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More from Saturn… including the discovery of a "Trojan" moon

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Particle Physics and Astronomy Research Council Press Release

For Immediate Release

25 January 2005

The joint NASA/ESA/ASI Cassini Huygens space mission continues to provide further insight into Saturn and its satellites with further results, including the discovery of a new moon and surprise findings about Saturn's magnetic field, being published in the journal Science today (25th February 2005). The papers, which include several UK scientists as co-authors, include the initial results from the Imaging Science Subsystem (ISS) team and the observations of Saturn's magnetic field by the Magnetometer team.

The discovery of a new moon (provisionally named Polydeuces), by the Imaging Science Subsystem team brings the total of confirmed moons in the Saturnian system up to 34. ISS team member Professor Carl Murray from Queen Mary, University of London explains,

"After the initial detections of the moon in October we were able to look back at earlier images and confirm its presence. This record enabled us to calculate a good orbit and confirm that that it is in fact a small Trojan moon (about 5 km or 3 miles across) in the same orbit as Dione."

Saturn is the only planet known to have Trojan moons. Trojan moons are those found near stable "Lagrange points" (see notes to editors) and are situated 60 degrees ahead or behind a larger moon in its orbit around a planet.

Professor Murray continues, "Unlike Helene, Dione's other Trojan moon, Polydeuces can get as close as 39 degrees to Dione and then drift as far as 92 degrees from it, taking over two years to complete its journey around the Lagrange point. The extent of this wandering is the largest detected so far of any Trojan moon."

The first observations of the Saturn's magnetic field at Saturn for 23 years reveal a very dynamic and changeable environment over times scales of minutes to days. Since the Pioneer and Voyager flybys in the 1970's there has been no noticeable change in the internal planetary field at Saturn. However, the magnetic field generated external to the planet was found to be very different.

Professor Michele Dougherty from Imperial College, Principal Investigator on the Magnetometer instrument explains about some of the other findings from her instrument,

"On approaching the 'E' ring we saw definite troughs in the magnetic field. These troughs were compensated for by peaks in plasma density - indicating that bubbles of plasma were moving past the spacecraft and away from the planet. This suggests that the 'E' ring is acting as a source of plasma for the magnetosphere. Further work is required to confirm this but it looks very similar to conditions observed at Io and Jupiter."

She continues, "We also recorded a first at orbit insertion! On flying through the engine exhaust plume which was generated by the firing of Cassini's main engine we observed the first detection of artificially induced plasma waves in a magnetosphere other than our own."

Further details about the Cassini findings as published in Science today can be found at:-


Gill Ormrod - PPARC Press Office

Tel: 01793 442012. Mobile 0781 8013509


Carolina Martinez  - Jet Propulsion Laboratory, Pasadena, Calif.

Tel: 00 1 (818) 354-9382. Email:

Professor Carl Murray - Co-Investigator on Imaging Science Subsystem team

Queen Mary, University of London

Tel: 020 7882 5456. Mobile: 07976 243883


Professor Michele Dougherty - Principal Investigator, Magnetometer

Imperial College London

Tel: 020 7594 7757. Mobile: 07990 973761.


Notes to Editors

Cassini moon discoveries

Prior to 2000 the total number of Saturnian moons was 18. Prior to Cassini the total has risen to 31 with the extra 13 being so-called irregular moons orbiting at the outskirts of the Saturn system.

The first two new "Cassini" moons were discovered in August from images taken in June 2004. These have been provisionally named Methone and Pallene. They orbit between the moons, Mimas and Enceladus. This brought the total to 33.  The next two objects discovered (again in August 2004) are as yet unconfirmed to be moons.

The latest discovery of Polydeuces was first made in October 2004. It is in the same orbit as Dione and along with Helene, is an example of a Trojan moon. This brings the current total to 34.

Naming of Moons

Satellites in the Saturnian system are named after Greco-Roman Gods and they either have Gallic, Inuit or Norse names depending on their orbit group. Polydeuces was the twin brother of Castor, son of Zeus and Leda; he was also the brother of Helen of Troy. For a full list of Saturn's moons, the origin of their names and their discoverers see:-

Trojan moons

The term "Trojan moons" comes from the example of Trojan asteroids that are in the same orbit as Jupiter. Dione is not the only Saturnian moon to have Trojan moons associated with it. Tethys has Telesto and Calypso.

Lagrange point

Lagrange points arise as a special solution of the "three body problem" - the behaviour of three masses moving under their mutual gravitational effect.  If there is a central planet with a moon in orbit around it then there are five special points (the Lagrange points) where the gravitational effects of the planet and moon are balanced.  At these points a small third object could remain in a fixed configuration.  Three of these points (L1, L2 and L3) lie along the line joining the planet and the moon and are known to be unstable such that any object located nearby would drift away.  However, it can shown that the remaining two points are positions of stability, provided the moon's mass is less than about 4% of the planet's mass.  These are the L4 and L5 points located in the orbit of the moon but 60 degrees ahead of and 60 degrees behind the moon.  In reality objects do not remain precisely at these points but undergo slow, pendulum-like oscillations around the points.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.  The imaging team is based at the Space Science Institute, Boulder, Colo.

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.