This documentary is about the end of the Cassini-Huygens space-research mission commonly called 'Cassini'. This was a collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. From its launch in October 1997 to nearly 20 years later in September 2017, the craft has nearly run out of fuel and reached the end of its original and extended missions. 'Horizon' (BBC) visits some of the main scientists working on the 'Cassini' ...