Glossary

Cassini–Huygens mission: A joint mission to Saturn conducted by NASA, the European Space Agency (ESA), and the Italian Space Agency (Agenzia Spaziale Italiana, ASI). The spacecraft launched on 15 October 1997, arrived at Saturn in July 2004, and operated until 15 September 2017, when the Cassini orbiter made a planned descent into Saturn’s atmosphere. The Huygens probe landed on Titan on 14 January 2005 — the first landing ever made in the outer Solar System.

Cassini spacecraft: The orbiter component of the Cassini–Huygens mission. It conducted 13 years of close observations of Saturn, its rings, and its moons.

Galilean moons: The four largest moons of Jupiter — Io, Europa, Ganymede, and Callisto — discovered by Galileo Galilei in January 1610.

Giant planets: The four outer planets of our Solar System — Jupiter, Saturn, Uranus, and Neptune — also called the Jovian planets.

Huygens probe: The lander carried by Cassini that descended through Titan’s atmosphere on 14 January 2005, transmitting data for 72 minutes from the surface.

Jovian planets: The four outer planets — Jupiter, Saturn, Uranus, and Neptune — all much larger and less dense than the Terrestrial planets, and composed primarily of hydrogen, helium, and other light elements in supercritical fluid form.

Juno mission: A NASA mission to Jupiter launched on 5 August 2011 and inserted into polar orbit on 4 July 2016. Juno studies Jupiter’s gravity, magnetic field, auroras, and deep atmospheric structure. Its mission was extended to 2025, including close flybys of Io, Europa, and Ganymede.

Planetary rings: Thin, flat systems of orbiting ice and dust particles found around all four Jovian planets. Saturn’s rings are the brightest and most massive.

Terra: The Latin name for Earth.

Terrestrial planets: The four inner planets of the Solar System — Mercury, Venus, Earth, and Mars — characterized by rocky surfaces and dense, metallic cores.

Tidal heating: The process by which gravitational interactions with a massive body (such as Jupiter or Saturn) stretch and compress an orbiting moon, generating internal friction and heat. This energy source powers volcanic and cryovolcanic activity on moons such as Io and Enceladus.

Tiger stripes: Long fractures in Enceladus’s south polar region from which cryovolcanic plumes of water ice and vapor are ejected into space, feeding Saturn’s E ring.

Voyager 1 and 2 missions: Twin spacecraft launched by NASA’s Jet Propulsion Laboratory (JPL) in 1977. Voyager 1 flew past Jupiter and Saturn before becoming the first human-made object to enter interstellar space in August 2012. Voyager 2 remains the only spacecraft to have visited Uranus and Neptune, completing its “Grand Tour” of all four outer planets before exiting the Solar System in 2018.

Europa Clipper mission: NASA mission launched in October 2024, scheduled to arrive at Jupiter in the early 2030s. It will make dozens of close flybys of Europa to investigate its subsurface ocean and potential habitability.

JUICE mission: The European Space Agency’s Jupiter Icy Moons Explorer, launched in April 2023 and due to arrive at Jupiter in 2031. It will study Ganymede, Callisto, and Europa to understand the formation of giant planets and their moons.

Dragonfly mission: NASA rotorcraft lander scheduled to launch in 2028 and arrive at Titan in the mid-2030s. It will fly from site to site across Titan’s surface to study its chemistry and potential for life.