Galileo Millennium Mission Status 21 Feb 2001

Engineers are narrowing down possible causes for an
intermittent problem with the camera on NASA’s Galileo spacecraft
that may be related to effects of Jupiter’s radiation belts.

The spacecraft signaled an alarm from the camera system
three times while Galileo passed close to Jupiter from Dec. 28,
2000, to Jan. 1, 2001. Each time, the camera either restored
itself to normal functioning or was restored by commands from the
ground. The incidents appear to be related to a single similar
event five months earlier, and the underlying cause may be
cumulative exposure of electronic components to the intensely
radioactive environment near Jupiter, said Dr. Eilene Theilig,
Galileo project manager at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif. Galileo, now in its sixth year of what was
originally planned as a two-year mission orbiting Jupiter, has
weathered more than three times the radiation dose it was
designed to withstand.

“We are able to clear the fault by power-cycling the
instrument — turning the power off and on — and reloading its
memory. The fact that the camera can fix itself without our
intervention is puzzling but provides valuable information to
analyze what is happening,” Theilig said.

Engineers have examined a small sampling of the camera data
recorded while Galileo passed through the inner portion of the
Jupiter system in late December. The sampling indicates that more
than half of the 120 pictures taken during that encounter period
were captured properly, including all the ones taken Dec. 28 as
the spacecraft flew by the moon Ganymede during an eclipse. In
pictures taken while the camera fault was present, however,
images are blank, as if entirely saturated with light. The first
transmissions of complete images from the encounter will come
later this month.

Experiments at JPL with an engineering model of the camera
system are aiding analysis of events on the spacecraft. The main
suspect is an amplifier in the circuitry that processes signals
from the camera’s CCD (charge-coupled device), a light-sensor
grid akin to the ones in video cameras. “The investigation is
continuing,” Theilig said. “When we get a better understanding of
the fault and what triggers it, we should be able to identify
some workarounds, such as planning ahead to power-cycle the
camera at appropriate times, so we can minimize the impact to our
imaging objectives.”

Galileo’s next encounter will be a flyby of Jupiter’s moon
Callisto on May 25. Additional information about the mission is
available at http://galileo.jpl.nasa.gov . Galileo was launched
in 1989 and began orbiting Jupiter in 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.