"卡西尼"使命在13年中对土星及其卫星作出重大科学发现。(NASA/JPL)

美国国家航空航天局（NASA）”卡西尼”号（Cassini）无人探测器于9月15日完成20年太空飞行，坠入土星高空大气层，流星般燃烧陨毁。

“卡西尼”号以此 “最后篇章”结束了航程达80亿公里的使命，其中包括对土星及其光环之间未曾探索过的空间的一系列环绕观测。

NASA科学家琳达·斯皮尔克尔（Linda Spilker）曾表示，探测器在燃料全部耗尽前，会努力完成最后动作，将天线保持对准位于澳大利亚的卫星接收器，从而将“卡西尼“向土星大气层俯冲时的景象几乎即时传送回来。

科学家也刻意要使探测器彻底销毁，从而让任何可能曾悄然在探测器上一路跟随的地球微生物同归于尽。

“卡西尼”使命是NASA，欧洲航天局（European Space Agency）和意大利航天局（Italian Space Agency）的合作项目，也是更广泛合作的成果结晶——有27个国家参与了这项使命。

“卡西尼”号探测器于1997年从佛罗里达州卡纳维拉尔角（Cape Canaveral, Florida）发射升空。它的成就包括：

观察土星环的复杂构成。科学家现在确定，最美丽的土星特征——它的光环，是经许多过程形成。
发现六颗新卫星。其中一颗外观犹如海绵，另一颗形似《星球大战》（Star Wars）影片中的”死星”（Death Star）。
在外太阳系第一颗卫星上着陆。欧洲航天局探测器“惠更斯”号（Huygens）在土星的土卫六“泰坦”（Titan）上着陆，发现了河流和液体甲烷形成的峡谷。
发现了土卫二“恩刻拉多斯”（Enceladus）的喷流。这很可能意味着在这颗小型卫星的表面冰层下存在海洋。

斯皮尔克尔说，“‘卡西尼’彻底改变了我们会如何看待生命的方式”。过去没有人认为行星的卫星有可能成为让生命存在的地方。但是，由于“卡西尼”的发现，斯皮尔克尔现在计划开展前往土卫二的使命，更加仔细地搜寻生命迹象。

“卡西尼”发现了土星土卫二“恩刻拉多斯”在喷射冰晶水汽——地表下有海洋的迹象。(NASA)

NASA的厄尔·梅兹（Earl Maize）说，“‘卡西尼’使命的结束是一个伤感的时刻”。但确定无疑的是，它将在长远的未来继续激发更多科学发现。

可点击NASA视频观看“卡西尼”号的最后时刻。

After epic mission, Cassini takes final plunge into Saturn [video]

NASA’s Cassini spacecraft will end its 20-year mission September 15 by diving into Saturn’s upper atmosphere, where it will burn up like a meteor.

The unmanned spacecraft’s “grand finale” caps off an 8-billion-kilometer mission that included a series of orbits into the unexplored space between Saturn and its rings.

The craft will use its last drops of fuel in its final maneuvers, fighting to keep its antenna pointed at a satellite dish in Australia. The craft will send data in near real time as it rushes headlong into Saturn’s atmosphere, NASA scientist Linda Spilker said.

Scientists planned to burn up the ship completely to destroy any microbes that might have hitched a ride from Earth.

The Cassini mission — a project of NASA, the European Space Agency and the Italian Space Agency — testifies to even broader cooperation, as 27 countries helped make it a success.

Cassini was launched from Cape Canaveral, Florida, 20 years ago, in 1997. Here is a list of accomplishments from the mission:

Observing the complexity of Saturn’s rings. Scientists have determined that Saturn’s most beautiful feature, its system of rings, was created by multiple processes.
Discovering six new moons. One newly discovered moon looks like a sponge, and another is shaped like the “Death Star” in Star Wars movies.
Landing on the first moon in the outer solar system. The European Space Agency’s Huygens probe landed on Saturn’s moon Titan, where it found rivers and canyons carved by liquid methane.
Finding jets of water shooting from Enceladus. This discovery likely signifies an ocean beneath the tiny moon’s crust of ice.

“Cassini has changed the paradigm of where we might look for life,” Spilker said. No one thought that a moon orbiting a planet might be a great candidate for living things. But thanks to Cassini, she plans a return mission to Enceladus to make a detailed search for signs of life.

Cassini spotted Saturn’s moon Enceladus as it shot jets of ice and water vapor — signs of a subsurface ocean. (NASA)

“The end of Cassini’s mission will be a poignant moment,” said Earl Maize, of NASA. But one thing is for sure: It will continue to spark scientific discovery for many years to come.

You can follow NASA’s coverage of Cassini’s grand finale online.

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