Last year, after almost a decade of studying Saturn and its moons, the plasma spectrometer onboard the Cassini probe broke down. This was far beyond its planned lifespan of four years – and what’s more, it may yet have second life, with plans to revive it near the end of the mission in 2017.

In addition to its long service at Saturn, the instrument had also survived the long journey through space that had begun in 1997 – a journey in which it was subjected to the harsh environment of interplanetary space, passed Venus twice and made measurements during Earth and Jupiter swingbys.

Artist’s impression of Cassini at Saturn. Credit: NASA (public domain)

Scientists at UCL Mullard Space Science Laboratory took a lead role in the hardware development and the science team for the electron spectrometer, and while it is no longer operating, there is still plenty of work to do on the data it gathered. In particular, the detector’s studies of Saturn’s moon Titan are expected to yield further secrets.

Cassini – a NASA-led Saturn orbiter – released an ESA-built lander called Huygens to land on Titan when it arrived in the Saturn system in 2005. Huygens took the first ever picture on the surface of a body in the outer Solar System. (It is also the only landing to-date on another planet’s Moon.)

Huygens photo of Titan’s surface. Credit: NASA/JPL/ESA/University of Arizona (public domain)

But what the Cassini probe is able to do from space, which Huygens could not on the ground, is analyse the complex interaction of Titan’s thick atmosphere with the magnetic field of Saturn. Titan is surprisingly Earth-like, despite being quite small. It has a thick atmosphere and an icy, rocky surface, as well as mountains, seas, lakes and rivers (though these are made of liquid hydrocarbons such as methane, rather than water).

Hydrocarbon lakes on Titan, observed by the radar onboard Cassini. Radars do not produce colour images – in this picture, the smooth areas (lakes and rivers) have been coloured blue to improve contrast. Credit: NASA/JPL-Caltech/USGS (public domain)

But while Earth’s atmosphere is cocooned well inside our planet’s magnetic field, protecting it from the Solar wind and from interactions with other Solar System bodies, Titan’s is not. Titan spends most of its orbit within Saturn’s hot magnetosphere, meaning its atmosphere interacts with the giant planet it orbits, including the plasma and magnetic field that surrounds it. This makes Titan’s atmosphere very ‘leaky‘ compared to Earth’s.

A recent fly-by of Titan by Cassini while the moon was orbiting outside Saturn’s magnetosphere reveal another intriguing phenomenon: the Solar wind blowing part of Titan’s atmosphere away, leaving a comet-like plume of gas coming from it.

UCL is hosting the Alfvén conference this week, in which experts in planetary magnetic fields and plasmas are coming together to discuss the latest results from missions including Cassini.

Planet Earth is a vast magnet with a strong magnetic field surrounding it. As well as making compasses point north and guiding solar particles to the poles, creating the aurorae, the magnetic field protects our atmosphere. Shielded from the solar wind and from solar and galactic cosmic rays, Earth’s atmosphere is thick and life-sustaining and is not at any risk of being blown into space.

Mars – an Earth-like planet in many other respects – has lacked such a global magnetic field for about 3.8 billion years. Its atmosphere is thin – less than 1% of the atmospheric pressure we have on Earth – leaving a cold, hostile and barren surface.

The magnetic fields of Earth (left) and Mars (right). Earth has a strong, planet-wide magnetic field that shields our atmosphere from the Solar wind. Mars has only small localised areas of magnetism called crustal fields, meaning its atmosphere interacts directly with the Solar wind. Credit: NASA/GSFC (public domain)

The lack of magnetic field makes Mars different from Earth in other respects too. In fact, in some ways Mars behaves much like a comet: since the Solar wind interacts directly with the atmosphere a comet-like tail of gas is constantly being blown off the planet (albeit not to the extent of a comet as it passes near the Sun).

UCL researchers have studied the behaviour of Mars and its interaction with the Solar wind for many years, including through participation in the European Space Agency’s Mars Express mission. Orbiting Mars since 2003, the mission has been a huge success (unlike the Beagle 2 lander it carried), and like many missions it carries UCL hardware contributions. This comes in the form of a sensor in the ASPERA-3 instrument, for which UCL carried out calibration and blackening (to prevent internal reflections degrading observations) prior to launch.

Computer rendering of the Mars Express probe. Credit: NASA/JPL (public domain)

The ASPERA-3 instrument has helped UCL scientists study Martian magnetic anomalies called crustal fields. These are small, localised magnetic fields on the surface of Mars which interact with the Solar wind and material escaping the Martian atmosphere. It has also observed how the faint ‘tail’ of material streaming from Mars has photoelectrons caused by the Sun.

UCL is also involved in the exploration of the Martian surface – it leads the team building the main camera for the ExoMars Rover, Europe’s first Mars rover, which touches down in 2019.

The Alfvén conference, being held at UCL this week, is bringing together experts from around the world in the field of electrical, magnetic and plasma interactions in the Solar System. Today’s focus is Mars. Among the presentations will be new research on the data sent back by Mars Express, as well as the latest news from NASA’s MAVEN mission and India’s Mars Orbiter Mission, both on route to the red planet with expected arrival in September 2014.

This evening there will be a public lecture at UCL about the Rosetta mission, which is currently approaching Comet C-G. Matt Taylor, the scientific leader of Rosetta, will outline the probe’s amazing ten year journey, outline Rosetta’s scientific goals and explain why comets are such an important window into the distant past of our Solar System. The lecture is free and open to all, but please reserve your seat as spaces are limited.

The European Space Agency’s Rosetta mission, coming to its climax only now, a decade after launch, will have a number of scientific firsts: the first probe to follow a comet around the Sun while it orbits, the first to orbit a cometary nucleus, and the first to deploy a lander on the comet’s surface.

UCL has made a major contribution to the scientific programme the spacecraft, thanks to its long-standing expertise in plasma physics. Much of this expertise was gained in a previous space mission.

Artist’s concept of the Giotto spacecraft approaching Comet Halley. Credit: Andrzej Mirecki (CC-BY-SA 3.0)

In 1986, the Giotto probe flew by Comet Halley, giving the first ever close-up view of a comet’s nucleus. UCL built the probe’s Fast Ion Sensor and led the Johnstone Plasma Analyser team. Thanks to this detector, scientists were able to study the interaction between the comet and the solar wind as the comet ploughed through it, including the enormous bow-shock in front of the comet that extended for around a million kilometres, and the processes by which cometary ions are ‘picked up’ by the solar wind.

Rosetta should give us a lot more than this. Two decades of technological progress mean sharper photos and more sensitive instruments. But the major improvement is in how long the probe will be able to study the comet for, as well as its close-up study of the comet’s ionosphere. Giotto made only a relatively short flyby of Halley, in large part because the comet’s orbit is highly inclined and moves in the opposite direction to Earth’s, and it only passes through the plane of the Solar System on two brief occasions every 76 years. Comet Churyumov-Gerasimenko, Rosetta’s target, is in an orbit that is easier to rendezvous with, which means – with some admittedly difficult and time-consuming orbital mechanics – Rosetta can be brought into orbit around it.

This, in turn, means the changes in the comet, including the growth of its coma, and plasma (gas) and dust tails, can be charted in detail as it proceeds around the Sun.

Computer rendering of the Rosetta spacecraft. Credit: public domain (Celestia)

UCL scientists at the Mullard Space Science Laboratory are particularly closely involved in the Rosetta Plasma Collaboration, a scientific group which will study the electrical properties of the comet as it interacts with the Solar wind. This is a direct continuation of the work done with Giotto nearly three decades ago.

The wait won’t be long now – Rosetta is less than 30,000km from its goal (less than the distance between Earth and geostationary communications satellites), and will begin manoeuvring into orbit in early August. Early data will come sooner, with the first detection of the comet’s plasma expected perhaps as early as this week.

Rosetta is one of a number of missions being discussed at the Alfvén Conference this week at UCL. Named after Hannes Alfvén, a Swedish plasma physicist and Nobel laureate, the conference is bringing together experts from around the world in the field of Solar System plasma interactions, such as those Rosetta will study.

As part of the conference, Matt Taylor, the scientific leader of the Rosetta project will be giving a public lecture tomorrow (Tuesday 8 July) at 6pm. He will outline the probe’s amazing ten year journey, outline Rosetta’s scientific goals and explain why comets are such an important window into the distant past of our Solar System. The lecture is free and open to all, but please reserve your seat as spaces are limited.