If I had to pick a theatrical way to go out, then being quickly vaporised in a fiery blaze of glory whilst plunging into Saturn’s atmosphere early one September morning would rank quite high on that list. It certainly proved a fitting end for NASA’s Cassini probe, which culminated its twenty-year voyage of discovery on 15 September after providing numerous significant contributions to our understanding of Saturn and its surrounding bodies.
The Cassini was launched from Cape Canaveral on 15 October 1997 on a Titan IVB Centaur, set to arrive at Saturn by 2004. The initial stages of its voyage out included several calculated gravitational assists from other planets; this saw Cassini pass Venus twice, the first time achieving an acceleration of 7km/s. A flyby of the Earth on 17 August 1999 gave the craft an additional 5.5km/s boost. Alongside its own survey equipment, the Cassini also carried the ESA’s Huygens probe, which hitched a ride in ‘sleep mode’ en route to its eventual destination on Titan, where it would go on to conduct its own research mission.
the probe which boldly went where no probe had gone before.
In December 1999, Cassini became only the seventh spacecraft to pass through the asteroid belt. The area is not actually considered a hazard, and whilst there the probe was able to utilise its onboard CDA (Cosmic Dust Analyser) to study the region, providing influential new data. By 31 October 2002, the team were able to shoot their first test images of Saturn from a range of 285 million kilometres – nearly twice the distance between the Earth and the Sun, and still twenty months before the probe’s projected arrival.
On Thursday 1 July 2004, the Cassini/Huygens combined probe became the first vessel to orbit Saturn, an occasion which only somewhat overshadowed its discovery of two new moons – Methone and Pallene, three and five kilometres in diameter respectively – the day before. After over seven years travelling together, the Huygens detached on 23 December; its eventual landing on the surface of Titan some three weeks later, an outstanding success for the ESA, marked it the only landing to date in the outer solar system and setting the record for the furthest successful landing from planet Earth.
Although the immense distances travelled by Cassini are undeniably impressive, it is the information gathered by the probe once in Saturn’s system that is truly exciting, and the groundbreaking facts revealed about the moon Enceladus which are perhaps most interesting. On 16 February 2005, scientists noticed surprising readings from the craft’s magnetometer, indicating something – potentially an atmosphere – pushing back at Saturn from the vicinity of the moon. Use of the CDA revealed clouds of tiny particles, and further investigation led to examination of the moon’s southern pole.
A depiction of the mission in question. Source: NASA/JPL-Caltech
This area, the topography of which radiated heat and consisted of various fractures, craters, and fissures, proved to be not only the source of the particle clouds but also of considerable amounts of water vapour. From these ‘geysers’ the team surmised the presence of subterranean liquid water pockets; further sampling revealed a veritable cocktail comprising volatile gases, water vapour, carbon dioxide, carbon monoxide, and organic materials, some twenty times denser than expected.
Further analysis of Enceladus revealed Earth-like tectonic activity beneath its surface, and later sampling from Saturn’s outer rings revealed deposits of sodium – which, due to their location, could only have come from Enceladus – of such significant size that the moon’s subterranean water deposits were concluded to be of previously unexpected volume in order to have dissolved and held such quantities of the element.
program manager Earl Maize said that the probe had performed “just about perfectly”
By late 2009, discovery of ammonia in the area provided the final piece of evidence for large amounts of liquid water beneath Enceladus, and in 2016 the presence of hydrogen – pumped through Enceladus’ subterranean waters by hydrothermal activity – proved the moon perhaps the most likely location for extraterrestrial life in our solar system.
Other notable discoveries by Cassini include yet another moon in orbit around Saturn (the adorably tiny Aegaeon, at 0.3km in radius), the presence of an oxygen and carbon dioxide infused exosphere around Rhea, and readings from ‘interstellar dust’ believed to originate far outside of our solar system. Reflecting on the mission, program manager Earl Maize said that the probe had performed “just about perfectly”, adding that the team had managed to accrue “every last second of data”.
By April 2017, the craft had executed the ‘Ring Dive’ manoeuvre and began its final course towards the planet’s surface. NASA’s officially timeline cheerfully proclaimed that Cassini, after two decades of faithful service, would be “crushed and vaporized by the pressure and temperature of Saturn’s final embrace”. When signal was finally lost at approximately 1:45am GMT on 15 September, we said a final farewell to Cassini – the probe which boldly went where no probe had gone before.
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