WASHINGTON A software malfunction prevented a key piece of equipment on the Cassini spacecraft from recording data as it flew through the plume from a geyser shooting off a moon of Saturn, NASA said late on Thursday.
NASA called the problem "an unexplained software hiccup" that came at a very bad time, preventing Cassini's Cosmic Dust Analyzer instrument from collecting data for about two hours as it flew over the surface of the moon Enceladus on Wednesday.
A key objective of the fly-by was to determine the density, size, composition and speed of particles erupting into space from the moon's south pole in a dramatic plume.
Bob Mitchell, Cassini program manager, said the problem meant that the instrument did not collect data as the craft flew through the plume -- a process lasting under a minute.
"When it went through the plume, it was not working properly," Mitchell said in a telephone interview, expressing disappointment. "We had tested that software very carefully. We don't know why it didn't work properly."
The Cassini spacecraft, studying the giant gaseous planet Saturn and its moons in a joint U.S.-European mission, flew as close as 30 miles over the surface of Enceladus (pronounced en-SELL-ah-dus) on Wednesday.
"During the fly-by, the instrument was switching between two versions of software programs. The new version was designed to increase the ability to count particle hits by several hundred hits per second," NASA said in a statement.
"The other four fields and particles instruments on the spacecraft, in addition to the ion and neutral mass spectrometer, did capture all of their data, which will complement the overall composition studies and elucidate the unique plume environment of Enceladus," NASA said.
The Cassini spacecraft, which first spotted the geysers in 2005, flew over the moon at 32,000 miles per hour (51,500 kph) in the first of four fly-bys of Enceladus scheduled this year.
One of about 60 moons of Saturn, Enceladus is considered among the most intriguing bodies in the solar system, owing to the geysers that spurt from fractures in the surface at the south pole and spew material about 500 miles into space at about 900 miles per hour (1,450 kph).
Enceladus, whose diameter is 310 miles, is one of the solar system's brightest objects. Encased in ice, it reflects almost all of the sunlight that strikes it.
Some scientists surmise hot water must exist under the surface to eject these plumes. The presence of liquid water raises the possibility Enceladus may boast conditions conducive to the development of life, perhaps in the form of microbes.
NASA said images taken by the spacecraft showed that the north polar region is much older and more pockmarked with craters of various sizes than much of the southern hemisphere, in particular the south pole.
The images show craters that were caused by impacts of objects onto the moon's icy surface in varying stages of alteration by tectonic activity and probably from past heating from below the surface, NASA said.
Enceladus is one of Saturn's innermost moons. The eruptions from its geysers seem to be continuous, generating an enormous halo of fine ice around the moon and supplying material to one of Saturn's famed rings.