2017-09-08 16:10:02
Out There: Cassini Flies Toward a Fiery Death on Saturn

16:10, September 08 76 0

The Cassini spacecraft that has orbited Saturn for the last 13 years would weigh 4,685 pounds on Earth and, at 22 feet high, is somewhat longer and wider than a small moving van tipped on its rear. Bristling with cameras, antennas and other sensors, it is one of the most complex and sophisticated spy robots ever set loose in interplanetary space.

On Friday morning, the whole world will hear it die.

At the Jet Propulsion Laboratory in California, the scientists of the Cassini mission will figuratively ride their creation down into oblivion in the clouds of Saturn. They will be collecting data on the makeup of the planet’s butterscotch clouds until the last bitter moment, when the spacecraft succumbs to the heat and pressure of atmospheric entry and becomes a meteor.

So will end a decades-long journey of discovery and wonder.

The Cassini-Huygens mission, as it is officially known, was hatched in the 1980s partly to strengthen ties between NASA and the European Space Agency and partly because, well, where else in the solar system would you want to go? With mysterious, mesmerizing rings and a panoply of strange moons (62 and counting), Saturn was the last outpost of the known planets before the discoveries of Uranus, Neptune and Pluto.

As launched in 1997, the spacecraft consisted of two parts: an orbiter, built by NASA, and a lander, the Huygens probe, built by the European Space Agency to explore Saturn’s largest moon, Titan. The names were a testament to a golden age of European Renaissance astronomy.

Giovanni Domenico Cassini was a sharp-eyed 17th century astronomer who first discerned a dark gap in Saturn’s enigmatic rings and then discovered four moons. Christiaan Huygens discovered Titan and recognized Saturn’s rings for what they are.

The orbiter and lander arrived in July 2004 like wide-eyed tourists at Saturn, the realm of mystery and rings. Shortly thereafter, in December 2004, Huygens departed the mother ship and made the first landing on an alien moon, touching down in the hydrocarbon slushes of Titan three weeks later.

Cassini was just settling in for a long stay, circling Saturn like a pesky interplanetary paparazzo.

A list of its greatest hits would include movies of the six-sided storm that hugs the planet’s north pole; detailed views of Saturn’s spidery golden rings, woven into warps, braids and knots by the gravity of tiny moonlets; the discovery of plumes that look like snow-making machines shooting from the surface of the moon Enceladus. Not to mention postcards of lakes and seas on Titan.

NASA, not shy about sharing its accomplishments, recently released a blizzard of numbers summarizing the mission: 4.9 billion miles traveled, 294 orbits of Saturn completed, 2.5 million commands executed, 635 gigabytes of science data collected, 453,048 images taken, 3,948 science papers published, 27 nations participating and two oceans discovered.

To which must be added: $2.5 billion to build and launch Cassini and Huygens, split between NASA, E.S.A. and the Italian Space Agency, and another $1.4 billion to run them for 20 years in flight.

Like great scientific endeavors, Cassini raised as many questions as it answered. What, for example, is going on in those oceans on Titan and Enceladus?

Titan, the only moon in the solar system to have a thick atmosphere — even thicker than the Earth’s — is now the only body in the universe known to have liquid on its surface. That liquid is not water, but methane and ethane — hydrocarbons. The air on Titan is almost pure nitrogen. In addition, there may be an ocean of water or some other liquid substance deep under the surface.

If you think that Life As We Don’t Quite Know It could be based on some liquid other than water — a possibility suggested by Steven Benner, a biochemist at the Foundation for Applied Molecular Evolution in Florida — Titan has the potential to be Exhibit Number One. In recent years, proposals have been floated to send balloons, boats and even a submarine to Titan to check out whatever chemistry might be going on in its frigid wastes.

Other astronomers have seized on Enceladus as the most likely place to find extraterrestrial critters. The plumes erupting from its southern region suggest that there is a warm salty ocean beneath the ice. Where there is water, so the mantra goes, there may be life.

Microbes might be hitching a ride to space on those plumes, free for the taking by a spacecraft designed to detect them. Which Cassini was not.

Nevertheless, Cassini flew right through one of the plumes in October 2015 and found evidence that chemical and thermal reactions deep in the ocean were producing energy in the form of hydrogen gas. Similar environments on Earth, like deep-sea vents, are hotbeds of microbial activity.

Thanks to Cassini, then, the far worlds of Saturn have leapt to the top of the lists of alien-life hunters. Recently NASA circulated a call for proposals for future missions out there.

But nothing lasts forever. The scientists could have left Cassini for dead when its time came, a derelict in space. But that would have risked contamination on Saturn’s pristine and now very interesting moons, should the spacecraft hit them. So it had to go, and anyway there was still more to be learned by crashing it into Saturn.

Cassini’s fate was sealed last April. Using Titan’s gravitational pull, Cassini changed course oh so slightly onto a trajectory that would take it on the first of 22 passes inside Saturn’s rings, where no spacecraft has ever gone.

On September 11, Cassini will get one more “goodbye kiss” from Titan, a last fatal gravitational nudge directing the spacecraft into Saturn itself.

The cameras will turn off on the 14th, after one final look around the environs Cassini has called home for the last 13 years. But most of the spacecraft’s instruments will keep working, gathering and analyzing samples of the planet’s atmosphere as the spacecraft blazes into the clouds, which should tell us something about how the giant planet formed and evolved. Have the rings always been there, or are they a more (cosmically) recent addition?

Scientists and the press, in all its social media glory, will assemble at the Jet Propulsion Laboratory to witness the demise of Cassini, estimated to happen on September 15 at about 7:54 a.m. Eastern Daylight Time. Last week Cassini sent back what Carolyn Porco, of the Space Science Institute in Boulder, Colo., and imaging team leader on the project, called “one of our last best looks at Enceladus...that small moon at Saturn with the big possibilities.”

“Brace yourselves,” she added in an email. “The end is near.”

The news for all those gathered will arrive as a sudden silence. Cassini will break up and burn like a meteor into a wisp of stray atoms lost in the clouds. Dr. Porco said that the entry point would be visible from Earth and that some amateur astronomers were hoping to see some sign of Cassini’s entry.

But the odds are against them. And so Cassini will wave goodbye with a flash of unearthly light that no humans, at least, may ever see.