Cassini-Huygens is one of the most successful and ground-breaking space missions ever launched, and has just ended with a fiery descent into Saturn’s atmosphere. The mission spanned decades, revolutionised our understanding of the planet Saturn and its system of rings and moons, and launched the careers of a generation of planetary scientists. In this post we look back at the major achievements and legacy of Cassini-Huygens.
The mission began with discussions in the early 1980s and is the result of a major international collaboration involving both the USA’s National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Cassini-Huygens is named after two 17th century astronomers who made discoveries about the Saturn system simply by looking through telescopes. Members of the science team can be found worldwide, including many in the UK, and Imperial College London has overall responsibility for one of the seven scientific instruments that flew on the spacecraft.
Cassini-Huygens launched in 1997, took about seven years to reach Saturn in 2004, and only met its end this year, 2017. The range of major science results from the mission include:
The Huygens probe lands on Titan: Saturn’s largest moon, Titan, has a thick, nitrogen-rich atmosphere, leading many to refer to Titan as “primordial Earth in a freezer”. Shortly after arrival at Saturn the ESA Huygens probe detached from the spacecraft and descended to the surface of Titan on 14 January 2004, becoming the first probe to land on a world in the Outer Solar System. Thanks to Huygens and many Cassini spacecraft flybys over more than a decade we now know that it rains methane on Titan, forming rivers and lakes. Titan’s atmosphere is a soup of complex hydrocarbons, making Titan a favoured destination for future exploration.
Discovery of the plumes of Enceladus: Saturn’s moon Enceladus was not expected to be the site of one of the biggest discoveries in solar system science. During a flyby of this icy moon the UK-led magnetometer instrument (a “space compass”) measured a disturbance in Saturn’s magnetic field, and the rest is history. The disturbance was shown to be evidence of a dynamic atmosphere now called the plumes of Enceladus. Further observations of more flybys showed that these are plumes of water coming from cracks in Enceladus’ icy surface near the moon’s south pole, which likely originate from an ocean beneath the ice. Enceladus is now regarded as somewhere we may one day find life that did not start on Earth.
Revealing how active Saturn’s rings are: Thanks to many years in orbit Cassini’s cameras have shown us how active and dynamic the planet’s distinctive ring system really is. Cassini has seen structures in the rings that look like propellers, and witnessed what might be the birth of a new moon. Also, while we knew Saturn’s rings were thin, the changing seasons experienced by Cassini allowed us to see shadows on the rings that gives researchers what they need in order to calculate just how thin they are, to greater accuracy than ever before.
Storms and the hexagon in Saturn’s atmosphere: The long time spent at Saturn by Cassini has revealed that the mysterious hexagonal-shaped jet stream in Saturn’s atmosphere around the north pole is stable over years, as are storms at both poles, and scientists are still not in agreement on why there are these structures in the giant planet’s atmosphere. Cassini also saw an enormous storm emerge in the atmosphere in 2010, which lasted for almost a whole year.
How long is a day on Saturn?: We still don’t know! This might seem like an easy thing to figure out, but the different bands of Saturn’s atmosphere rotate at different rates, only constraining the length of a day to somewhere between 10 and 11 hours. The wobble of the planet’s magnetic field and periodic changes in radio waves coming from near Saturn were expected to give us a better idea, but Cassini has shown that this gives a different length of day in the North compared to the South, and these both change as Saturn travels around the Sun. Revealing such mysteries to be tackled by present and future scientists is the hallmark of the most pioneering missions, like Cassini.
Even at the end of the mission when Cassini skimmed Saturn’s atmosphere before its final dive to destruction the spacecraft was taking and transmitting data that will lead to more major discoveries in the near future, which we will need to add to the above list.
At the very end when the last communication was received from the spacecraft there were mixed emotions across the science team. Many of our scientific careers were launched by Cassini, including my own, and so we shared in a sense of sadness at the end of such a spectacular mission. However, I felt we also had a strong shared sense of pride in what we had collectively achieved as an international community, and gratitude to all those who also did their bit to make it possible.
The design, operation, and exploitation of data from Cassini represents a monumental and historic achievement, and has driven us to define new and ambitious future missions that will continue our exploration of the Solar System. Scientists in the UK remain part of a strong community, and continue to build instruments for future missions. Our focus is now starting to shift to Jupiter with its almost-planet-sized moons Europa and Ganymede, and a certain future mission.