Goodbye Cassini

For the past 13 years, the Cassini probe has been a lone explorer in the system of Saturn — making discoveries about the planet and its moons just like its namesake, Italian-French astronomer Giovanni Domenico Cassini, did four centuries earlier.

But Cassini’s historic mission is about come to a dramatic end


 

 

With its fuel dwindling, NASA decided to crash the seven-metre spacecraft into the ringed planet’s atmosphere, lest it smash into one of Saturn’s moons and contaminate a scientifically pristine site with something Cassini might have brought from Earth. Next week, it will hurtle into Saturn’s atmosphere at 120,000 kilometres per hour, where it will be violently torn apart in less than a minute.

The last signals will be received by Canberra’s Deep Space Communications Complex at 9:53pm AEST on Friday, September 15 — a final message from a spacecraft that began its journey aboard a Titan IV rocket on October 15, 1997.

“It’s changed our view of ocean worlds, other places that could support life we can explore … and who knows what we might discover that’s still in the data?” Project scientist Linda Spilker said.

And there’s still science to be done as the final chapter of Cassini’s epic journey begins.

September 9: The last loop

In April this year, Cassini dived between Saturn and its rings for the first time, coming closer to the planet than any spacecraft before it.

After sending back historic images of this first plunge, it completed another 20 loops depleting the last of its fuel reserves.

 

An artist’s concept of the final orbits of Cassini as it repeatedly dives between the innermost ring of the planet and Saturn itself.
(Supplied: NASA Jet Propulsion Laboratory)

It will make its 22nd and final loop between Saturn and its innermost rings this weekend.

This peculiar elliptical orbit will give scientists information about the gravity — and the mass — of the planet and its rings.

The more massive, the older they’re likely to be.

It was thought the rings could have formed around the same time as Saturn, 4.6 billion years ago. But Cassini’s early results suggest they are about 100 million years old — far younger than expected, Dr Spilker said.

That suggests some object — like a comet or a moon — came close to Saturn and broke apart not long ago, providing the material to form the rings.

September 12: Goodbye Titan

After a final rollercoaster loop, Cassini says its farewell to Saturn’s largest moon around 6.30AM AEST on September 12.

That last flyby — dubbed Titan’s “goodbye kiss” — will use the moon’s gravity to slow the spacecraft down, ensuring it dives deep enough into Saturn’s atmosphere to be destroyed.

Titan was a critical site of scientific research in the Cassini mission. The spacecraft dropped a probe, Huygens, into Titan’s atmosphere in December 2004 that collected images and data which revealed Titan was home to lakes and seas that contain liquid methane.

 

A near-infrared, colour mosaic from Cassini shows the sun glinting off of Titan’s north polar seas.
(Supplied: NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho)

“On Titan, methane plays the role that water plays on the Earth,” Dr Spilker said.

“It’s intriguing to think there’s so much liquid methane on the surface — could it perhaps be an environment capable of supporting life?”

Cassini also hinted an organic molecule called vinyl chloride existed on Titan. This was confirmed earlier this year by the sensitive Atacama Large Millimeter Array radio telescope in Chile. Coupled with the liquid habitat, the presence of this molecule suggests Titan has the ingredients suitable for the formation of cell-like membranes vital for life — albeit not life as we know it.

September 15: Cassini’s last look at the Saturn system

A few days later, Cassini will shoot its final gallery of the Saturn system’s highlights.

Among them is Enceladus, Saturn’s sixth-largest moon. One of Cassini’s major discoveries was that the icy, smooth surface of this moon hides a warm, salty ocean beneath it — complete with organic chemicals.

 

Ice plumes coming from surface of Enceladus.
(Supplied: Cassini Imaging Team, SSI, JPL, ESA, NASA)

Scientists are keen to learn more about Enceladus, and several new missions to the moon have been proposed by various space agencies, including NASA and the European Space Agency.

“The discovery of plumes of icy water that jet from the surface of the moon, the global salty ocean beneath it, is probably Cassini’s biggest discovery,” Dr Spilker said.

“It shows there’s other places now that could be potential habitats for life. So that’s tremendously exciting.”

Cassini will also take its last shots of the bizarre hexagon at Saturn’s north pole.

 

GIF: The hexagonal jet stream at Saturn’s north pole.

Measuring 30,000 kilometres across, this weather pattern is made up of flowing air currents, produced when solar radiation hits the atmosphere.

Astronomers still don’t know why it’s six-sided — but they have been studying the swirling vortex at its centre to get a better understanding of how hurricanes on Earth come about.

After taking all of this in, Cassini will turn its antenna to Earth about 7:45am AEST on Friday, September 15 and begin to dump any data it’s been carrying back home.

Measuring 30,000 kilometres across, this weather pattern is made up of flowing air currents, produced when solar radiation hits the atmosphere.

Astronomers still don’t know why it’s six-sided — but they have been studying the swirling vortex at its centre to get a better understanding of how hurricanes on Earth come about.

After taking all of this in, Cassini will turn its antenna to Earth about 7:45am AEST on Friday, September 15 and begin to dump any data it’s been carrying back home.

 

The Canberra Deep Space Communication Complex will track Cassini to the end. (CSIRO: Robert Kerton)

It’ll also establish a communications link that won’t cease until the probe is destroyed.

The Canberra Deep Space Network station will pick up the signal about 1:15pm AEST and will monitor the spacecraft’s progress right up to the end.

September 15, 6:37pm AEST: The dive begins

At this point, it’s about three hours until Cassini’s planned destruction. The probe will manoeuvre its ion and neutral mass spectrometer (INMS) — the instrument it uses to sniff gases in the environment to determine their make-up — into prime position for the collection of data.

 

An extraordinary view of Saturn from the Cassini spacecraft. Thank-you, Cassini!
(Cassini Imaging Team, SSI, JPL, ESA, NASA)

The INMS will be one of eight instruments transmitting data back to Earth in real time.

But in many respects, it’s the most important for these final hours — it’ll directly sample the composition of Saturn’s atmosphere as the probe draws closer to the planet.

Dr Spilker said the samples would help determine the ratio of helium to hydrogen in Saturn’s atmosphere — key to understanding the internal structure of the planet and its evolution

September 15, 9:54pm: The end

It’s at this point Cassini enters Saturn’s atmosphere. The craft’s antenna is still pointed to Earth, transmitting back data from the various instruments on board. But not for long.

One minute later, the atmosphere of Saturn gets so thick it prevents the probe’s thrusters from doing their job — and Cassini starts to tumble.

“Very shortly thereafter, Cassini will just vaporise in the atmosphere,” Dr Spilker said.

 

An artist’s impression of Cassini breaking up as it enters Saturn’s atmosphere.
(Supplied: NASA/JPL-Caltech)

“It’s a bittersweet time,” she said.

But though it’ll be gone, Dr Spilker said the legacy of the mission would live on through future explorations.

“The flight team, the team operating Cassini, as well as the Cassini scientists, we’ve gotten to know each other quite well — we’re a family,” Dr Spilker said.

“You’re seeing the end of the spacecraft itself but those family members will all go their separate ways too. “They’ll take their knowledge and experience on to other missions, and carry on Cassini’s legacy in a different way,” she said.

 

Goodbye Cassini. Sitting here on that little dot at the bottom right of this image, we thank you.

(Supplied: NASA/JPL-Caltech/Space Science Institute)

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