November 14, 2011

The Cassini Probe and Saturn’s Rings

Dr. Carl Murray
Report by: Chris Gadsden

Carl Murray is a member of staff at Queen Mary, University of London, and is part of an international team of astronomers and scientists working on the Cassini-Huygens mission to Saturn. His speciality is the study of Saturn’s ‘F’ ring.

The lecture started with the projection of an interesting slide showing the ‘G’ and ‘E’ rings with a view of an extremely small planet Earth as viewed from Saturn.

Prof Murray explained that the Cassini-Huygens project is a joint NASA/ESA robotic spacecraft mission to study the planet Saturn, its moons and rings.  An Announcement of Opportunity was made in October 1989, and the spacecraft was launched in October 1997.  Flyby of Jupiter, when the spacecraft received a ‘slingshot’ from Jupiter’s gravity, occurred in December 2000.  The craft entered into orbit around Saturn in July 2004 to begin what was then planned as a 4-year tour of the Saturn system.

Carl went on to talk about the UK involvement in this project, which consists mainly in the supply of 6 of the 12 Cassini instruments, still in orbit, and 2 of the 6 Huygens instruments.  Huygens was the Titan lander.

He projected a slide of the Cassini team, including himself, followed by a series of fascinating slides about the mission and Saturn:-

> The route to Saturn and the Saturnian System

> Titan, image taken with filters to enable vision through the thick atmosphere

Huygens discovered Titan in 1655, one of the more than 60 moons of Saturn.  Titan is the largest moon and has an atmosphere similar to the early atmosphere of Earth. Its atmosphere is thick and pressure is 50% greater than on Earth.

> Radar image of Titan

> Ice plumes (“jets”) being ejected from Enceladus – evidence of water

Enceladus, Saturn’s sixth largest moon, has a “young” surface with smooth areas; it does not have an atmosphere.  There is water on the surface, and large quantities of icy material are released in the form of “jets” from its southern surface

> View of terminator with plumes at S pole and the Moon at N

> Artist’s impression of rings (stunning!)

> Sequence of movement in the rings

> Rings from on top and from afar

Prior to the Cassini probe, we had no close-up of the rings of Saturn. The rings have structure and it is difficult for starlight to penetrate.

> Evidence for orbital evolution of Bleriot “propeller moon”

There are many “propeller moons” which are too small to be seen but which create disturbances within Saturn’s rings, such as Bleriot in the ‘A’ ring.

> Daphnis in the Keeler gap

> Regular structure across D & C rings

> Several images of the F rings between 2006 & 2008 showing jets and variations in the F ring shape

> Finally, a slide showing a new moon of Saturn: the discovery of Polydeuces

Carl then discussed a most interesting topic: “How to discover a Moon”, detailing the following required steps:-

> Initial detection of a moving object

> Check that it behaves in accordance with Kepler’s Law

> Draw preliminary orbit, check that it is a freely precessing ellipse

> Additional detections

> Draw improved orbit

> Fit observations to numerical integration

> Produce “final” orbit

> Detect it again!

There then followed a short sequence showing the discovery of the moon Anthe on 22 June, 2007, followed by another sequence showing Anthe’s orbital evolution.

And finally, in a lighter vein to wind up the lecture, Carl projected two slides: His room when an undergraduate at Queens’ University, Belfast; and a picture of him standing next to Richard Garriot (astronaut and visitor to the ISS) and Buzz Aldrin (lunar module pilot of Apollo 11).

One more fascinating snippet of information from Carl before closing the lecture: The Cassini Mission is now expected to end in 2017 – a long way past its planned service expiry date of 2008!

As befits such an absorbingly interesting lecture, many questions were thrown up from the floor of the packed lecture hall, to which Carl responded with his usual clarity and humour.

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