After 13 years in orbit around Saturn, NASA's Cassini spacecraft is about to plunge itself into the planet's atmosphere and disintegrate. NASA decided to put an end to the mission on Friday because the probe is almost out of fuel.
Cassini has provided exquisite details about the second-largest planet in our solar system.
Take the hurricanes at Saturn's poles, for example. "These hurricanes are large enough they'd cover about half the continental United States, about 50 times larger than a typical Earth hurricane," says Cassini project scientist Linda Spilker of NASA's Jet Propulsion Laboratory.
Then there are the remarkable, hexagonal-shaped jet streams at the north pole. They've been there since before Cassini arrived in 2004.
"We have jet streams here on the Earth, but they change almost daily," Spilker says. "So we're really puzzled. It's the only place we know of in our solar system that has a long-lived hexagonal jet stream."
Spilker's special interest is Saturn's rings, and she says Cassini has revealed some unexpected things about them. There are places, for example, where the particles that form the rings clump together.
"The clumpiness has a unique character. Sometimes it looks kinda clumpy and speckly, other times it looks streaky," she says. And in other places, the particles float freely and don't appear to have any structure.
"How you can keep those areas separated?" she says. "That's an interesting and curious puzzle."
For all that Cassini has revealed about Saturn, there are still plenty of mysteries.
"It's a little bit embarrassing to confess, but we don't know how long a day is on Saturn," says Michele Dougherty of Imperial College in London. She's the scientist in charge of Cassini's magnetometer, an instrument that measures Saturn's magnetic field.
"In some ways," she says, "you can almost use a magnetometer to see inside a planet and get a better understanding of its internal structure."
Cassini's final orbits are taking it closer to the planet than ever before. Dougherty is hoping this will let her instrument see a telltale tilt in the magnetic field that should resolve the uncertainty over the length of a Saturnian day. "If we don't, we might not be able to work out the exact length of a day is on Saturn," she says.
Some of Cassini's most interesting discoveries involve Saturn's moons.
"Enceladus is this little moon. It's about the size of the United Kingdom," says Carly Howett of the Southwest Research Institute in Boulder, Colo. Scientists were amazed to see giant plumes of salty water vapor belching from Enceladus' south pole, suggesting liquid seas under a frosty crust that could maybe, possibly, harbor life.
Howett's instrument on Cassini, the composite infrared spectrometer, has revealed that the surface of Enceladus is extremely porous. "Much more porous than freshly fallen snow," she says. "If you were to put your hand on top of this and push down, your hand would go a long way into the surface. It wouldn't put up much resistance at all."
That Cassini is still functioning so well after 13 years in orbit isn't a big surprise to JPL mission engineer Julie Webster. She says the spacecraft came prepared.
"We carry two computers, two radios, two gyroscopes, two sun sensors, two star scanners, so we had our backups," she says.
A good indication of just how well Cassini has worked is the number of times it's gone into "safe" mode. When a spacecraft detects some type of software or hardware problem on board, it shuts down all nonessential equipment, turns its main antenna toward Earth and basically calls home to ask for instructions.
"We've only done that six times in 20 years, and only twice since 2003," Webster says. "So most of the 'safings' were early as we were learning the spacecraft."
NASA's decision to end the Cassini mission has an interesting back story. It seems mission managers were worried that without fuel to change its orbit, the probe could crash into one of Saturn's moons sometime in the future. The space agency was loath to let that happen, because it can't be certain that Cassini isn't carrying some hardy microbial spores from Earth. There's good reason to believe that some bacteria could survive 20 years in space.
The last thing NASA would want to do is send a future probe to one of Saturn's moons, only to find it colonized by bacteria from Earth.
The end of Cassini is going to be a sad day for the many thousands of scientists, engineers and technical staff who have worked on the mission.
"I've gone through all the stages of mourning, all the stages of grief," says Webster, who has worked on the mission since its launch in 1997.
"It's going to a sad day," agrees Howett, who has been working on the Cassini mission since 2005. "I'm heavily, emotionally invested in this mission in a way that doesn't normally happen. It's basically been the backbone of my career."
Dougherty says she'll also be sad to see the last radio signal from Cassini. "But it's going to a really proud moment, too," she says. "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."
KELLY MCEVERS, HOST:
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. Transcript provided by NPR, Copyright NPR.