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In Depth

Space

Cassini-Huygens

The history and discoveries of the Saturn probe

May 23, 2007

The Cassini spacecraft, with the Huygens probe seen on the right, sits atop a Titan IVB/Centaur expendable launch vehicle at the Cape Canaveral Air Station in this photo taken prior to the 1997 launch. (NASA) The Cassini spacecraft, with the Huygens probe seen on the right, sits atop a Titan IVB/Centaur expendable launch vehicle at the Cape Canaveral Air Station in this photo taken prior to the 1997 launch. (NASA)

Saturn has always inspired awe.

Jupiter might be the biggest planet in our solar system, but thanks to its vast and elaborate ring system, Saturn has always had a certain cachet among astronomers.

In the 17th century, two of those astronomers, Holland's Christiaan Huygens and Italian Giovanni Domenico Cassini, turned their telescopes to Saturn and discovered its five largest moons. Huygens claimed the largest, Titan. But even with the best telescopes there is only so much of Saturn you can see from the Earth.

In astronomy there's nothing better than actually going there, so more than three centuries after Cassini and Huygens first peered at Saturn, they are being honoured with a spacecraft and a probe that carry their names. The mission: visiting the planet and its largest moon.

The Cassini spacecraft, which arrived in Saturn's neighbourhood in 2004, is not the first probe to look at the ringed planet — the Voyager missions flew by Saturn in the early 1980s. But Cassini is special because it's going in for a closer look.

One of the scientists working on the Voyager missions, Linda Spilker of NASA's Jet Propulsion Laboratory, is also working on the Cassini mission.

"It's just very exciting to be part of a mission like Cassini," Spilker said. "You know there will be new discoveries, you just don't know what they will be."

Brett Gladman, a planetary scientist at the University of British Columbia (UBC), said discoveries are to be expected since Cassini is the first modern probe to successfully orbit another planet.

While NASA sent the Galileo probe to orbit Jupiter between 1995 and 2003, problems with its antennae meant it couldn't perform as scientists had hoped, he said. So far, the Cassini spacecraft has worked almost perfectly.

Equipped for a long-range mission

The Cassini spacecraft is the largest and most complex interplanetary spacecraft ever built. It's the result of a co-operative project that includes NASA, the European Space Agency and the Italian Space Agency. The mission is managed by the NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif.

When it was fully loaded and fueled, the spacecraft weighed more than five and a half tonnes, and measured more than seven metres high and four metres wide. It contained 1,630 interconnected electronic components, 22,000 wire connections, and more than 14 kilometres of cabling.

Loaded with a dozen instruments, Cassini has everything from cameras that can take exquisite pictures of alien landscapes, to radar that can peer into the depths of alien atmospheres. It can measure magnetic fields and analyze the quantity and composition of dust particles.

Cassini's most important instrument is the Huygens probe, whose mission was to land on the Saturnian moon, Titan.

Given Saturn's distance from the Earth — an average of 1.43 billion kilometres — transmissions to or from Cassini take anywhere from 67 to 85 minutes. The craft will send more than 300 gigabytes of data back to Earth during its mission, where more than 250 scientists from around the world will examine it.

One of Cassini's more exciting observations has been short-lived spokes in the rings of Saturn. They form in minutes and can stretch as long as 16,000 kilometres. (NASA) One of Cassini's more exciting observations has been short-lived spokes in the rings of Saturn. They form in minutes and can stretch as long as 16,000 kilometres. (NASA)

Cassini was launched in 1997, heading first to Venus for one of a series of "slingshot" maneuvers that used gravity to boost its velocity and send the craft on its way to the outer solar system. Even so, the journey to Saturn took about seven years.

During that time, the spacecraft managed to make itself useful by performing experiments and taking pictures, including the most detailed global colour portrait of the planet Jupiter.

Reaching the goal

On July 1, 2004, the spacecraft arrived at Saturn, where Cassini shifted most of its focus to the moon Titan.

Scientists hope that Titan will provide them with a window into the Earth's past. The second-largest moon in the solar system, it is the only moon to have clouds and a planet-like atmosphere.

While the Earth's atmosphere is currently made of about four parts nitrogen to one part oxygen, Titan's atmosphere is made up almost entirely of nitrogen with about two per cent hydrocarbons (such as methane). Scientist's speculate that Titan's atmosphere resembles Earth's from several billion years ago.

In October 2004, the Cassini spacecraft made the first of 45 close flybys of Titan, at one point passing only 1,200 kilometres above the moon.

On Chistmas Day 2004, the Huygens probe separated from the Cassini orbiter, landing three weeks later, sending data for about 90 minutes after reaching the surface. The probe touched down on land, near the border of the Shangri-la and Adiri regions. Based on pictures taken by Cassini, the landing site appeared to be, for lack of a better word, shoreline, Spilker said.

"Without a doubt, the place we have learned the most about, at least it's surface, is Titan," Spilker said.

"What we've found is that there are many Earth-like proceses going on on Titan. It's starting to feel more like home, but instead of water, you have methane. Instead of sand dunes, you have Titan dunes."

To Spilker, Huygens' landing on Titan was a highlight of the Cassini mission.

"There have been some suprises as well," she said. "We thought Titan was covered with liquid seas, but now they seem to be only on the poles."

The Cassini probe took this detailed shot of Jupiter as the craft was on its way to Saturn. (NASA) The Cassini probe took this detailed shot of Jupiter as the craft was on its way to Saturn. (NASA)

It was the summer of 2004 that the Cassini orbiter took radar images of Titan's surface and saw beyond the moon's hazy orange and yellowish atmosphere for the first time.

Two years later, lakes of liquid hydrocarbons were discovered in Titan's northern latitudes. They are the only known lakes outside of the Earth, and range in size from about 100 kilometres in diameter to one that's even bigger than any of the Great Lakes.

Moons

UBC's Gladman has focused his research on the moons of Saturn. When Cassini was launched, people only knew about 18 moons, but during its seven-year journey Gladman helped to discover a number of new ones, bringing that number up to 31.

The Cassini spacecraft managed to find another four moons, giving Saturn at least 35 named moons, and another 20 unnamed ones.

Most of the moons Gladmann helped discover are in the outer part of the Saturnian system, while the Cassini spacecraft has been focusing on the inner moons. While he regrets that their research hasn't overlapped more, Gladman still marvels at the amazing suprises the Casinni mission has produced.

For Spilker, one such surprise is the discovery of geysers of water on the moon Enceladus, which then feeds into one of Saturn's rings.

"The fact that this active moon is responsible for letting out particles that are responsible for Saturn's E Ring is amazing," she said.

The presence of geysers also indicates that Enceladus is geologically active.

"It's only 500 kilometres long, hence small," says Spilker.

"We would have expected it to be dead like Saturn's other moon Mimas, but these geysers means that it has water, liquid water most likely. Now we want to know what other chemicals are around, because anywhere there is liquid water, then there is a chance that the conditions are right for some type of life."

The next Enceladus flyby is in March 2008.

Rings

While Saturn might have some interesting moons, its rings are what capture the imagination. Extremely thin but spread over more than 480,000 kilometres, Saturn's rings are made up of billions of ice particles and rocks dancing together around the planet.

"This is the ultimate celestial mechanics laboratory," Gladman said. "You have so many particles in a ring system, you can see consequences of how gravity works to incredible precision."

One of Cassini's more exciting observations has been short-lived spokes in the rings. They form in minutes and last an hour or two before disappearing again, but can be as long as 16,000 kilometres.

The Voyager probes first detected these spokes during flybys of Saturn more than 25 years ago, but until the Cassini mission, the phenomenon had never been verified. As yet, no one knows why they happen.

Saturn

The Cassini spacecraft has also turned part of its attention to the planet Saturn itself.

In November 2006, scientsits found a huge storm brewing at the south pole of the planet.

The storm had a distinct eyewall, similar to that of a hurricane on Earth. This Saturnian storm is so big — 8,000 kilometres across — that it would stretch all the way across Canada with several thousand kilometres to spare. Its winds blow at 560 kilometres per hour, several hundred kilometres faster than any storm ever recorded on Earth.

Even with all the scientific discoveries that the Cassini mission's instruments have given us, what really grabs the public's attention are the breathtaking photographs - and there have been many.

Much like when the Russians brought back the first images of the dark side of the moon, the Cassini spacecraft has been able to get some amazing shots of Saturn from angles never seen before.

What's left to do

Cassini has already orbited Saturn for more than three years and its primary mission is set to end in 2008. Thankfully, the spacecraft is scheduled to receive at least a two-year extension, with more still possible.

The Galileo spacecraft managed to survive nine years after arriving at Jupiter before it was sent to its end in the Jovian atmosphere. But at the end of Cassini's mission, scientists have to be sure to avoid contaminating the moons Enceladus or Titan with the craft's radiological waste, as both moons might contain the beginnings of life.

Due to the danger of hitting something in Saturn's rings and losing control, NASA is looking at other eventual possibilities for disposing of Cassini — such as a high-altitude orbit or crashing the craft on one of the smaller moons.

But even after the spacecraft is gone, the job of the scientists won't be over, Spilker said.

"Right now we are looking for the most obvious and new discoveries, but over time we will put all the data together and analyze it. It will take anywhere between 10 and 20 more years."

While Gladman isn't working on the Cassini mission — there are currently no Canadian scientists working directly on the project — he still keeps a close eye on the data it is generating. Once a month, he goes in and skims through the mission websites, pulling out the most magnificent of photos and grabbing the latest discoveries as examples to use in his university classes.

Gladman feels the Cassini mission has many benefits for humanity, and hopes he isn't the only one.

"All you have to do is look at some of the images to realize how wonderful the universe is," he said.

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