Cassini: Mission’s End at Saturn
September 5, 2017

Professor Nicholas Achilleos of University College London (UCL) Physics & Astronomy talks to Europlanet about Cassini’s final days and what working on the mission has meant to him  professionally and personally.

On the 15th of September, 2017, the Cassini spacecraft will plunge into the atmosphere of the planet Saturn, bringing to an end its mission as a dedicated orbiter of this beautiful ringed world, which it has been orbiting now for 13 years.

It is both an exciting and very sad occasion. Exciting because we know that the mission has been a resounding success, not only in terms of the scientific advancements it has enabled but also because it has accumulated an enormous legacy dataset, whose analysis will be keeping future generations of scientists occupied for many years to come. Sad because it means saying goodbye to the project itself.

I am a science co-investigator with the Cassini Magnetometer Team, led by Principal Investigator Professor Michele Dougherty from Imperial College London. The magnetometer, or MAG, is an instrument whose sensors are perched on an 11-metre boom which extends out from the side of the spacecraft – this arrangement is necessary for us to minimise the ‘contamination’ of the magnetic field measurements by field from spacecraft electronics.

Artist’s impression of Cassini in orbit at Saturn, with the magnetometer boom extending to the left. Credit: NASA/JPL-Caltech

Since 2004, MAG scientists have been mapping the magnetic field which is generated inside Saturn itself, and the field due to external sources which we find in Saturn’s magnetosphere, an enormous region surrounding the planet where physics is strongly influenced by the planet’s own magnetic field. Saturn’s internal field is almost perfectly symmetric about the planet’s rotation axis – making it almost unique among the magnetised planets, of which the Earth is of course another example.

The spacecraft is currently executing its final ‘proximal orbits’, which bring it closer to the planet than it has ever been, and which are critical for MAG analysis. We are hoping to accurately determine the apparently very small non-symmetric part of the field – this is crucial not only because it will give us information about how the field is being produced by the ‘magnetic dynamo’ in Saturn’s interior, but also because it will finally allow us to unambiguously pin down the rotation period of Saturn itself – that is, the exact length of a ‘Saturn day’.

There are two other important discoveries associated with MAG. Firstly, Cassini confirmed the earlier discovery by Voyager of a periodic ‘signal’ in the magnetic field throughout the planet’s magnetosphere; we have good reason to believe that this signal is an indication of energy being transferred from flows in the planet’s atmosphere out to its magnetosphere. This energy is transported over distances of millions of kilometres, and the magnetic field acts as the ‘wire’ along which this energy is transported.

Secondly, MAG was the first instrument to report something unusual at the first flybys of Saturn’s tiny icy moon, Enceladus. The field measurements indicated that Enceladus seemed to have something like a very extended ‘atmosphere’ of sorts. These data were sufficient to convince mission control to fly even closer to Enceladus on the next flybys and, indeed, during those flybys we obtained images of the incredible water plumes, or geysers, which continually throw out water molecules and ice grains from cracks in the icy surface of Enceladus. We now know that this source of water is also the principal source of plasma (charged particles) in the planet’s magnetosphere – thus making the small moon Enceladus the ‘little, but powerful engine’ that drives the much more enormous magnetosphere of its parent planet.

This artist concept shows the detection of a dynamic atmosphere on Saturn’s icy moon Enceladus. The Cassini magnetometer instrument is designed to measure the magnitude and direction of the magnetic fields of Saturn and its moons. Credit: NASA/JPL-Caltech.

The Cassini mission has given me, and many others, the privilege of collaborating with many greatly talented scientists, engineers and mission planners. I have played the roles of mission planner, support scientist and now co-investigator. I have had the good fortune to work with very talented graduate students who have written theses about the MAG and other Cassini datasets, and it has been a great pleasure to watch them blossom into talented scientists. The Cassini community, I’m sure, will continue to collaborate together in different ways in the future. For now, the approaching end of mission should be viewed as a commemoration of an enormously successful international,  scientific project – and a timely reminder of what humans can achieve when we respect each others’ abilities and differences, so that we can work together towards a common goal.