How and why NASA will kill Cassini spacecraft at Saturn

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cassini huygens mission spacecraft probe nasa jpl caltech PIA21438
An illustration of the
Cassini spacecraft over Saturn’s north pole with its
hexagon-shaped storm.


NASA/JPL-Caltech


  • The Cassini spacecraft, which launched toward Saturn in
    1997, is running low on fuel.
  • To avoid running out and accidentally crashing into and
    contaminating a nearby moon that may harbor alien life, NASA is
    going to destroy the robot.
  • Before Cassini perishes, it will fly between Saturn and
    its rings and record as much new data as possible.

For nearly three decades, researchers have worked to design,
build, launch, and operate an unprecedented mission to explore
Saturn.

Called Cassini-Huygens — or Cassini, for short — the golden

nuclear-powered spacecraft
launched in October 1997, fell
into orbit around the gas giant in July 2004, and has been
documenting the planet and its dizzying variety of moons ever
since.

But all good things must come to an end. And for NASA’s $3.26 billion probe, that day is Friday, September
15, 2017.

During a press conference held by the US space agency
on April 4, researchers explained why they’re killing off their
cherished spacecraft with what they call the “Grand
Finale
.” The maneuver will use up the fleeting reserves of
Cassini’s fuel, putting it on a collision course with Saturn.


enceladus
False-color
image showing plumes erupting from Enceladus’
surface.


NASA/JPL/Space
Science Institute



“Cassini’s own discoveries were its demise,” said Earl Maize, an
engineer at NASA’s Jet Propulsion Laboratory (JPL) who manages
the Cassini mission.

Maize was referring to a warm, saltwater
ocean
that Cassini found hiding beneath the icy crust of
Enceladus, a large moon of Saturn that spews water into space.
NASA’s probe flew through these
curtain-like jets of vapor and ice
in October 2015, “tasted”
the material, and indirectly discovered the subsurface ocean’s
composition — and it’s one that may support alien life.

“We cannot risk an inadvertent contact with that pristine body,”
Maize said. “Cassini has got to be put safely away. And since we
wanted to stay at Saturn, the only choice was to destroy it in
some controlled fashion.”

But Maize and a collaboration of researchers from 19 nations
aren’t going to let their plucky probe go down without a fight.

They plan to squeeze every last byte of data they can from the
robot, right up until Cassini turns into a brilliant radioactive
comet above the swirling storms of Saturn.

‘We’re going in and we’re not coming out’

Long before Cassini began orbiting Saturn in 2004, mission
managers carefully plotted out its orbits to squeeze in as many
flybys of the gas giant planet, its moons, and its expansive icy
rings as possible.

Their goal: Get lots of chances to record
unprecedented new images
, gravitational data, and magnetic
readings without putting the spacecraft into harm’s way or
burning up too much of its limited propellant.

But after 13 years of operation at nearly 1 billion miles (1.45
billion kilometers) away from Earth, Cassini’s tank is running
close to empty.


TitanSkye Gould/Business Insider

“We’re coming to the end. As it runs out of fuel, the things it
can do are quite limited — until we decided on a new approach,”
Jim Green, the leader of NASA’s planetary science program, said
during the press conference.

NASA could have propelled Cassini to some other planet — perhaps
Uranus or Neptune. But in 2010, mission managers decided to keep
it around Saturn, reasoning they could squeeze more science out
of the mission there. But this effectively doomed the spacecraft
to a fiery death.

Cassini’s death spiral will officially begin on April 22, 2017.
That’s when it will, for the last time, fly by Titan — an icy
moon of Saturn that’s bigger than our own, has a thick
atmosphere, seas of liquid methane, and even rain.

Titan’s gravity will slingshot Cassini over Saturn, above the
planet’s atmosphere, and — on April 26 — through a narrow void
between the planet and the innermost edge of its rings.

“That last ‘kiss goodbye’ will put Cassini into Saturn,” Maize
said. “This is a roller-coaster ride. We’re going in, and we are
not coming out — it’s a one-way trip.”

Cassini’s science-packed finale


saturn interior layers metallic hydrogen rocky core nasa jpl caltech
An illustration of
Saturn’s internal structure.


NASA/JPL-Caltech


The void between Saturn and its rings is about 1,200 miles wide,
or roughly the distance from northern Washington state to the
southern tip of California.

“As we’re skimming close to the planet, we’ll have the best views
ever of the poles of the planet,” Linda Spilker, a Cassini
project scientist and a planetary scientist at NASA JPL, said
during the press briefing. “We’ll see the giant hurricanes at the
north and south poles.”


saturn hexagonal storm north pole cassini nasa jpl caltech jason major
Saturn’s
hexagon-shaped jet stream at the planet’s north
pole.


NASA/JPL-Caltech/Space
Science Institute; Jason Major/Lights in the
Dark



During its final orbits above Saturn, Cassini will get its
closest-ever views of the hexagon-shaped feature of Saturn’s
north pole, which Spilker said is “two Earth diameters across”
yet poorly understood.

“Perhaps by getting close with Cassini, we’ll answer the
question, ‘What keeps the hexagon there in this particular
shape?'” she said.

Spilker said Cassini will also photograph the auroras of Saturn’s
poles, measure how massive the planet’s rings are, “taste” the
icy material they’re made of, and even probe deep below its thick
clouds to see how big its rocky core is.

Sensitive magnetic and gravitational measurements that Cassini
couldn’t make before may also answer lingering questions about
the internal structure of Saturn, including how big its rocky
core is, plus how fast a shell of
metallic hydrogen
around it spins.

“How fast is Saturn rotating?” Spilker asked. “If there’s just a
slight tilt to the magnetic field, then it will wobble around and
give us the length of a day.”

Hours before it takes its final plunge on September 15, 2017,
Cassini will beam back its last batch of images — then prepare
for the end.

The fiery end of a longtime robotic friend

Cassini is a 2.78-ton robot with delicate instruments that was
not designed to ram into icy ring material at 70,000 mph. It also
wasn’t made to plunge into the thick atmosphere of a gas giant
and live to tell the tale.

Nevertheless, scientists behind the mission say they are going to
do their best to shield its instruments from damage and keep the
data flowing until the moment it dies.


cassini spacecraft clean room wokrers nasa jpl caltech KSC 97PC1111 orig
The
Cassini spacecraft being prepared for flight in
1997.


NASA


They’ll do this primarily by using the cone-shaped primary
antenna as a shield to protect cameras, magnetometers, and more.

“If we get surprised, well, we’ve got a bunch of contingency
plans … We’ll milk the best out of this,” Maize said. He added
that even if icy bits take out Cassini’s ability to talk to
Earth, the spacecraft “will still finish out exactly where we
planned, but we’ll have a little less science than we hoped for.”

When Cassini begins its final plunge, it will use its last
propellant to fight atmospheric drag and keep the antenna pointed
at Earth. During that time, it will “taste” the composition of
Saturn’s atmosphere as it descends into the gases, broadcasting
its readings in real time back to satellite dishes on Earth.

But the measurements won’t last long.

“It will break apart, it will melt, it will vaporize, and it will
become a very part of the planet it left Earth 20 years ago to
explore,” Maize said.

While members of the Cassini team said they’re looking forward to
the Grand Finale, they weren’t without remorse.

“It’s really going to be hard to say goodbye … to this plucky,
capable little spacecraft that has returned all of this great
science,” Spilker said. We’ve flown together a long time.”

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