By Dr Claire Bretherton, Curator Science

On 15th October 1997, Cassini-Huygens blasted off from Cape Canaveral in Florida on its way to Saturn, the first spacecraft to enter orbit around the ringed planet. In the two decades since Cassini-Huygens has completely changed our knowledge of the planet, its stunning rings and dynamic moons, and taken some of the most awe-inspiring images of the Solar System we have ever seen.

The Cassini mission has always been dear to my heart. It launched on the 15th October 1997. I was 18 at the time, and just a couple of weeks into my degree at Leicester University studying Physics with Astrophysics. Seven years, and a long long journey later, which for Cassini took it half way across the solar system, and for me took me through an MPhys, teacher training and half of my PhD, we were both ready to do some real science. We’d both dabbled a little along the way, but now, as it entered orbit around Saturn, to begin its initial four year science mission, Cassini was ready to revolutionise our understanding of the mysterious ringed planet and change the course of future planetary exploration.

Cassini 1

Cassini-Huygens was a collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) and involved scientists and engineers from 27 different countries. NASA and ASI were responsible for the Cassini orbiter, named after the 17th century Italian astronomer Giovanni Domenico Cassini, who discovered four of Saturn’s moons and identified a gap in Saturn’s rings, which is now known as the Cassini Division.

View of Titan
First colour view from the surface of Titan taken by the ESA Huygens probe. NASA/JPL/ESA/University of Arizona

ESA contributed the Huygens lander, named after the Dutch astronomer Christiaan Huygens, who discovered Saturn’s largest moon, Titan. On the 14th of January 2005, Huygens was dropped onto Titan, braking in the moons dense atmosphere, and parachuting down to the surface. On its way down, the lander collected ground-breaking new data on Titan’s atmosphere, clouds and surface features.

Meanwhile, Cassini continued to study the Saturn system from orbit, providing tantalizing evidence of the diversity and dynamism of its many moons and studying the planet’s atmosphere, magnetic fields and ring system in unprecedented detail.

With its initial science goals completed in 2008, Cassini’s mission was extended. The two-year Cassini Equinox mission included 60 additional orbits of Saturn and over 30 flybys of some of its moons. It also allowed the spacecraft to observe Saturn’s rings as they were lit edge on by the Sun, revealing stunning new detail and previously unseen structure.

In 2010, the mission was extended again. Known as the Cassini Solstice Mission, this second, seven year long extension would take us all the way past Saturn’s northern hemisphere summer solstice. Having originally arrived just after the northern hemisphere winter solstice, this allowed us to observe a complete seasonal period and to complete more in-depth studies of both Titan and Enceladus.

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A huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet. NASA/JPL-Caltech/Space Science Institute

There are so many highlights from the Cassini-Huygens mission that it’s hard to pick a favourite. The mission discovered seven new moons, revealed new structure and activity in Saturn’s rings, observed the changing colour of the planet’s north-pole hexagon, and witnessed the aftermath of a massive reoccurring storm seen in 2010. But to me the mission’s greatest successes come from its in-depth study of some of Saturn’s many moons, which have resulted in some truly surprising discoveries.

These natural colour views compare the appearance of Saturn’s north-polar region in June 2013 and April 2017. 

Cassini-Huygens has found rivers, lakes and seas of liquid methane and ethane covering Titan’s polar regions, complete with rain from hydrocarbon clouds, whilst the moon’s equator is marked by icy ‘sand dunes’ hundreds of kilometres long and 100 metres high.

Near-infrared, colour mosaic from NASA’s Cassini spacecraft shows the sun glinting off of Titan’s north polar seas. NASA/JPL-Caltech/University of Arizona/University of Idaho

It has tasted Titan’s thick atmosphere to find a rich and complex chemical soup that still can’t be fully explained and revealed conditions that may be similar to the early Earth. Meanwhile, gravity experiments have revealed evidence for an ocean of liquid water and ammonia deep underground.

Exploration of the icy moon Enceladus has revealed a hot spot at the southern pole, icy jets spewing out from the surface and a vast salty ocean below the ice.  These jets contribute directly to Saturn’s diffuse E-ring and further study of the ring’s chemistry suggests the existence of hydrothermal vents, not dissimilar to those found on Earth. Together, these astounding new results promote Enceladus to one of the top candidates in our search for life beyond our own planet.

Cassini’s last Enceladus plume observation. NASA/JPL-Caltech/Space Science Institute. 

In April this year, after almost 13 years of discovery, and with its fuel running low, Cassini embarked on its Grand Finale, a series of death defying dives between the planet and its rings, before plunging into Saturn on the 15th September, just a month out from its 20th birthday. Even in its final moments Cassini continued to send data. Whilst the space craft is gone, its legacy lives on in our new understanding of the jewel of our Solar System.

In Saturn’s shadow. Saturn eclipses the Sun allowing Cassini to capture this spectacular view. Look out for the tiny pale blue dot just outside the brighter rings – this is our own planet, the Earth. NASA/JPL/Space Science Institute.