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Last Updated: Tuesday, 17 August, 2004, 13:05 GMT 14:05 UK
Cassini finds new Saturn moons
NASA/JPL/Space Science Institute
The moons take the total number of Saturnian satellites to 33
The Cassini-Huygens mission in orbit around Saturn has discovered two new moons circling the ringed planet.

The new discoveries take Saturn's total tally of natural satellites to 33.

The moons are about 3km (2 miles) and 4km (2.5 miles) across and located 194,000km (120,000 miles) and 211,000 km (131,000 miles) from Saturn's centre.

They are provisionally named S/2004 S1 and S/2004 S2, though one of the new moons may have been spotted before in a single image from the Voyager probe.

This tells me I should take more holidays
Sebastien Charnoz, University of Paris
Space officials said they may be the smallest bodies yet seen orbiting the gas giant.

The new satellites are between the orbits of two other Saturnian moons, Mimas and Enceladus.

S/2004 S1 and S/2004 S2 were first seen by Dr Sebastien Charnoz, a colleague of Cassini imaging team member Andre Brahic at the University of Paris, France.

Dr Charnoz said that, in his opinion, the circular orbits of the moons on the equatorial plane of Saturn suggest they originated around the planet and are not captured asteroids or Kuiper Belt objects.

"In general captured objects, like (Saturn's moon) Phoebe, have an elongated and inclined orbit, which is not the case with S/2004 S1 and S2," he told BBC News Online.

"But the moons are so small it is very possible that, under the gravitational influence of the rings and other big satellites, their orbits could have evolved significantly during the past.

"The gravitational attraction of big satellites - such as Mimas, Enceladus, Titan and others - may have moved the orbits of S1 and S2 in the past. So it is very possible that the moons did not form where there are observed today."

'Gratifying discovery'

Imaging team leader Dr Carolyn Porco commented: "It's really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons."

Scientists expected moons as small as S/2004 S1 and S/2004 S2 might be found within gaps in Saturn's rings and perhaps near the F ring.

Mimas, Nasa
The moons orbit between Mimas (above) and Enceladus
But they were surprised these small bodies are between two major moons.

The smallest previously known moons around Saturn are about 20km (12 miles) across.

Small comets speeding around the outer Solar System would be expected to collide with small moons and pulverise them. The fact that these moons exist where they do might place limits on the number of small comets in the outer Solar System.

This is vital for understanding the zone beyond Neptune known as the Kuiper Belt, which is filled with small, icy bodies and is thought to be a source for comets.

It also sheds light on the cratering histories of the moons around giant planets such as Saturn. Cratering is used by some scientists as an indicator of the age of planetary surfaces.

Drifting bodies

"If small comets are rare, as they seem to be in the Jupiter system, the new [Saturn] moons might have survived since the early days of the Solar System," said Dr Luke Dones, an imaging team member from the Southwest Research Institute in Boulder, Colorado, US.

Moons surrounding the giant planets are not generally found where they formed because tidal forces from the planets can cause them to drift from their original locations.

In drifting, they may sweep through locations where other moons disturb them, making their orbits eccentric or inclined relative to a planet's equator. One of the new moons might have undergone such an evolution.

"Discovering these faint satellites was an exciting experience, especially the feeling of being the first person to see a new body of our Solar System," said Dr Charnoz.

"I had looked for such objects for weeks while at my office in Paris, but it was only once on holiday, using my laptop, that my code eventually detected them. This tells me I should take more holidays."

S/2004 S1 could be an object spotted in a single image taken by Nasa's Voyager spacecraft 23 years ago, which was at the time given the name S/1981 S14.

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