Illustration of the Huygens probe after landing on the surface of Titan. Image credit: ESA/C. Carreau

Jan 14, 2015 Reflections on Titan ten years after Huygens

Sen—Can it really be 10 years this month since we landed on the surface of Titan, Saturn’s largest moon, with our beloved Huygens Probe—and with the technology of the late 1980s?

Well, it must be, as I have a certificate from Guinness World Records which states “In recognition of the participation of the Open University (UK) in the most distant landing in the Solar System. The Huygens spacecraft landed successfully on Titan on 14 January 2005”!

How come I was involved? Once the European Space Agency (ESA) and NASA had decided to embark on this adventure together to explore Saturn, its moon Titan and the Saturnian System, they threw open to the scientific community the opportunity to provide the scientific instruments which are of course the raison d’être for the mission. I put together a team, way back in the early 1990s, and we bid to provide the Surface Science Package. And I was selected along with 5 other scientists (from France, Germany, Italy and the USA) as Principal Investigators (PIs), each of us leading international teams. So that is how I came to be at the European Space operations Centre (ESOC) on 14 January 2005.

That was a day that I will never forget. I experienced every emotion imaginable as the 350 kg spacecraft hit the top of Titan’s thick atmosphere at 6.8 km/s, slowed down by atmospheric drag then floated down to the surface by parachute over the next 2½ hours. At least that was what we hoped would happen.

We had worked towards that outcome for the previous 15 years—us academics in universities and the teams in industry and in ESA and NASA. But when it came to this final, most critical phase, we were totally powerless. This was because from the instant that Huygens was released from its Cassini mothercraft on 25 December 2004, it was totally autonomous—it was on its own. It didn’t have a radio receiver, so that even if we had wanted to control it we couldn’t. It was set on a collision course with Titan and had been programmed to deal with anything that Titan could throw at it. That was the plan.

We knew what should happen, but knowing both the spacecraft and being aware of the uncertainty of our knowledge of Titan, we were only too aware of the potential hazards. Would the winds and turbulence in the atmosphere be more extreme than we had thought? Would the heat protection system be able to cope with the enormous heat pulse generated by the extremely high speed entry? And as for the landing, of course we had no idea what surface we would land on. Would it be solid ice, as hard as concrete at the surface temperature of around -170 °C that we knew from Voyager observations existed on the surface? Or would it splash down, and sink, in the hypothesised sea of liquid methane proposed by some scientists to exist there? And maybe even worse, if Huygens did survive, would it find a bland, boring, wishy-washy place that we had maybe wasted the previous 15 years working towards making sense of?

Well bland and boring it is not! With what Huygens told us at the time and the subsequent periodic flybys of Titan by Cassini, we now know that Titan is truly a remarkable place. It has 2 large seas and numerous lakes of liquid hydrocarbons—hills, rivers, dune fields, perhaps cyrovolcanoes, clouds and weather.


This image was returned on 14 January 2005, by ESA's Huygens probe during its successful descent to land on Titan. This colour view, following processing to add reflection spectra data, gives a better indication of the actual colour of the surface. Initially thought to be rocks or ice blocks, they are more pebble-sized. The two rock-like objects just below the middle of the image are about 15 centimetres (left) and 4 centimetres (centre) across respectively, at a distance of about 85 centimetres from Huygens. Image credit: ESA/NASA/JPL/University of Arizona

As is so often the case in space exploration, what we find actually poses as many new questions as old questions that it answers.  

So as we start 2015, my New Year’s resolution is to redouble our efforts to go back to Titan—this time, with a floating probe so that we can explore the only other surface seas in our Solar System. 

When we gather at ESOC later this month, it will be to celebrate this adventure that we have taken together, to review what we have achieved and also to look forward to future exploration of this wonderful place that we have just started to reveal.

Though I’m sure our meeting at ESOC will be a cheerful event, it will also be bittersweet. Of the original six Principal Investigators, we have lost one, Hasso Niemann, who was happily able to see the fruits of his labours as he was at ESOC to oversee his instrument measure in great detail the composition of Titan’s atmosphere. Sadly the ESA Project Manager, who had driven us hard but fairly, throughout the Project, witnessed the launch but didn’t make it through to the landing on Titan. We shall be remembering them and other colleagues whom we have lost along the way.


Ligeia Mare in a false-colour image from Cassini flybys between February 2006 and April 2007. Ligeia Mare is the second largest known body of liquid on Saturn's moon Titan measuring roughly 420 km x 350 km and its shorelines extend for over 3,000 km. It is filled with liquid hydrocarbons, such as ethane and methane. Image credit: NASA/JPL-Caltech/ASI/Cornell

Animation of Huygens' descent and touchdown created using real data recorded by the probe’s instruments as it descended to the surface of Titan, Saturn’s largest moon, on 14 January 2005. Animation credit: ESA–C. Carreau/Schröder, Karkoschka et al (2012). Image from Titan’s surface: ESA/NASA/JPL/University of Arizona