NASA has doomed its $4.36 billion Saturn probe -- here's what Cassini may discover on its death spiral

Cassini saturn grand finale illustration artwork nasa jpl caltech 14NASA/JPL-CaltechAn illustration of NASA’s Cassini probe as it plunges into the clouds of Saturn.

NASA’s $A4.36 billion Cassini probe is doomed.

The nuclear-powered robot called Cassini has orbited Saturn for nearly 13 years but has run dangerously low on fuel. NASA doesn’t want to risk crashing Cassini into any of Saturn’s icy moons, since it could contaminate their hidden oceans. So the space agency just kicked off a death spiral that will send the spacecraft straight into Saturn.

On April 22, Cassini paid a final visit to Saturn’s largest moon, Titan, which set the robot on a path to make an unprecedented dive between the Saturn and its innermost rings on April 26.

The new orbit will lead Cassini to a spectacular death on September 15, 2017.

“This is a roller-coaster ride,” Earl Maize, an engineer at NASA’s Jet Propulsion Laboratory who manages the Cassini mission, said during a press briefing on April 4. “We’re going in, and we are not coming out. It’s a one-way trip.”

In the intervening months, however, the robot will go where none has gone before it, and beam back a treasure trove of photos and data that researchers have thus far only dreamed about.

“It’s Cassini’s blaze of glory,” Linda Spilker, a Cassini project scientist and a planetary scientist at NASA’s Jet Propulsion Laboratory, told Business Insider. “It will be doing science until the very last second.”

Spilker walked us through what Cassini may see and discover during its final moments.

Launched in 1997, NASA's Cassini spacecraft spent seven years flying to Saturn. It sank into orbit in July 2004 -- but the probe has since run low on propellant.

NASA fears it will crash into a moon like Enceladus, which conceals a habitable ocean beneath its icy crust. Cassini discovered the ocean by flying through the Enceladus's watery jets.

So NASA scientists decided to put Cassini on a 'Grand Finale' mission: a death spiral that began with the probe's final flyby of Saturn's giant moon Titan.

Cassini photographed the frozen world closely one last time before the moon's gravity changed its path through space.

'You can think of Titan like a giant additional fuel tank,' Spilker said. 'By using its gravity, we can bend and shape Cassini's orbit.'

That orbit will sail Cassini high above Saturn's north pole ...

... And give the spacecraft another look at a hexagonal storm that's about two times wider than Earth.

But Spilker says the main event will be the first 'ring crossing,' when Cassini dives through a gap between Saturn and its rings of ice. The probe will fly through at about 76,800 mph -- or 45 times faster than a speeding bullet.

The gap exists between Saturn's atmosphere and its D ring.

The space is about 1,200 miles wide, or roughly the distance from northern Washington to the southern tip of California. That may seem roomy, but on a cosmic scale it's tiny.

NASA/JPL-Caltech; Business Insider

No one knows how many ring particles might be there, or how big they will be.

Luckily, NASA figured out a way to shield Cassini from potentially dangerous debris using the probe's high-gain antenna dish, which is normally the piece that communicates with Earth.

NASA/JPL-Caltech; Business Insider

'In the half-hour or so along the closest point of the flyby, we're pointing the high-gain antenna down, like a giant shield, to protect the instruments behind it,' Spilker said. 'We call this position 'shield-to-ram'.'

If tiny particles hit Cassini, its instruments will record those strikes. Scientists can then 'hear' them to deduce their size and numbers. 'It's like hail hitting a tin roof,' Spilker says. The data might help confirm the estimated age of the rings, which are thought to be 4.4 billion years old.

Cassini might also 'taste' ring particles to figure out what they're made of. Spilker says they're likely 99% ice, though that the other 1% is a mystery. NASA has wanted this data for decades, but deemed it too risky until now.

A false-colour view of Saturn's rings.

Cassini will pull off 22 orbits and ring crossings from April through September. Spilker says the team predicts about a 1% chance of a mission-ending failure throughout all of them.

If the first ring crossing goes well, scientists will put down the makeshift shield (the position limits what they can do) and kick off a jam-packed schedule of unprecedented experiments. The goal: Solve as many mysteries of Saturn as possible.

The views returned by Cassini's camera during some of the future crossings should be spectacular, Spilker says.

A view Cassini might see as it crosses Saturn's rings.

During a few ring crossings in May, Cassini will slowly spin. This will calibrate sensitive radio instruments that can 'listen' to Saturn's magnetic field. Such data could help scientists figure out how long the planet's day is.

A diagram of the Cassini spacecraft and its main instruments.

Its day may be about 10.5 hours, but no one is certain. Thousands of miles of thick clouds obscure the core, and any wobble in the planet's rotating magnetic field has proven too subtle to detect.

Auroras at Saturn's poles are caused by its magnetic field.

Source: Business Insider

However, flying so close to Saturn could help record it. 'If we can see ... the magnetic field pole wobbling, like it does around Earth, that could tell us how fast Saturn really spins,' Spilker said.

Cassini will make its closest passes to Saturn's D ring during orbits in late May and early June. The probe will try to 'taste' ring particles then, when it's most likely to bump into some.

An illustration of Cassini's 'Grand Finale' at Saturn.

The mystery that Spilker is looking forward to solving the most: the mass of Saturn's rings, which she has tried to figure out since the 1970s. The answer 'won't come in an instant,' she says, but will take many orbits of Cassini to crack.

'This is the closest we'll get to (the rings),' Spilker says. 'Are they less massive than we think, and therefore young? How did they form -- was it the breakup of a moon? A Kuiper Belt object? A comet?'

Saturn's rings obscure part of Titan.

Cassini could find out by gradually measuring the rings' gravity. Similar measurements should also reveal the size and structure of Saturn's core, and how deeply the planet's winds blow.

This illustration shows the interior of Saturn, including its core of heavy elements (orange), liquid metallic hydrogen layer (grey), and gaseous molecular hydrogen envelope (brown). The planet's magnetic field is thought to originate from the top of the metallic hydrogen layer.

Starting August 14, Cassini will begin five final orbits of Saturn, which will bring the probe close enough to sniff the planet's outer atmosphere as it zooms by.

An infrared view of storms on Saturn.

Titan will doom Cassini on it's last orbit via a distant gravitational nudge. 'That final orbit gives us Titan's goodbye kiss,' Spilker says.

An infrared view of Saturn's moon Titan.

On September 15, 2017, Cassini will plunge into Saturn's clouds. The probe will burn its last fuel to keep its antenna pointed at Earth while it sends real-time measurements of Saturn's gases.

An artist's view of what Cassini might see during its final plunge into the clouds of Saturn.

But Cassini will lose that battle after about a minute. The probe will ultimately break into pieces, burn up, and become one with 4.5 billion-year-old gas giant.

'I don't think of this as killing Cassini,' Spilker says. 'I see it as a glorious end to an incredible mission.'

The Cassini spacecraft being prepared for flight in 1997.

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