On Friday, September 15th, 2017, just one month short of the 20th anniversary of its launch, the NASA/ Italian Space Agency (ASI) space probe Cassini will plunge into the upper reaches of Saturn’s atmosphere, bringing to a close the momentous NASA / ASI / European Space Agency Cassini-Huygens mission.
It will be a bitter-sweet moment for many the world over – most of all the vast international team who devoted up to fourteen years of their time on the mission – even before it launched. The Cassini vehicle has not only revealed so much about Saturn, its myriad moons, the rich complexities of the gas giant’s ring system- it has also helped inform us on the potential for life to exist elsewhere in the solar system and has even helped test Einstein’s work. It has also over the years returned some of the most stunning and evocative images of other worlds we have yet witnessed. Many of these images have been gathered together by National Geographic and have been put together in a superb interactive web presentation on the mission by Nadia Drake and Brian Jacobs.
In all seventeen countries have been directly involved in the conception, design, construction and operation of the Cassini-Huygens mission, both in terms of the Cassini orbiter and the Huygens Titan lander and the science instruments they carry. NASA carries primary responsibility for the orbiter’s design and construction, with the Italian Space Agency providing the all-important, dual-purpose high-gain radio antenna and its associated communications equipment, together with the low-gain communications suite which would provide continuous communications with Earth through the mission. ASI also incorporated a compact radar system in to the high-gain antenna systems, allowing it to function as a synthetic-aperture radar, a radar altimeter, a radiometer, and provide the visible channel portion of the VMS spectrometer package carried by the probe.
ESA was responsible for the Huygens lander, with France designing the vehicle itself, with the descent parachute system provided by Martin-Baker of America, while the science and communications packages were supplied by several European countries and the United States.
The mission was named for the Italian-French astronomer Giovanni Domenico Cassini, who first observed the divisions within Saturn’s rings system (and after whom one of the divisions is also named) as well as for of the planet’s moons, and Christiaan Huygens, the Dutch mathematician, physicist and astronomer, who first observed Titan, Saturn’s largest moon.
Work actually commenced on the mission in the 1980s, the goal being to develop a mission which could determine the three-dimensional structure and dynamic behaviour of Saturn’s ring system, investigate Saturn’s atmosphere and magnetosphere, determine the composition and likely structure of Saturn’s moons, including the nature and origin of the dark material on Iapetus‘s leading hemisphere, and, in conjunction with the Huygens lander, characterise Titan’s atmosphere, including the variability of the cloud haze, and characterise the moon’s surface at a regional level.
Initially, the mission was funded for a 10-year period from late 1997 through mid-2008, which included a journey of seven years to reach Saturn. The voyage took so long because at the time of launch, there was no launch vehicle combination capable of sending Cassini directly to Saturn. Instead, it completed a mini-tour of the inner solar system; six months after launch, Cassini flew by Venus, using the planet’s gravity to accelerate it into a wide elliptical orbit. A second encounter in June 2000 again accelerated the spacecraft, slinging it on to a further gravity-assist flyby of Earth in August 2000, which in turn accelerated it and bent it onto an trans-Jovian flight path.
In late 2000, Cassini reached the vicinity of Jupiter, making its closest approach to the planet on December 30th of that year. As well as using Jupiter’s gravity to sling it onwards to its final destination, Cassini used the encounter to study Jupiter and its faint system of rings. In all some 26,000 images of Jupiter, its moons and its rings were taken during the 6-month period of the flyby (October 2000 – March 2001). Cassini’s science suite was powered-up for the flyby, and resulted in some significant discoveries concerning Jupiter’s turbulent atmosphere, including breaking a long-held view. Jupiter’s banded atmosphere comprises a series of alternate bands of darker and lighter zones, in part caused by Jupiter’s rapid rotation. It had also been thought that the lighter bands were the result of the atmosphere rising upwards, giving rise to lighter cloud formations, before circulating downwards once more.
However, Cassini revealed the dark bands were peppered with individual storm cells of upwelling bright-white clouds too small to see from Earth, suggesting the vertical circulation of Jupiter’s atmosphere to be far more uniform than thought. The probe’s findings also showed that Jupiter’s thin and dusty rings to be made up from small, irregularly shaped particles, most likely created by ejacta from micrometeorites impacting the Jovian moons.
Cassini reached Saturn in 2004, officially entering orbit around the planet on July 1st of the year. Prior to doing so, the vehicle was part of a test of Einstein’s theory of general relativity. This states that any massive object like the Sun causes space-time to curve, causing a beam of light or any other form of electromagnetic radiation that passes close to it to travel farther (the Shapiro time delay). In 2003, with the Sun coming between Earth and Cassini, scientists on Earth measured the frequency shift in radio signals being received from the spacecraft. Similar experiments had been carried out with the Voyager and Viking missions, but Cassini provided for much more refined measurements to be taken, and firmly validated Einstein’s theory.
Cassini‘s primary mission at Saturn commenced as it approached the planet for orbital insertion. Although the orbiter was capable of functioning – all things being equal – through until around 2017, this primary mission was scheduled to last just 3 years and 261 days, ending in mid-2008. This was sufficient time for the primary goals of the mission to be achieved, but Cassini was always designed to achieve so much more. With this in mind, the programme was granted two funding extensions. The first, called the Equinox Mission, funded the project through until the end of 2010, and gave a particular focus on Titan (15 flybys) Saturn’s ice-covered moon Enceladus, thought two of the locations in the Saturnian system where life might have taken hold.
The second extension, granted funding in 2010 to the tune of around US $60 million a year, is referred to as the Solstice mission (as it would end a few months past Saturn’s summer solstice). It guaranteed that, avoiding any spacecraft failures, the mission would continue through to the point where Cassini’s manoeuvring propellants would be practically depleted. This phase of the mission allowed for a more extended study of Saturn, its rings and moons. It meant Cassini could witness never before seen seasonal changes in the planet’s atmosphere and study. It also meant Cassini could study Saturn’s atmosphere and magnetosphere at exactly the same time as NASA Juno mission studied Jupiter’s atmosphere and magnetosphere, allowing a direct comparison of the two. Finally, the extension would carry the mission through its 5-month “grand finale”, probing the region between Saturn and its complex ring system.
