JUDY WOODRUFF: This Friday, the Cassini spacecraft is set to end its long tour of Saturn with
a fatal plunge into the planet.
It's been a workhorse and source for much of what we know about Saturn.
It will beam back images until its final moments from some 800 million miles away.
William Brangham has more.
It's the focus of this week's Leading Edge.
WILLIAM BRANGHAM: Some of the numbers involved in the Cassini mission are truly mind-blowing.
More than 290 orbits of Saturn, nearly five billion miles traveled, 450,000-plus images
taken.
There have been nearly 4,000 papers published about the work.
And it included the participation of 27 nations.
Our science correspondent, Miles O'Brien, has this appraisal.
This report was produced in partnership with our friends at "NOVA," whose program "Death
Dive to Saturn" airs tonight on PBS.
MILES O'BRIEN: Twenty years after it began its detailed tour of Saturn, its rings and
moons, NASA's Cassini spacecraft is winding its way toward a suicide plunge into the planet.
It's the end of an epic space odyssey, Cassini's grand finale.
LINDA SPILKER, NASA Jet Propulsion Laboratory: As the orbits progress, we get closer and
closer to Saturn's atmosphere.
MILES O'BRIEN: Project scientist Linda Spilker joined the Cassini team at the Jet Propulsion
Laboratory before launch in 1997.
Their challenge now?
Thread a cosmic needle, sending the spacecraft in between Saturn and its rings, eking out
some final morsels of data before the mission is over.
LINDA SPILKER: The mysteries we want to solve with the grand finale mostly have to do with
revealing Saturn from the inside out.
MILES O'BRIEN: Surprising discoveries are nothing new for Cassini.
The team has been pushing the frontiers of science for years and sharing spectacular
images captured by the spacecraft.
CAROLYN PORCO, NASA: It's just such a surreal looking planet.
Really, it wins the beauty contest in the solar system.
That's for sure.
MILES O'BRIEN: Carolyn Porco might be just a little biased.
She's the lead scientist in charge of Cassini's cameras.
Most recently, they recorded a storm on the north pole of Saturn that changes color from
turquoise in the winter to golden brown in summer.
Scientists believe sunlight interacts with molecules in the atmosphere, creating a sort
of Saturnian smog.
Over the years, Cassini has shown us the rings of Saturn in unprecedented, stunning fashion.
They are about 175,000 miles across, but, in most places, only 30 feet thick.
CAROLYN PORCO: We get to see lots of places just really densely packed, where the particles
are protruding two miles above the ring plain.
I mean, it's astonishing.
MILES O'BRIEN: And Cassini has also turned its instruments to Saturn's many moons.
Porco's team captured images of plumes erupting from the icy moon Enceladus.
CAROLYN PORCO: This is what we saw.
We saw dozens of fine jets shooting off the south pole of Enceladus.
MILES O'BRIEN: They later determined the geysers were made of water ice and were loaded with
organic compounds.
They also found tiny nanosilica particles.
LINDA SPILKER: What's so amazing is that those nanosilica grains could only form in really
hot water.
All of the sudden, the pieces started to fall into place, and so we're thinking, maybe you
have hydrothermal vents on the seafloor of Enceladus.
MILES O'BRIEN: Hydrothermal vents, organic compounds and liquid water, the combination
is very intriguing for scientists, because it is likely life began on Earth under similar
circumstances.
CAROLYN PORCO: It doesn't get any better than this, to go to Saturn and come away having
discovered what we think might be the best place in the solar system to go to search
for life.
MILES O'BRIEN: The prospect of life on Enceladus prompted the Cassini team to change plans
for how the mission should end.
If the spacecraft smashed into the icy moon, it might bring hitchhiking microbes from our
planet with it.
So, instead, Cassini will auger straight into Saturn, swallowed up by the giant gas planet
where there is no possibility for the development of life.
Still, Cassini's fiery end is no small task to engineer.
The spacecraft has to be in tip-top shape, so it can beam data back to Earth for as long
as possible.
JULIE WEBSTER, NASA: We are responsible for the health and safety of the spacecraft.
MILES O'BRIEN: Julie Webster is the manager of the Cassini Spacecraft Operations Center.
Using exact replicas of the vintage electronics on board the spacecraft...
JULIE WEBSTER: Everybody talks about gigabits these days.
We're down to kilobits.
MILES O'BRIEN: ... she and her team are simulating scenarios for Cassini's final dive.
MAN: No red alarms, and we are go for orbit trim maneuver 467.
JULIE WEBSTER: The timing of everything is highly choreographed.
WOMAN: The accelerometer is powered on at this time.
JULIE WEBSTER: Because we are doing something almost every second on the spacecraft, and
certainly every minute.
WOMAN: The wind roll turn has started.
JULIE WEBSTER: To have either an anomaly on the spacecraft or a sequence that isn't quite
right, there's very little time to figure out what's wrong, fix it, clean it back up,
put the sequence back on board the spacecraft.
We don't have a lot of time to recover at that point.
We're game on.
(LAUGHTER)
MILES O'BRIEN: And then it will be game over, a bittersweet moment.
Saying goodbye isn't easy.
JULIE WEBSTER: The sense of an impending end is the hardest experience I have had to experience
in a long time.
MILES O'BRIEN: It will be a long time before NASA gets back to Saturn and its moons.
As a matter of fact, there are no plans on the books to return -- William.
WILLIAM BRANGHAM: Miles, are there other planned missions to go to some of the moons of other
planets?
MILES O'BRIEN: Yes.
A similar moon, which orbits Jupiter, Europa, also ice-covered, also has a liquid ocean
beneath, is one of NASA's targets.
The Europa Clipper is slated to launch in the 2020s, and it's similarly intriguing to
scientists, in the sense that they believe it might harbor life.
So, stay tuned for that one, I guess.
WILLIAM BRANGHAM: Miles, before we let you go, I understand you're also hosting a very
special PBS special tonight about the rediscovery of the USS Indianapolis.
This was a famous World War II vessel.
They have now found it.
Viewers will be able to see some of this wreckage live from the bottom of the ocean.
Can you tell us a little bit about this?
Why is this such an important find?
MILES O'BRIEN: Well, the USS Indianapolis was sunk right at the tail end of World War
II; 880 men were lost.
It's the worst disaster in U.S. Navy history.
The vessel had only a few days prior delivered the components of the Little Boy bomb, the
atomic bomb that was dropped on Hiroshima.
They were transiting over to the Philippines, sunk by a Japanese torpedo.
The story that people may be familiar with, though, is that no one knew that they were
sunk.
They were forgotten.
And 800-plus men were in the water bobbing for four-and-a-half days.
They suffered from hypothermia, dehydration.
And they were attacked by sharks.
It was a dramatic event.
And the wreck has only been found three-and-a-half weeks ago.
WILLIAM BRANGHAM: Why so long to find this?
MILES O'BRIEN: It's in about 18,000 feet of ocean.
It's one of the deepest spots on the planet.
And so technology has only gotten to the point to make it even practical to hunt for something
like this in such deep, rugged, completely lightless terrain beneath the sea.
It was found by a group led by Microsoft co-founder Paul Allen, who has decided to make it one
of his missions in life to find these historically important shipwrecks.
They were determined to do it, and they did.
And you will see live pictures from 18,000 feet below the Philippine Sea live tonight
on PBS, 10:00 p.m.
WILLIAM BRANGHAM: Sounds really great.
Miles O'Brien, as always, thank you so much.
MILES O'BRIEN: You're welcome, William.