Cassini-Huygens is a NASA/ESA/ASI mission designed to explore the Saturn system, including its rings and moons, with a special focus on Titan. After its launch on 15 October 1997, the nominal mission at Saturn began on 1 July 2004. By the time the nominal mission was completed in July 2008, Cassini had completed 75 orbits around Saturn and 44 Titan flybys.
The nominal mission lasted until July 2008, one of its milestones being the landing of ESA's Huygens probe on Titan. Huygens descended through Titan's atmosphere and touched the surface on 14 January 2005. It obtained invaluable data during its descent and on the surface of the moon, making important contributions towards fulfilling the mission objectives.
A first mission extension, called the 'Equinox mission', started on July 2008 and lasted for two years until October 2010. A major objective of this mission phase was to observe the Saturn system while the Sun crossed Saturn's equatorial plane. The second extension, called the 'Solstice mission' is now underway, and will last until 2017. It's major objective is the monitoring of seasonal changes induced by varying solar illumination, until the Sun reaches its highest elevation above Saturn's equatorial plane.
Cassini mission overview showing the Huygens landing, the number of Saturn orbits and flybys of different Moons. Credit: NASA/JPL-Caltech (Image by David Seal)
The scientific objectives of the nominal, or 'Prime' mission included a thorough study of the Saturn system. These objectives can be grouped in 5 categories: Saturn, its ring system, the magnetosphere, the icy moons, and Titan. Huygens' science objectives
Cassini's science objectives
- Determine the abundance of the atmospheric constituents, including noble gases, establish isotope ratios for abundant elements, and constrain scenarios of formation and evolution of Titan and its atmosphere;
- Observe the vertical and horizontal distribution of trace gases, search for more complex organic molecules, investigate the energy sources for atmospheric chemistry, model photo-chemistry of the stratosphere, study the formation and the composition of aerosols;
- Measure the winds and global temperatures, investigate cloud physics, general circulation and seasonal effects in Titan's atmosphere, search for lightning discharges;
- Determine the physical state, topography and composition of the surface and infer Titan's internal structure;
- Investigate the upper atmosphere, its ionisation, and its role as a source of neutral and ionised material for Saturn's magnetosphere.
- Determine the vertical structure of the atmosphere, in particular, how its composition, cloud properties, density, and temperature vary with height;
- Understand the horizontal motions of the atmosphere: its waves, eddies, and storms -- where they are located and how they form, grow, evolve, and dissipate;
- Determine the deep structure of the atmosphere, how it rotates, and how it relates to the upper atmosphere;
- Study how the atmosphere varies with time, both on short (daily) and long (seasonal) time scales;
- Investigate the relationship between the ionosphere, the magnetic field, and the plasma environment;
- Investigate the sources of lightning.
Ring science objectives:
- Map the composition and size distribution of ring material;
- Study the configuration of the rings and the dynamic processes responsible for their structure;
- Investigate the relationships between the rings and the embedded moons;
- Search for new ring-embedded moons;
- Study the interaction between the rings and Saturn's magnetosphere, ionosphere, and atmosphere.
Icy satellite science objectives:
- Map their surface geology and composition and determine their geologic histories;
- Determine the physical processes responsible for the surface and subsurface structure;
- Determine their bulk compositions and internal structure;
- Investigate their interactions with Saturn's magnetosphere and ring system.
- Determine the global configuration and dynamics of hot plasma in the magnetosphere of Saturn through energetic neutral particle imaging of ring current, radiation belts, and neutral clouds;
- Study the sources of plasmas and energetic ions through in situ measurements of energetic ion composition, spectra, charge state, and angular distributions;
- Search for, monitor, and analyze magnetospheric substorm-like activity at Saturn;
- Use imaging and composition studies to determine the magnetosphere- satellite interactions at Saturn, and understand the formation of clouds of neutral hydrogen, nitrogen, and water products (such as protons, oxygen atoms or hydroxyl radicals);
- Study how satellite surfaces and atmospheres are modified due to plasma and radiation bombardment;
- Study Titan's cometary interaction with Saturn's magnetosphere (and the solar wind) via high-resolution imaging and in situ ion and electron measurements;
- Measure the high energy (Ee > 1 MeV, Ep 15 MeV) particle component in the inner (L < 5 RS) magnetosphere to assess cosmic ray albedo neutron decay (CRAND) source characteristics;
- Investigate the absorption of energetic ions and electrons by the satellites and rings in order to determine particle losses and diffusion processes within the magnetosphere;
- Study magnetosphere-ionosphere coupling through remote sensing studies of the aurora and in situ measurements of precipitating energetic ions and electrons
After completion of the nominal mission in July 2008, the Cassini mission was extended until 1 July 2010. During this extension, which was called the 'Equinox mission', Cassini flew an additional 65 orbits around Saturn and performed 27 Titan flybys and seven flybys of Enceladus, an icy moon where active cryo-volcanism was discovered in 2005.
The objectives of the Equinox mission were modified to best exploit Saturn's new location in its orbit and answer new questions based on the findings and knowledge gained during the nominal mission. Cassini continued to study all members of the Saturn system including Titan, Saturn, its magnetosphere and rings, icy satellites, and gather information needed for future missions. Cassini science objectives
- Follow-up on Huygens' in situ investigations by studying seasonal changes in Titan's methane/ hydrocarbon hydrological cycle, and in the high-latitude atmosphere;
- Based on new knowledge, determine the types, composition, distribution, and ages of surface units, determine the internal and crustal structure, and measure aerosol and heavy molecule layers and properties.
- Observe seasonal variations in temperature, clouds, and composition, and in the winds at all accessible altitudes;
- Determine the planet's rate of rotation and internal structure, study the life cycles of newly-discovered atmospheric waves, and measure the spatial and temporal variability of trace gases and isotopes.
- Exploit the prime opportunity presented by Saturn's equinox (11 August 2009) to study its rings and determine spoke formation mechanisms and micro-scale properties of the ring structure;
- Study time-variability of ring phenomena on decadal timescales;
- Constrain the age of the rings, understand how narrow gaps are cleared, and determine variations in particle composition at high resolution.
- Study long-term seasonal changes on the mid-sized moons (Enceladus, Rhea, Tethys, Dione, Mimas, Iapetus);
- Based on new information obtained during the nominal mission, determine whether Enceladus harbours an ocean, and search for possible anomalies in its internal structure;
- Determine whether Rhea has a ring of its own;
- Search for low-level activity on Dione, such as a tenuous exosphere.
- Continue the study of Saturn's magnetosphere that was started at the beginning of the nominal mission to observe it over a full solar cycle, and determine the variability of Enceladus' plumes;
- Study Saturn's magnetotail to determine its dynamics, conduct in situ studies of Saturn's ionosphere, and investigate magnetospheric periodicities.
At the end of the Equinox mission in July 2010, the Cassini mission was granted a second extension. The mission had arrived at Saturn just after the planet's northern winter solstice. This latest extension will continue until a few months past northern summer solstice in May 2017, marking the beginning of summer in the northern hemisphere and winter in the southern hemisphere.
This gives Cassini the chance to carry out unprecedented observations over a full seasonal period in detail. The Solstice mission is scheduled to complete an additional 155 orbits of Saturn, 54 flybys of Titan and 11 flybys of Enceladus.
The main objective of the Solstice mission is to study seasonal-temporal changes and investigate new questions posed by discoveries made during earlier mission phases for each member of the Saturn system: Titan, Saturn, the rings, the icy satellites and the magnetosphere, in a previously unobserved seasonal phase from equinox to solstice. Cassini science objectives
- Study seasonal changes in the methane-hydrocarbon hydrological cycle by monitoring the lakes, clouds, aerosols and their seasonal transport;
- Study seasonal and temporal change with an emphasis on surface lakes and other materials, internal structure, aerosols and heavy molecules, upper atmospheric density, surface topography, surface temperature, clouds and winds;
- Study seasonal changes in upper atmospheric properties, specifically the temperature and formation and break-up of the winter polar vortex;
- Study Titan's plasma interaction as it moves from south to north of Saturn's solar wind-warped magnetodisc, from one solstice to the next;
- Determine the internal structure of Titan - identify and understand origins of surface features and associated material (such as volcano-like formations with material apparently deposited around them that could support cryo-volcanism, or lakes and their surroundings)
- Observe the atmosphere for changes in temperature, clouds, storms and composition in three spatial dimensions and in the winds at all accessible altitudes;
- Study seasonal change with an emphasis on: rotation rate, polar storms, trace gases, lightning, ionosphere, and the internal structure;
- Observe the magnetosphere, ionosphere and aurorae at all timescales, from minutes to years, and how they are affected by seasonal and solar cycle forcing;
- Determine Saturn's rotation rate and internal structure;
- Determine the dynamics of Saturn's magnetotail.
- Monitor long-term variability - in particular, seasonal variations in cryo-volcanic activity on Enceladus to help provide new constraints on the mechanism behind plume formation;
- Study potential temporal variability especially in Enceladus's ocean and interior structure, Dione's activity, Rhea's rings, Tethys's magnetospheric interactions and Rhea's differentiation state;
- Carry out a comparative study of Saturn's mid-sized satellites (Mimas, Enceladus, Thetys, Dione, Rhea, Hyperion, Iapetus and Phoebe), their cratering and comparative geology and surface composition.
- Study the production, appearance and variability of spokes on the rings in correlation with varying solar elevation;
- Determine the mechanism of formation of spokes and describe the micro-scale properties of the rings, their opening angle and temporal variability, with an emphasis on ring age and mass, clearing gaps, variations in compositional, microstructure, propeller structures. The innermost Saturnian ring, the D ring, will be observed in situ during Cassini's final orbits, also called the proximal orbits);
- Study variability of ring phenomena on decadal time-scales;
- Study the structure of the F-ring, embedded moonlets, clumps and transient objects;
- Determine the amount of micro-meteoroids and dust from the Zodiacal Cloud falling into the Saturn system, especially Saturn's rings, to constrain their age.
- Complete observations of Saturn's magnetosphere over a whole solar cycle (11 years), from one solar minimum (at the beginning of the nominal mission) to the next (by the end of the Solstice mission).