From 1997 to 2017 – an intriguingly complex mission comes to an end

Our series of articles will look back on the mission, its experiments, the scientific concept, its most important findings and the contributions provided by DLR and other scientific institutes in Germany. You will find all articles here.

On 15 October 1997 the Cassini spacecraft took off from Cape Canaveral and embarked on an almost seven-year journey to the Saturn System, atop a Titan 4B rocket. It circled the planet and its numerous glacial moons for almost 13 years, from 2004 to 2017. When it was first launched, however, the spacecraft headed in the other direction, towards Venus, to gather travelling speed during two flyby manoeuvres. The final Earth flyby in August 1999 propelled Cassini towards the outer Solar System, initially in the direction of Jupiter.

The Jupiter flyby took place exactly on New Year's Eve 2000 and will forever be remembered as the 'Millennium Flyby'. At the same time, the Galileo orbiter was still present in the Jovian system, which enabled the first joint exploration of Jupiter by two spacecraft that produced comparative measurements. It took Cassini another three-and-a-half years to travel from Jupiter to the Saturn system. The first spectacular flyby of one of Saturn's natural satellites took place three weeks before arrival, when Cassini passed the small moon Phoebe, the largest of the planet's outer moons.

Daring manoeuvre on arrival

The Cassini-Huygens spacecraft was travelling at a breakneck speed when it arrived at Saturn. It fired one of its main engines in order to slow down enough to be captured by Saturn's gravity. The six-ton spacecraft approached Saturn from below the ring plane, and crossed through the gap between the F and G rings. The spacecraft's main engine fired shortly after passing through the rings to slow Cassini-Huygens enough to be captured by Saturn's gravity. The main engine was turned to face the direction of travel, and the resulting thrust acted as a braking device, slowing down the spacecraft as it entered Saturn's orbit. The spacecraft was turned around and sent through the rings with its antenna pointed in the direction of travel, to protect the probe from any incoming particles of ice and dust.

That marked the start of its four-year nominal mission. Following an initial flyby past Titan in October 2004, the European landing craft Huygens separated from the orbiter on 25 December 2004. Slowed down by a system of parachutes, Huygens touched down on Saturn's largest moon three weeks later, on 14 January 2005. Cassini's trajectory was constantly modified during the many other flybys past Titan, as were its orbital inclination and distance from Saturn. These manoeuvres were necessary to allow observation of Saturn and its rings from above, even at the higher latitudes. The greatest distance from Saturn was used for a tight flyby past the distant moon Iapetus. The spacecraft was forced to return repeatedly to the equatorial plane, as all of Saturn's larger moons are located there.

Saturn's equinox and summer solstice – the second and third mission phases

At the end of its four years in orbit, NASA decided to extend the mission for another two years in 2008. This extension was called the 'Equinox Phase', as it marked the start of spring in the northern hemisphere of the planet, after the actual equinox. This enabled imaging of Saturn's northern hemisphere and the northern latitudes of its moons. In addition, it was possible to observe the rings at precisely the moment when sunlight struck their edges. The orbit was changed continuously by Titan flybys during this period as well.

In 2010, NASA approved another mission extension – this time for another seven years – as all instruments and the spacecraft itself were working smoothly at the end of the preceding extended mission phase. NASA named this period the 'Solstice Phase' to mark the natural phenomenon unfolding in the northern hemisphere during this time. The Sun's favourable position allowed the first ever observation of Saturn's North Pole region.

The inevitable end of the mission

The 2010 extension was always meant to be the final one. On 15 September 2017 the orbiter will be directed straight at Saturn to meet its fiery demise. The reason for this is that the fuel supply is coming to an end and NASA will lose control of the spacecraft in the foreseeable future. Scientists fear that the uncontrolled spacecraft might collide with one of Saturn's icy moons that could be harbouring life. At the end of its mission. Cassini will have completed 22 orbits truly spectacular final orbits: from April 2017, the orbiter criss-crossed the rings, coming closer than ever to Saturn and its blanket of clouds. These orbits have allowed for high-resolution observation of the irregular moons, otherwise hidden away in Saturn's rings.

For just under 20 years, the 12 instruments on the Cassini spacecraft and the six instruments in the Huygens lander captured an immense volume of data that was transmitted back to Earth. To be on the safe side, particularly important data was transmitted twice. The spacecraft's four-metre antenna managed this data volume by sending it almost exclusively to NASA's three large, 70-metre antennas. Among other things, the data set included 6337 images from the high-resolution camera, 2486 images from the wide-angle camera, and 404,835 datasets from the visible and infrared spectrometer. All of the data was archived in NASA's Planetary Data System (PDS) and is now available to scientists worldwide, as well as to the general public. This is not just raw data, but also compiled data such as image mosaics and maps.

Intricate imaging plans for 12 orbiter experiments

Planning the imaging for all experiments was an extremely complex task. Firstly, collecting more data than would be possible to beam down to Earth was pointless – but naturally all operators of the 12 instruments were keen to acquire more data than could actually be transmitted. The second problem was that all of the instruments were firmly fixed in place, and there was no platform to rotate them individually, so pointing all of the instruments at the surface simultaneously was not an option. Only the remote sensing instruments were rotatable, enabling synchronous observation. Turning the spacecraft by 90 degrees took around 30 minutes, so it was necessary to decide which experiment would be the main instrument before each of the rapid flybys. The operators of the particle detectors often wanted to look in different directions than the scientists controlling the optical instruments. During flybys it was necessary to point the main antenna towards Earth to measure the mass of the moons using radio signals. But this made imaging the surface practically impossible.

Scientists were assigned to four large focus groups in order to resolve these conflicts. In a first stage, they agreed on the most important issue to be resolved in a particular orbit and how much data volume would be available. This led to detailed planning, involving issues such as: which instrument will plan the spacecraft's alignment and what volume of data will it be assigned? This was an iterative process that began long before arrival at Saturn and did not end until just recently. Numerous facilities in the United States and Europe – including the To the Institute's website – were involved in the planning process, so strict coordination and control by the Jet Propulsion Laboratory, which conducted the mission on behalf of NASA, were imperative. JPL reviewed all plans to ensure their feasibility before transmitting commands to the spacecraft.

The DLR Institute of Planetary Research planned the point- and second-exact target sequences for four medium-sized moons (Dione, Rhea, Iapetus and Phoebe) allowing the Imaging Science Subsystem (ISS) to take black-and-white and colour images of the atmosphere, rings and moons of Saturn. Most geological interpretations of the moons' surfaces were made based on these images. Another focus was the cartographic treatment of the image data to produce geometrically accurate maps. The DLR Institute of Planetary Research calibrated the instrument, planned the image capture sequences and conducted scientific analysis of spectrometer data for the Visible and Infrared Mapping Spectrometer (VIMS) used to produce spectral maps of the chemical composition and structure of the surfaces, atmospheres and rings of Saturn and its moons.

Related Links

• Press kits for the launch and most important phases of the Cassini-Huygens mission (NASA)
• Mission overview (NASA diagram)
• Overview of moon flybys 2004–2017 (NASA diagram)
• Cassini-Huygens facts and figures (NASA poster)

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