Galileo speeds by Jupiter's moon Callisto in this computer animation. Photo: NASA/JPL

NASA's Galileo spacecraft on Wednesday passed the closest point to Jupiter of the spacecraft's current orbit of the giant planet, and remains healthy as it heads for a flyby of Callisto, the outermost of Jupiter's four largest moons.

Galileo swung within about 460,000 kilometers (about 285,000 miles) of Jupiter's cloud tops at 10:33 a.m. PDT time, according to engineers managing the spacecraft from NASA's Jet Propulsion Laboratory, Pasadena, Calif. Flying that close to Jupiter exposes the spacecraft's electronics to potential harm from intense radiation belts.

"We have indications Galileo is bearing up well to the harsh environment, but it is still in a challenging environment," said Dr. Eilene Theilig, Galileo project manager at JPL. "As anticipated, we are seeing an intermittent anomalous behavior in the camera, similar to what we saw during Galileo's last encounter five months ago. Prior to high-priority observations, we plan to cycle power to the instrument off and on to decrease the risk of losing images. Cycling the power has cleared the intermittent anomaly in the past."

Galileo is on course to pass within about 123 kilometers (76 miles) of Callisto at 4:24 a.m. PDT on Friday. Galileo has succeeded at more flybys of assorted worlds -- including Venus, Earth, and two asteroids as well as Jupiter's four largest moons -- than any other spacecraft, and Friday's will be its closest yet.

As of 2 p.m. PDT Wednesday, Galileo had recorded about 30 percent of the scientific data that its instruments had been programmed to collect during this swing through the inner portion of the Jupiter system. The images and other data will be transmitted back to Earth over the next two months, with an interruption of three weeks in June when Jupiter and Galileo will be behind the Sun from Earth's perspective.

Galileo passes above Jupiter's moon Callisto in this computer animation. Photo: NASA/JPL

The scheduled observations so far have included studies of Jupiter's clouds in infrared wavelengths, to improve understanding of the structure and dynamics of the planet's atmosphere and distant observations of Io, innermost of Jupiter's large moons, to monitor its volcanic activity. High- resolution images of Callisto's surface are planned for studies of how loose debris on the surface may be obscuring some of the smaller craters. Callisto is about the size of Mercury, with a heavily cratered surface that reveals billions of years worth of information about the size and frequency of comets and other objects hitting Jupiter and its moons.

Galileo has already received more than three times the cumulative radiation exposure it was designed to withstand and has continued making valuable scientific observations more than three years after its original two-year mission in orbit around Jupiter. Its nuclear electrical power source -- two radioisotope thermoelectric generators -- continues to provide power to the instruments, computers, radio and other systems on the spacecraft.

Galileo was launched in 1989 and has been orbiting Jupiter since 1995. JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.