The objects were detected as flashes of light by NASA's Galileo spacecraft in September, when the craft flew near Amalthea. Galileo's science team has since been disbanded and the craft is bound for a purposeful September collision with Jupiter that will end nearly a decade of exploration of giant planet and its environs. The probe is nearly out of propellant.

The data were only recently analyzed, and not enough was gathered to pin down the sizes or orbital behavior of the objects. Two of them might have been spotted twice, accounting for the uncertain quantity.

Paul D. Fieseler, an engineer at NASA's Jet Propulsion Laboratory, made the discoveries along with Shadan Ardalan.

"We cannot say if they are in orbit or are merely temporary residents tossed up in some collision," Fieseler told SPACE.com today. "If they are in a stable orbit, I personally would call them moons. If not, perhaps 'ejecta' would work. For myself, I've just been referring to them as 'rocks' or a 'debris field.'"

The discovery comes during a year when already 18 new, small Jovian moons have been found by ground-based telescopes. Some of the moons are as small as 0.62 miles (1 kilometer). A new moon of Saturn was also discovered this week. Some astronomers have said recently that the profusion may soon force them to consider if there is a difference between moons and debris, especially when the Cassini spacecraft arrives at Saturn next year and, likely, finds small but substantial objects embedded in the rings.

Fieseler said the latest analysis of Amalthea indicates it might be little more than a rubble pile. It is thought to contribute to faint rings of debris that orbit Jupiter. The rocks spotted by Galileo could have been blasted out in a collision, he said. He figures they are "somehow associated with the give and take of material between Amalthea and the rings."

Galileo detected light reflected by the rocks with its star scanner, a telescope used to determine the spacecraft's orientation.

"It is only because Galileo was very close to these objects that we were able to detect them," Fieseler said.

The size, orbit and nature of each rock is likely to remain a mystery for many years.

"The Galileo spacecraft will pass far from Amalthea on its final suicide dive this fall so there is no real chance of a follow-up observation with this spacecraft," Fieseler said.

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