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On Friday, NASA's Cassini spacecraft will plunge into Saturn's atmosphere and disintegrate. NASA decided to end the mission after 13 years in orbit around the gas giant. NPR science correspondent Joe Palca reports that Cassini has answered many of scientists' questions about Saturn and raised some new ones, too.
JOE PALCA, BYLINE: Cassini is a big mission. It costs nearly $4 billion, has a dozen scientific instruments and carried a separate probe that landed on Saturn's moon Titan. Thousands of scientists and engineers have worked on Cassini over the years, many of whom have been with the mission for quite a while.
CARLY HOWETT: I got involved in Cassini in 2005.
MICHELE DOUGHERTY: 1997.
JULIE WEBSTER: When I first started working directly for Cassini was January 1995.
PALCA: That was Carly Howett, Michele Dougherty and Julie Webster. But they're relative newcomers to Cassini compared to project scientist Linda Spilker. Spilker works at NASA's Jet Propulsion Laboratory. She started working on Cassini in 1988, when it was still just a glimmer in NASA's eye. I asked her if after nearly three decades she was getting kind of sick of Saturn and Cassini.
LINDA SPILKER: Oh, no. Oh, no. I think we've learned such a tremendous amount about Saturn.
PALCA: And she delights in telling people about it. Take the giant storms Cassini found at Saturn's poles.
SPILKER: These hurricanes are large enough they'd cover about half the continental United States, so about 50 times larger than a typical Earth hurricane.
PALCA: Spilker's special interest is Saturn's rings. She says Cassini has revealed some unexpected things about the rings. For example, there are places where the particles that form the rings clump together.
SPILKER: The campiness even has a unique character. Sometimes it looks kind of clumpy and speckly. Other times it looks streaky.
PALCA: I love when scientists use technical terms like streaky and clumpy and speckly.
SPILKER: (Laughter) That's what it looks like.
PALCA: For all that Cassini has revealed about Saturn, there are still some basic facts about the planet scientists just don't know.
DOUGHERTY: It's a little bit embarrassing to confess, and that is we don't know how long a day on Saturn is.
PALCA: Michele Dougherty is at Imperial College in London. She's the scientist in charge of Cassini's magnetometer, an instrument that measures Saturn's magnetic field. Cassini's final orbits are taking it closer to the planet than ever before. Dougherty is hoping that will let her instrument see a telltale tilt in the magnetic field that could resolve the uncertainty in the day length.
DOUGHERTY: And if we don't, we might not be able to work out what the exact length of a day on Saturn is.
PALCA: Some of Cassini's most interesting discoveries involve Saturn's moons. Take Enceladus.
HOWETT: Enceladus is this little moon. It's about the size of the U.K., which - I like the analogy for obvious reasons.
PALCA: Carly Howett is at the Southwest Research Institute in Boulder.
HOWETT: It has plumes coming out of it. And that's been a big discovery of the Cassini mission. And studying that has been a big part of my job.
PALCA: The plumes are made of salty water, suggesting that there are liquid seas under Enceladus' frosty crust that could maybe, possibly, maybe harbor life. That Cassini is still functioning so well after 13 years in orbit is not a big surprise to JPL mission engineer Julie Webster. She says the spacecraft came prepared.
WEBSTER: We carry two computers, two radios, two gyroscopes, two sun sensors, two star scanners. So we kind of had our backups.
PALCA: Like everyone I've spoken with who worked on the Cassini mission, Webster says it will be tough watching the mission end.
WEBSTER: I've gone through all the stages of mourning, all the stages of grief.
HOWETT: It's going to be a sad day. It's going to be a sad day.
DOUGHERTY: But it's a very proud moment, too, because the instruments and the spacecraft are still doing spectacularly well. And so to end in this almost blaze of glory that we're going to end in I think is the way to go.
PALCA: For her part, Linda Spilker sees the end of Cassini as just a beginning.
SPILKER: I think Saturn is a great place. And I hope to be part of something to go back.
PALCA: Joe Palca, NPR News.
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