Cassini’s amazing space odyssey to Saturn
Scientists says Cassini is helping them undestand how our solar system developed.
By: Joseph Hall News reporter, Published on Sun Jul 06 2014
Of the astronomically profound discoveries it’s made over a decade of circling, the startling hint this April of a new moon being formed in the rings of Saturn is merely the latest.
Indeed, the spacecraft Cassini — which inserted itself into orbit around the giant gas planet in July, 2004 — has transmitted imagery and sensory data back to Earth that has given us a new understanding of our bejewelled neighbour three doors down.
“It’s one of the most successful (space) missions probably ever,” says University of Toronto astrophysicist Hanno Rein, whose own work has been significantly informed by the tiny craft’s output.
“Fantastic … the stunning images, this is unprecedented. They’re an order of magnitude more exciting than anything we’ve seen before,” Rein says.
Rein is especially intrigued by the natal moon — tentatively named Peggy — that Cassini detected in Saturn’s outermost A Ring this spring.
Because he believes we could be witnessing in the icy satellite’s birth a small-scale version of the planetary formation that created our solar system billions of years ago.
“The kind of theories we develop for Saturn we then apply to other systems like the (ancient) disks (around the sun) in which planets formed,” Rein says.
“But for Saturn, we can observe how it forms in situ. It’s really exciting to see it in action.”
Yet the proto-moon simply joins in the metronomic output of discovery that Cassini has hurled back to Earth since reaching Saturn a decade ago — and during its seven-year, 3.5-billion kilometre journey to get there.
Jesse Rogerson, a PhD candidate in astrophysics at York University, has followed the Cassini project for years and says it’s been revolutionary for our understanding of the sixth planet.
The two NASA Voyager probes that flew past in the early 1980s were able to collect good data on the planet for only a few weeks each, Rogerson says.
“The only way to really learn about it is to have something sit there,” he says. “What Cassini has done for the Saturnian system, it’s like shining a light bulb in a darkened room.”
Speaking from NASA’s Jet Propulsion Laboratory in Pasadena, Calf., Cassini project manager Earl Maize says he’ll leave the superlatives to others.
“But … it’s been a wonderful ride,” Maize concedes. “The things that the project has discovered (it’s been) surprise after surprise over the last 10 years.”
Those surprises have fundamentally changed the scientific thinking about Saturn that began when Dutch astronomer Christian Huygens described the rings in 1655.
“In fact I would go even a little bit further and say some of the things Cassini has discovered have changed our view of our own solar system,” adds Linda Spilker, the project’s top scientist.
In particular, two moons in the Saturn system — which thanks in part to Cassini is now known to have at least 53 of them — have provided oceans of material for the craft’s camera’s and probes to explore.
The planet’s largest moon, Titan, which was also discovered by Huygens, has been obscured to astronomers by an orange haze for the 350 years since.
But beneath the fog, the Cassini mission revealed that Titan is among the most Earthlike worlds in the solar system, Spilker says.
The craft, which has made dozens of flybys of the moon, some within 100s of kilometres, also carried and released the European Huygens probe, which parachuted on to Titan’s surface in January, 2005.
Together the twined crafts have shown a moon with liquid methane seas, lakes and rivers and a vibrant, active atmosphere that have shaped its surface in very earthy ways. There’s even an occasional rain.
“We revealed that this really is … very analogous to Earth,” says Maize, who counts the exploration of Titan as the mission’s top achievement so far.
“We have rain, we have lakes, we have rivers, we have weather patterns … it’s absolutely fascinating.”
Continuing flybys will use Cassini’s radar and sensors to further map the moon’s topography, search for seasonal climate changes and attempt to confirm the presence of a huge ocean of water thought to lurk below Titan’s surface.
If Cassini showed Titan to be a topographical and atmospheric mirror of Earth, however, it showed the ice-moon Enceladus to be a tantalizing host for potential life.
Cassini pictures of the moon shocked scientists when they revealed massive geysers of salt water erupting from its southern pole, raising the possibility that Enceladus’ icy surface could be covering vast seas of water below.
With water being a prerequisite of life, tiny Enceladus — just 480 kilometres in diameter — leaped to the head of the solar system’s class of bodies where primitive organisms may one day be found.
“In Esceladus, with its icy jets, and Titan, with its methane lakes and liquid water ocean underneath, we’ve in a sense helped redefine the (solar system’s) habitable zone,” Spilker says.
“It (potential life sites) is no longer just sort of that Goldilocks zone, that place where the Earth orbits where you can have liquid water on the surface.”
In Saturn’s rings themselves, Cassini has revealed a credible analogy for the creation of the solar system’s planets, Spilker says.
Like the shifting clouds of ice, rock and dust that encircled our sun billions of years ago, Saturn’s rings are composed of materials, big and small, that are colliding and melding into larger bodies.
“We saw shadows of large mountain-sized particles that are accreting and accumulating in the rings and giving us ideas about maybe how planets formed in the solar system,” Spilker says.
“It’s just intriguing to think that maybe some of the tiny moons we see just outside the rings might have actually started in the rings themselves.”
It’s this moon-formation process that has most intrigued Rein, who believes that Saturn’s rings represent a current template for the solar system’s beginnings.
“The moons are so small that you can’t actually see the individual moons; there are maybe a few hundred that are maybe a kilometre in size,” he says. “But they create this small disturbance in the rings and because Cassini has such a good resolution you can actually see this perturbance.”
Following these ring disturbances should allow scientist to predict — applying the laws governing gravitational orbits — precisely where new moons will be when Cassini looks for them again, Rein says.
“But it turns out it’s not where (the laws) predicted it would be, it’s moved a bit. And it’s moving because there are strong interactions with the rings in which it’s embedded.”
This is the first time astronomers have seen objects in orbit change their course, Rein says.
And he says the alterations are likely caused by collisions between the tiny moons and even smaller bodies of ice and dirt within the rings. These herky-jerky pathways would be similar to the ones Earth and the other planets took during their formations.
Rein is currently running numerical simulations on the Cassini moonlet data to better understand these interrupted motions so as to better predict what may happen on a planetary scale.
“Really (however) it’s with Cassini just being there long enough to look for and discover these new things … that we’re rewriting the book not only about Saturn but about our solar system as well,” Spilker says.
In its decade at Saturn, the $3.3 billion (U.S.) Cassini mission has logged more than 3.2 billion kilometres in 206 orbits and generated more than 3,000 scientific papers back on Earth.
Some of its greatest orbital hits include:
The discovery of seven new moons around the planet.
The recording of a great northern storm that erupted in late 2010 and within months had grown to ring the planet in a swirling band of orange tumult. During the storm, which petered out soon after its head caught up with its tail, Cassini detected the largest temperature rise ever recorded on a planet and elements never seen before in Saturn’s atmosphere.
The discovery of massive hurricanes at both poles of the planet.
Photographing a shining Earth and its moon — while thousands of people on this planet waved up during the NASA’s “Earth Waves at Cassini” event — during a trip to the dark side of Saturn in July, 2013.
The first full recording of the massive, hexagonal jet stream structure circling at Saturn’s northern pole.
Rein speaks with awe about the ability of Cassini’s Earthbound handlers to position the craft for its orbital observations — manoeuvres that could give the term rocket science new lustre.
“The spacecraft has very little fuel … but they need to make very small changes to the spacecraft orbit with the little fuel they have,” he says. “And they’ve done a remarkable job with that.”
Instead of expending scarce fuel for orbit-altering thruster burns, Rein says, the probe’s remote pilots have plotted out close encounters with Saturn’s moons using their gravitational pull to sling the ship into desired positions above the planet.
“That they can do that is pretty amazing. It’s literally rocket science,” he says.
Its handlers employed similar gravitational strategies in guiding Cassini to its Saturn destination, using the pull of the sun and several of its other planets to manoeuvre and accelerate the craft during a journey of stupefying complexity.
The twined Cassini-Huygens package was launched from Florida’s Cape Canaveral Air Force Station atop a Titan IVB/Centaur rocket on Oct. 15, 1997.
And while the destination planet is a mere 1.5 billion miles from Earth on average, the craft had to travel more than twice that distance to get there.
The twinned Cassini-Huygens craft made two passes by Venus, where it added to already ample scientific probing of that planet.
It also made an Earth flyby on the way out to slingshot it in the right direction.
“Our biggest gravity assist (however) was Jupiter,” Maize says.
And it was at Jupiter that Cassini worked with the Galileo probe, already in orbit around that giant planet, to achieve a one-two science punch.
“While we were surfing inside and outside of the solar wind and Jupiter’s magnetic field, Galileo was deep inside,” Maize says.
“For the first time we were actually having two spacecrafts make simultaneous measurements of a giant planet’s magnetic field and that was just incredibly exciting.”
It also gave Cassini — named after Italian astronomer Giovanni Cassini — a chance to calibrate and test its 12-pack of instruments, two years before its Saturn encounter.
Later, by waking up Cassini’s propulsion system — which would insert it into Saturn’s orbit — at just the right time, the ground crew was able to take the first close-up readings and images of Phoebe, a Kuiper Belt planetoid that had been pulled into a Saturnal orbit.
If its beginnings were eventful, Cassini’s scheduled death a little more than three years hence promises to be breathtaking, Spilker says. The ship will be taken closer and closer to the planet until it’s finally plunged into its gaseous mass.
“Just imagine the great pictures we’re going to get of both the rings and the planet when we’re so close,” she says.
But before that grand finale, Maize says, he is certain of only one thing:
“We’re going to continue to be surprised.”