An artist's concept of Galileo during the approach to Io. Image: NASA TV/Spaceflight Now

The Galileo probe orbiting Jupiter ran into problems Thursday as the spacecraft neared its final close encounter with the moon Io. Similar to glitches suffered in the past, this most recent event prevented any observations from taking place during the closest part of the flyby.

Galileo entered into a "safe mode" at 1341 GMT (8:41 a.m. EST) Thursday, 28 minutes before the scheduled closest approach to Io. Ground controllers believe that such incidents are caused by intense radiation that surrounds Jupiter.

Software aboard the craft identified a computer reset, immediately resulting in the safe mode. Instruments were commanded to stop gathering data and were put into a standby mode until signals and instructions are received from the team on the ground.

"We're not totally surprised, because Galileo has already outlived expectations and we knew that it might encounter additional difficulties from the high-radiation environment on this flyby," said JPL Galileo project manager Dr. Eilene Theilig. "Galileo has already lasted more than four years past its original mission and has survived three-and-a-half times the radiation it was designed to withstand, so it's not unexpected that this flyby would be interrupted by a problem."

Scientists and engineers are sending signals to Galileo in an attempt to get the craft out of safe mode in time for the remainder of the flyby, which officially does not end until Sunday.

The Galileo team continues to be hopeful that they will be able to restore normal spacecraft functioning by transmitting new commands to Galileo to revive data collection, Theilig said.

Even with the cessation of normal spacecraft operations, the close encounter did accomplish its primary purpose of using Io's gravity to slingshot Galileo onto a collision course with Jupiter in September 2003.

Controllers opted to crash the veteran probe into the planet to avoid the slim chance that the craft could impact the moon Europa after the mission ends. Europa is thought to have an ocean of melted saltwater under its icy surface, and scientists do not want to contaminate Europa in interests of future studies on the possibility that it could harbor life.

Thursday's flyby was the 34th and last at any of Jupiter's four Galilean moons -- Io, Europa, Ganymede, and Callisto. Passing just 63 miles above Io's fiery volcanic surface, it was also the closest approach in Galileo's six years of orbiting the gas giant.