Death of NASA’s Cassini on Saturn has sad ring to it

Scientists to mourn coming plunge into planet’s atmosphere

In four days, NASA’s Cassini spacecraft will nose-dive into Saturn and burn up in the planet’s atmosphere.

It’s the final, suicidal step of a months-long dance through Saturn’s rings that has given scientists an unprecedented view of the sixth planet from the sun. It’s also the end of a mission that has revolutionized our understanding of Saturn and opened our eyes to two worlds that could be home to alien life — the moons Titan and Enceladus.

It is the end of an era. And Cassini fans are devastated.

To understand why, you have to understand Cassini — a plucky, school bussized spacecraft that has been orbiting Saturn since 2004.

First, a brief bio: Cassini launched on its billion-mile journey from Earth to Saturn on Oct. 15, 1997. It was named for the astronomer Giovanni Cassini, who discovered four of the planet’s moons and a gap in its rings. Cassini also carried a single passenger: The Huygens lander, built by the European Space Agency and named for the Dutch scientist who first spotted the moon Titan.

Cassini and Huygens arrived in Saturn’s orbit seven years after launch, in July 2004. Several months later, Huygens split off and touched down on the shore one of Titan’s lakes of liquid methane. It was humankind’s first-ever landing on a moon other than our own, and the first landing of any kind in the outer solar system. Cassini, meanwhile, was the first probe to orbit Saturn.

Cassini has been extraordinarily successful — one of the most successful planetary missions ever. Its flight was smooth, its instruments worked, its software rarely acted up.

In addition to Huygens’ perfectly stuck landing, Cassini probed the formation of Saturn’s ring system, discovered a 5,000 milewide hurricane at Saturn’s south pole and got the first close-up view of the planet’s hexagonal North Pole storm. Cassini revealed that Saturn’s rings have a lot of three-dimensional texture and contain bumps as big as the Rocky Mountains. Roughly 4,000 papers have been written using the 635 gigabytes of data collected by Cassini in nearly 300 orbits of Saturn.

Best of all were the revelations about Saturn’s ocean moons. In the haze around Titan, Cassini discovered molecules that could be precursors to — or even indicators of — biological activity on that planet. Zooming past the icy moon Enceladus, it found evidence of an underground ocean of water, and spotted geysers spewing out ingredients for life.

The spacecraft is not equipped with life-detecting instruments — no one could have imagined it might make such discoveries when it launched 20 years ago. But these moons are now considered two of the best places in the solar system to look for alien organisms, and they are the focus of several proposals for new NASA missions.

“There’s this tremendous legacy,” said project scientist Linda Spilker, who has worked on the mission since 1988. “Cassini has certainly rewritten the textbooks.”

Cassini is a victim of its own success. It’s precisely because of Cassini’s revelations about Titan and Enceladus that the spacecraft has been sentenced to die. Back in 2009, when it became apparent the spacecraft was running out of fuel, scientists at NASA’s Jet Propulsion Laboratory got together to assess their options. The craft couldn’t be left to float around in space, on the off chance that it might be knocked out of orbit and crash into one of the potentially habitable moons. If that happened, Cassini could potentially contaminate those worlds with Earthling microbes.