Cassini–Huygens in the context of Mariner program

Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet and are made almost entirely of water ice, with a trace component of rocky material. Particles range from micrometers to meters in size. There is no consensus as to when the rings formed: while investigations using theoretical models suggested they formed early in the Solar System's existence, newer data from Cassini suggests a more recent date of formation.

Though light reflected from the rings increases Saturn's apparent brightness, they are not themselves visible from Earth with the naked eye. In 1610, the year after his first observations with a telescope, Galileo Galilei became the first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn. The concept that Saturn's rings are made up of a series of tiny ringlets can be traced to Pierre-Simon Laplace.

Enceladus is the sixth-largest moon of Saturn and the 18th largest in the Solar System. It is about 500 kilometres (310 miles) in diameter, about a tenth of that of Saturn's largest moon, Titan. It is covered by clean, freshly deposited snow hundreds of meters thick, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon reaches only −198 °C (75.1 K; −324.4 °F), far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide variety of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrain.

Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecrafts, Voyager 1 and Voyager 2, flew by Saturn in 1980 and 1981. In 2005, the spacecraft Cassini started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapour, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totalling about 200 kilograms (440 pounds) per second. More than 100 geysers have been identified. Some of the water vapour falls back as snow, now several hundred metres thick; the rest escapes and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes are similar in composition to comets. In 2014, NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi). The existence of Enceladus's subsurface ocean has since been mathematically modelled and replicated.

There are 274 known moons of the planet Saturn, the most of any planet in the Solar System. Saturn's moons are diverse in size, ranging from tiny moonlets to Titan, which is larger than the planet Mercury. Three of these moons possess particularly notable features: Titan, Saturn's largest moon (and the second largest moon in the Solar System), has a nitrogen-rich, Earth-like atmosphere and a landscape featuring river networks and hydrocarbon lakes, Enceladus emits jets of ice from its south-polar region and is covered in a deep layer of snow, and Iapetus has contrasting black and white hemispheres as well as an extensive ridge of equatorial mountains which are among the tallest in the solar system.

Twenty-four of the known moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane (except Iapetus, which has a prograde but highly inclined orbit). They include the seven major satellites, four small moons that exist in a trojan orbit with larger moons, and five that act as shepherd moons, of which two are mutually co-orbital. At least two tiny moons orbit inside of Saturn's B and G rings. The relatively large Hyperion is locked in an orbital resonance with Titan. The remaining regular moons orbit near the outer edges of the dense A Ring and the narrow F Ring, and between the major moons Mimas and Enceladus. The regular satellites are traditionally named after Titans and Titanesses or other figures associated with the mythological Saturn.

The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2024, has continued with eight further spacecraft missions in the vicinity of Jupiter and two more en route. All but one of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but four were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On July 5, 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult due to large fuel requirements and the effects of the planet's harsh radiation environment.

The first spacecraft to visit Jupiter was Pioneer 10 in 1973, followed a year later by Pioneer 11. Aside from taking the first close-up pictures of the planet, the probes discovered its magnetosphere and its largely fluid interior. The Voyager 1 and Voyager 2 probes visited the planet in 1979, and studied its moons and the ring system, discovering the volcanic activity of Io and the presence of water ice on the surface of Europa. Ulysses, intended to observe the Sun's poles, further studied Jupiter's magnetosphere in 1992 and then again in 2004. The Saturn-bound Cassini probe approached the planet in 2000 and took very detailed images of its atmosphere. The Pluto-bound New Horizons spacecraft passed by Jupiter in 2007 and made improved measurements of its and its satellites' parameters.

The exploration of Saturn has been performed solely by crewless probes. Three missions were flybys, which formed an extended foundation of knowledge about the system. The Cassini–Huygens spacecraft, launched in 1997, was in orbit from 2004 to 2017.

The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide, and water. Although water is thought to reside deep in the atmosphere, its directly-measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three.

The atmosphere of Jupiter lacks a clear lower boundary and gradually transitions into the liquid interior of the planet. From lowest to highest, the atmospheric layers are the troposphere, stratosphere, thermosphere and exosphere. Each layer has characteristic temperature gradients. The lowest layer, the troposphere, has a complicated system of clouds and hazes composed of layers of ammonia, ammonium hydrosulfide, and water. The upper ammonia clouds visible at Jupiter's surface are organized in a dozen zonal bands parallel to the equator and are bounded by powerful zonal atmospheric flows (winds) known as jets, exhibiting a phenomenon known as atmospheric super-rotation. The bands alternate in color: the dark bands are called belts, while light ones are called zones. Zones, which are colder than belts, correspond to upwellings, while belts mark descending gas. The zones' lighter color is believed to result from ammonia ice; what gives the belts their darker colors is uncertain. The origins of the banded structure and jets are not well understood, though a "shallow model" and a "deep model" exist.

A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet.

Up until 1995, moonlets were only hypothetical components of Saturn's F-ring structure, but in that year, the Earth passed through Saturn's ring plane. The Hubble Space Telescope and the European Southern Observatory both captured objects orbiting close or near the F-ring. In 2004, Cassini caught an object 4–5 kilometers in diameter on the outer ring of the F-ring and then 5 hours later on the inner F-ring, showing that the object had orbited.

Phoebe (/ˈfiːbi/ FEE-bee) is the most massive irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on 18 March 1899 from photographic plates that had been taken by DeLisle Stewart starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru. It was the first natural satellite to be discovered photographically.

Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular moon of its size. Cassini's trajectory to Saturn and time of arrival were chosen to permit this flyby. After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.

The Day the Earth Smiled is a composite photograph taken by the NASA spacecraft Cassini on July 19, 2013. During an eclipse of the Sun, the spacecraft turned to image Saturn and most of its visible ring system, as well as Earth and the Moon as distant pale dots. The spacecraft had twice taken similar photographs (in 2006 and 2012) in its previous nine years in orbit around the planet. The name also refers to the activities associated with the event, as well as to the photographic mosaic created from it.

Conceived by the planetary scientist Carolyn Porco, the imaging team leader for Cassini, the concept called for the people of the world to reflect on their place in the universe, to marvel at life on Earth, and, at the time the pictures were taken, to look up and smile in celebration.

Huygens (/ˈhɔɪɡənz/ HOY-gənz) was an atmospheric entry robotic space probe that landed successfully on Saturn's moon Titan in 2005. Built and operated by the European Space Agency (ESA), and launched by NASA, it was part of the Cassini–Huygens mission and became the first spacecraft to land on Titan and the farthest landing from Earth a spacecraft has ever made. The probe was named after the 17th-century Dutch astronomer Christiaan Huygens, who discovered Titan in 1655.

The combined Cassini–Huygens spacecraft was launched from Earth on 15 October 1997. Huygens separated from the Cassini orbiter on 25 December 2004, and landed on Titan on 14 January 2005 near the Adiri region. Huygens's landing is so far the only one accomplished in the outer Solar System and on a moon other than Earth's.

Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) was an Italian-French mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard state. He discovered four satellites of Saturn and noted the division of its rings, later named the Cassini Division. Cassini was also the first of his family to begin work on the project of creating a topographic map of France. In addition, he also created the first scientific map of the Moon.

The Cassini space probe, launched in 1997, was named after him and became the fourth to visit Saturn and the first to orbit it.

Forward scattering is the deflection of waves by small angles so that they continue to move in close to the same direction as before the scattering. It can occur with all types of waves, for instance light, ultraviolet radiation, X-rays as well as matter waves such as electrons, neutrons and even water waves. It can be due to diffraction, refraction, and low angle reflection. It almost always occurs when the wavelength of the radiation used is small relative to the features which lead to the scattering. Forward scatter is essentially the reverse of backscatter.

Many different examples exist, and there are very large fields where forward scattering dominates, in particular for electron diffraction and electron microscopy, X-ray diffraction and neutron diffraction. In these the relevant waves are transmitted through the samples. One case where there is forward scattering in a reflection geometry is reflection high-energy electron diffraction.

The Science Programme of the European Space Agency is a long-term programme of space science and space exploration missions. Managed by the agency's Directorate of Science, The programme funds the development, launch, and operation of missions led by European space agencies and institutions through generational campaigns. Horizon 2000, the programme's first campaign, facilitated the development of eight missions between 1985 and 1995 including four "cornerstone missions" – SOHO and Cluster II, XMM-Newton, Rosetta, and Herschel. Horizon 2000 Plus, the programme's second campaign, facilitated the development of Gaia, LISA Pathfinder, and BepiColombo between 1995 and 2005. The programme's current campaign since 2005, Cosmic Vision, has so far funded the development of ten missions including three flagship missions, JUICE, Athena, and LISA. The programme's upcoming fourth campaign, Voyage 2050, is currently being drafted. Collaboration with agencies and institutions outside of Europe occasionally occur in the Science Programme, including a collaboration with NASA on Cassini–Huygens and the CNSA on SMILE.

Dione (/daɪˈoʊni/) is the fourth-largest moon of Saturn. With a mean diameter of 1,123 km and a density of about 1.48 g/cm, Dione is composed of an icy mantle and crust overlying a silicate rocky core, with rock and water ice roughly equal in mass. Its trailing hemisphere is marked by large cliffs and scarps called chasmata; the trailing hemisphere is also significantly darker compared to the leading hemisphere.

The moon was discovered by Italian astronomer Giovanni Domenico Cassini in 1684 and is named after the Titaness Dione in Greek mythology. Dione was first imaged up-close by the Voyager 1 space probe in 1980. Later, the Cassini spacecraft made multiple flybys of Dione throughout the 2000s and 2010s as part of its campaign to explore the Saturn system.

Iapetus, Saturn's third-largest moon, has a 20 kilometer (12 mi) equatorial ridge. The ridge runs along most of Iapetus's equator, and is the third tallest mountain structure in the Solar System. First discovered by the Cassini probe in 2004, the ridge's origin is unknown. There are bright areas on the sides of the equatorial ridge near Iapetus' bright trailing hemisphere, which were visible in Voyager 2 and were nicknamed the "Voyager Mountains".