jupiter when a planetary probe parachutes into the planet's upper atmosphere. The spacecraft carrying the probe, called Galileo, was launched from Earth in late 1989. When it arrives at its destination on Dec. 7, the craft will separate into two sections. One, the " bus," will remain in orbit around the planet, while the other unit, the probe, will float down to Jupiter on a one- way, better-get-it-right trip" sampling its surroundings as it descends.

"If all goes according to plan, the probe will radio information about Jupiter's atmosphere and clouds to the orbiting bus which will relay the signals to spacecraft controllers on Earth," Teske said. " Scientists expect the probe to stop working before it reaches the liquid hydrogen layer, so there will be no news from deep within the clouds. The bus will continue to observe Jupiter and its system of multiple moons while it remains in orbit for another 20 months. "