The Cassini–Huygens mission, commonly called Cassini, was a collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class unmanned robotic spacecraft comprised both NASA's Cassini probe, and ESA's Huygens lander which would be landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit. The craft were named after astronomers Giovanni Cassini and Christiaan Huygens.
Launched aboard a Titan IVB/Centaur on October 15, 1997, Cassini was active in space for nearly 20 years, with 13 years spent orbiting Saturn, studying the planet and its system after entering orbit on July 1, 2004. The voyage to Saturn included flybys of Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000). Its mission ended on September 15, 2017, when Cassini's trajectory took it into Saturn's upper atmosphere and it burned up in order to prevent any risk of contaminating Saturn's moons, which might have offered habitable environments to stowaway terrestrial microbes on the spacecraft.
The mission is widely perceived to have been successful beyond expectation. Cassini-Huygens has been described by NASA's Planetary Science Division Director as a "mission of firsts", which has revolutionized human understanding of the Saturn system, including its moons and rings, and our understanding of where life might be found in the Solar System.
Cassini's original mission was planned to last for four years, from June 2004 to May 2008. The mission was extended for another two years until September 2010, branded the Cassini Equinox Mission. The mission was extended a second and final time with the Cassini Solstice Mission, lasting another seven years until September 15, 2017, on which date Cassini was de-orbited to burn up in Saturn's upper atmosphere.
The Huygens module traveled with Cassini until its separation from the probe on December 25, 2004; it was landed by parachute on Titan on January 14, 2005. It returned data to Earth for around 90 minutes, using the orbiter as a relay. This was the first landing ever accomplished in the outer Solar System and the first landing on a moon other than our own.
At the end of its mission, the Cassini spacecraft executed the "Grand Finale" of its mission: a number of risky passes through the gaps between Saturn and Saturn's inner rings. The purpose of this phase was to maximize Cassini's scientific outcome before the spacecraft was disposed. The atmospheric entry of Cassini ended the mission, but analyses of the returned data will continue for many years.
Cassini 是最近的一艘前往外行星的「旗艦級」任務，和 New Horizons 或是 Juno 這種相對廉價、任務內容也單一的探測器相比，Cassini 的設計就是要搭載大量的儀器和設備，儘可能從更多的角度，去分析土星上的各種現象，並且還攜帶了一顆由歐洲太空署打造，名為「Hyugens」的著陸器，由衛星泰坦的表面，傳回重要的數據。
Cassini 針對土星的研究，最重要的發現，就是其北極詭異的六角形不僅是個固定的特徵，而且還會變色。經由 Cassini 的測量，我們大約可以在地球上的實驗中複製出類似的結構來，但無法確認是否土星上就是這樣的情況，還是有其他的機制。此外，由於 Cassini 在土星呆了足足有 13 年，已經接近土星公轉半圈的時間（土星年大約是 29 個地球年），因此 Cassini 得以觀察土星的季節變化，特別是夏季覆蓋整個行星的大風暴。這風暴似乎只發生在赤道及北半球，為什南半球沒有，又或是這是否只是個巧合，目前都是未知的。
在土星本身之外，被包在厚重大氣層之下的衛星泰坦（土衛六）是最重要的目標了。投放到表面的 Hyugens 及 Cassini 自身多次多達 100 次經過泰坦獲得的信息顯示，泰坦的表面有液態甲烷和乙烷組成的「海洋」，海洋會蒸發成雲、雲會再降下甲烷雨，並且在地表形成湖、河、山谷等地形。甚至在靠近赤道處，還有一片不太降雨的「沙漠」，有著由細碎的冰砂組成的沙丘呢。
最大的意外，則是吐衛二（Enceladus）了。說到冰層底下可能隱藏有一片液態水海洋的衛星，呼聲最高的當屬木衛二（Europa）了。不過因為 Cassini 持續的觀測，反倒是土衛二拔得了頭籌，成為第一顆確認地表下有液態水存在的衛星。不僅如此，土衛二還在表面上發現向太空中送出微粒的「噴泉」，顯示地表下有地熱活動，分析成份也發現有養活生物的條件呢。
目前 Cassini 已經最後一次揮別泰坦，9月15日拍下最後一張照片、將所有存儲的剩餘資料傳送完畢，之後將轉入「即時傳送」模式，在墜毀的過程中持續發送環境讀數，直到大約明天晚上失效為止。當然，Cassini 雖然不在了，它的旅途卻沒有終結 -- 其累積的大量資料，特別是最後大結局收集到的豐富信息，還將在未來許多年內，繼續為我們帶來新發現吧。 ——（摘www.engadget.com）