Animation of flyby (Quicktime, 7Mb)

Animation of flyby (Quicktime, 1.5 Mb)

PASADENA, Calif. NASAs Galileospacecraft successfully flew by Jupiters moon Io early Monday, an encounterthat will yield the closest look yet at the satellites volcano-studdedsurface.

The unmanned spacecraftflew within an estimated 380 miles (612 kilometers) of the moon at 1:06 a.m. ETat its closest approach. The flyby was Galileos 24^th since it firstarrived at Jupiter in December 1995, but only the second of Io. That first Ioencounter, made as Galileo entered orbit around Jupiter, was marred by arecorder glitch that prevented scientists from collecting any images of themoon, one of 15 jovian satellites.

Members of the mission team gatheredat the Deep Space Operations Center at the Jet Propulsion Laboratory earlyMonday said they had remained in constant contact with the spacecraft duringthe encounter and no further glitches were reported. They had feared that theintense radiation environment could trigger Galileo to go into "safe"mode, during which it shuts down its scientific instruments in order to protectitself from further damage, or even have caused them to lose contact with thespacecraft.

By 3 a.m., Galileo ProjectManager Jim Erickson confirmed that the spacecrafts data recorder hadfunctioned throughout the maneuver.

"Win, lose or draw,there will be some images that will knock your socks off, Erickson promised.

Because of the timerequired to receive and process the estimated 100 pictures, however, they willnot be released for about a week. (The image on this page is of Ra Patera, alarge shield volcano, and was taken by Voyager. It shows colorful flows up toabout 200 miles long emanating from the dark central volcanic vent.) The newpictures should show objects as small as 23 feet (7 meters) across.

Buffeted by radiation, thespacecraft suffered a glitch as it approached Io on Sunday. The intenseradiation, estimated at between 40 and 50 kilorads, caused an error in theonboard computers memory, causing some loss of data. However, engineersscrambled to upload new commands over the next 12 hours.

"The flight team didjust a wonderful job," said Deputy Project Manager Wayne Sible. "Thisencounter put us just above our design limits."

Mission members had fearedthe radiation would knock out some or all of Galileos 10 instruments, possiblyeven dooming the mission.

Besides the wealth ofimages of the moon, the most volcanically active body in the solar system,instruments aboard Galileo should yield up information about everything fromthe composition of Ios surface to the temperature of its lava flows. AlthoughIos surface is comparatively chilly minus 216 Fahrenheit (minus 138 Celsius) its lavas can reach 3,140 Fahrenheit (1,727 Celsius), far hotter than thosefound on Earth.

Among the features missionmembers hope Galileos cameras caught on tape is Prometheus, a volcano thatapparently has erupted continuously since at least March 1979, when Voyager 1spotted it during a Jupiter flyby.

"We dont know of any20-year volcanic eruption here on Earth," Erickson said.

Because Jupiters gravitationaltug is so large, Ios crust flexes with tidal regularity. And like an iron barthat is bent back and forth, the flexing heats the moon, causing its extremevolcanism.

Mondays flyby comes at theend of Galileos two-year extended mission, which was tacked onto the originaltwo-year mission that ended in December 1997. Galileo will swing by Io oncemore on Nov. 25, this time passing within 186 miles (300 kilometers) of itssurface.

After that, NationalAeronautics and Space Administration officials will have until late January todecide whether to further extend the mission. Erickson said he hoped Galileocould be operated in unison with the spacecraft Cassini during a Jupiter flybyplanned for December 2000.

Italian astronomer GalileoGalilei the spacecrafts namesake - discovered Io and fellow moons Europa,Ganymede and Callisto in 1610. Io is Jupiters third-largest moon, and isslightly larger than Earths own satellite.