The four small moons are the first known objects of their size detected in Saturn's rings.

Their presence supports the theory that the planet's spectacular rings were formed after the break-up of an icy moon. The theory predicted debris of all sizes, but only the very small and very large had been observed.

The observations are published in Thursday's issue of the journal Nature.

The propeller-shaped streaks in the rings were observed in July 2004, when NASA's Cassini spacecraft entered orbit above the planet's ring system. The features weren't noticed at first because the images were so faint.

Each pair of streaks in the planet's rings represents the gravitational effect of a moonlet on the surrounding particles. The researchers believe only objects from 40 to 120 metres in diameter would be able to form these shapes.

Smaller particles wouldn't have enough mass to affect their neighbours in such a way, and larger objects would have more obvious effects. Pan, a moon of Saturn that is 30 kilometres wide, is so massive that it carves out a gap in the rings all the way around the planet.

The propellers had been predicted in computer models of the rings, but were never observed until now. The researchers, led by Matthew Tiscareno of Cornell University, believe that Saturn's rings could have about 10 million such moonlets

The most prevalent theory about the origin of Saturn's rings is that an icy moon broke apart, either in a collision with a asteroid or comet or due to the power of Saturn's gravity.

Reports were received from Gunma, Ibaraki, Chiba and Shizuoka prefectures as well as Tokyo, and indicate that the unidentified object moved across the night sky from the west to the east.

"When dust from the cosmos falls to Earth, it emits a bright light in the air. It's likely that the figure was one such fireball that was especially large and bright," said a spokesman at the Gunma Astronomical Observatory in Takayamamura, Gunma Prefecture.

The cause most probably was space garbage of unknown origin. The result was instantaneous depressurization of the satellite's thermal control system fluid circuit, followed by "a sudden outburst of the heat-carrying agent."

The spacecraft subsequently lost its geostationary orientation and proper rotation. Although ground engineers were able to maintain marginal control, RSCC said the AM11 "started approaching the crucial values that can result in the total loss of the satellite."

Along with the effective loss of the satellite, the company said the presence of space garbage most likely also renders its orbital slot unusable. Therefore engineers have engaged "organizational and technical measures aimed at removing the Express-AM11 from 96.5 East into space disposal orbit."

RSCC has downloaded the satellite's backup capacities on the east orbital arc and as a result all Russian commercial TV and radio broadcasting has been restored. In addition, engineers have transferred all official communications channels to the Express-A2 (103 East), Express-AM2 (80 East) and Express-AM3 (140 East) satellites.