Last updated September 20, 2017 at 4:00 pm
It’s the last hurrah for one of NASA’s long running missions. Astrophysicist David Gozzard takes us through why Cassini was a game-changing satellite.
On 15th September NASA’s Cassini, a satellite currently orbiting Saturn, will plunge into the planet’s atmosphere and burn up – a fiery end to a spectacular 20-year mission of discovery.
Since April, Cassini has executed a series of 22 dives to pass between the planet and its iconic rings, a risky manoeuvre giving planetary scientists unprecedented close-up views of Saturn’s weather patterns and ring structure. These dives, culminating in the final plunge into the planet, have been dubbed Cassini’s Grand Finale. In honour of Cassini’s passing, let’s look back on this remarkable mission and the astounding things it has taught us.
To boldly go…
After the Pioneer and Voyager probes made spectacular but brief flybys of Saturn and the other giant planets in the 1970s and 80s, NASA pushed on with the next stage of the exploration of the outer solar system. They planned longer, orbital missions to study each planet in greater detail, but in the end, only two of these orbital missions made it to the launch pad. The Galileo probe studied Jupiter and its moons from 1995 until 2003, but missions to Uranus and Neptune were cancelled. Cassini emerged as a joint mission between NASA and the Italian Space Agency (ASI) to send an orbiter probe to Saturn. The European Space Agency (ESA) also joined the project, and developed the Huygens probe, a smaller craft carried by Cassini, designed to land on Titan, Saturn’s largest moon.
Costing US $1.4 billion, and packed with scientific instrumentation, Cassini is one of the most advanced robotic probes ever built. Not only does it have some of the best scientific equipment launched to space at the time, it is also highly automated. The radio signals controlling Cassini take 68 to 84 minutes to get from Earth to Cassini, meaning there is no way to control the spacecraft in real-time. To overcome this complication, the engineers gave Cassini the ability to carry out many tasks without further input.
Cassini-Huygens was launched on the 15th October 1997, however to reach Saturn it needed to take a round-about route involving several gravitational slingshots around other planets. Cassini-Huygens initially flew away from Saturn into the inner solar system where it made two flybys of Venus and one of Earth, gaining speed with each pass.
However, these flybys weren’t without controversy back on Earth. Because Saturn is 10 times further from the Sun than Earth, solar panels would be too big and heavy to supply power to the craft, so Cassini is powered a 32.7 kg lump of plutonium. Had the craft been slightly off its path on its approach to its slingshot around Earth, it could have crashed and broken up in the atmosphere, contaminating a huge area with its plutonium power source. In the end Cassini passed Earth safely and headed into the outer solar system where it received one last speed boost from Jupiter, sending it on its way to Saturn.
As an added bonus, mission scientists were also able to use Cassini to test, and once again prove, Einstein’s theory of General Relativity. While en route to Saturn, Cassini passed behind the Sun from the point of view of Earth. As the probe passed behind the Sun, radio signals reaching Earth from Cassini were deflected and warped by the Sun’s gravitational field in exactly the way predicted by Einstein, yet another proof of the nearly 100 year old theory.
After nearly seven years in space, Cassini-Huygens entered into orbit around Saturn on 1st July 2004 and began studying the planet in earnest. To be captured by Saturn’s gravity in the right way to begin its planned exploration Cassini had to pass through the largest gap in Saturn’s rings, a risky galactic threading-the-needle manoeuvre. Even though the ring gaps look empty, there was the real risk the area chosen to pass through contained thinly spread particles of dust and ice about the size of a grain of sand. At Cassini’s speed, colliding with something even this miniscule could significantly damage the craft and cripple it. To minimise the risk of damage as much as possible, Cassini dived into the ring with its large radio dish pointing forward – a highly expensive shield to protect the rest of the craft in the event of a collision.
Now in orbit around Saturn, Cassini could begin its scientific mission, including studying the structure and behaviour of Saturn’s rings to try to determine how they were formed; observing Saturn’s moons to discover their history; and analysing Saturn’s atmosphere and magnetic field in order to better understand the planet.
A whole new world
One of the most exciting targets for exploration was the moon Titan. Its name is fitting as it is the second-largest moon in the solar system and even larger than the planet Mercury. Measurements by ground-based telescopes and previous spacecraft showed that Titan had a thick atmosphere, making it an exciting and interesting target for Cassini.
Titan’s atmosphere is mostly nitrogen (like our own) but with large amounts of hydrocarbons such as methane. However the moon is so cold that these hydrocarbons behave like water does on Earth, evaporating from lakes and seas on the surface, forming clouds, and falling back down as rain. And all this makes Titan somewhat unique, as the only body other than Earth in the solar system to have lakes and seas on the surface. This resemblance to our home planet allows us to get a glimpse of some of the processes that may have happened on early Earth.
After Cassini entered into orbit around Saturn, it made a series of flybys of Titan, using radar and special imaging filters to peer through the thick, hazy atmosphere to study the terrain below, and map-out possible landing areas for Huygens. Then on Christmas day 2004 Cassini released Huygens to plunge into Titan’s atmosphere. After surviving a fiery entry, Huygens deployed a parachute and began taking images and readings as it drifted slowly towards the ground. During its descent, Huygens studied Titan’s wind and weather, analysed it atmosphere, and identified dunes, and drainage channels running from nearby high ground to a vast, flat floodplain.
Because Titan is much smaller than Earth, with much weaker gravity, but a thicker atmosphere, Huygens drifted down under its parachute for two and a half hours before bouncing to a stop on the surface. Huygens landed on damp ground in the floodplain where its camera showed a flat plain covered in pebbles made of water ice that are rounded due to weathering by liquid hydrocarbons when the plain is in flood.
That hazy atmosphere of Titan gives everything an orange hue and, being so far from the Sun, the light levels were similar to those experienced during twilight on Earth. The feeble sunlight also means that the rate of evaporation on Titan is 100 times slower than on Earth, but Titan’s atmosphere is able to hold nearly 100 times more vapour than Earth’s atmosphere before it rains. Together, this means Titan is expected to experience decades, or even centuries of drought, interspersed with torrential rains causing flash floods.
Huygens is, so far, the only probe to ever land on a body in the outer solar system, and holds the record for the furthest a spacecraft has ever landed from Earth.
In 2005 Cassini turned its attention to Enceladus, Saturn’s sixth-largest moon. Enceladus has an icy surface strewn with cracks and fissures, but very few impact craters, meaning that the moon is still geologically active. The moon’s interior is kept warm by variations in the gravitational tug from Saturn and this activity causes a series of large geysers at the moon’s south pole that spew ice and other chemicals out into space. The ice from these geysers falls into orbit around Saturn, creating Saturn’s faint outer E-ring.
In 2008 and 2015, Cassini skimmed low over the surface of Enceladus and passed through the plume from the geysers to analyse their make-up. What these passes revealed is that under its icy surface Enceladus has a salty ocean around 10 km deep encircling the entire globe. The warmth from the centre of the planet and this water and abundance of chemicals makes Enceladus one of the most likely places in the Solar System to find alien microbial life.
Lord of the Rings
Any planet can have rings, and all of the giant planets, including Jupiter, Uranus, and Neptune, have ring systems. However no planet beats Saturn’s for size and awe-inspiring appearance.
The rings are mostly made from trillions of small pieces of ice, and some dust, ranging in size from grains of sand to the size of small mountains. What is surprising though is just how incredibly thin Saturn’s rings are. While they are around a quarter of a million kilometres across, they are on average only tens of metres thick. If the rings were scaled down to the size of a football oval, they would be as thin as tissue paper.
How the rings formed was one of Saturn’s greatest mysteries. Astronomers couldn’t be certain whether the rings formed with the planet billions of years ago from the primordial dust cloud, or were the result of the cataclysmic breakup of a moon around the time of the dinosaurs. Cassini’s observations showed that different parts of the rings were very different ages, and that Saturn’s moons create processes that recycle and maintain the rings, making it likely that the rings formed at the same time as the planet and have been there ever since.
Since reaching Saturn, Cassini has discovered seven new moons, bringing Saturn’s total up to 62. Cassini even discovered a future moon, in the process of forming from material in Saturn’s rings. Much of the shape and structure of Saturn’s rings is governed by its moons. Moons such as Pandora and Prometheus help to maintain the rings by shepherding the ring material, preventing it from spreading out. The gravitational influence of some of the moons causes wave-like features in the rings, and also explains most of the gaps in the rings, though the causes of some of the gaps remain unexplained.
Saturn’s rings also act like a mini solar system, allowing us to observe what might have happened as the ring of dust and gas around our young Sun formed into the planets, asteroids, and comets. The ring material clumps and breaks apart in the same way as the early proto-planets, and the new moons forming in Saturn’s rings create the same clumps and gaps in the rings that early planets would have made in the Sun’s dust cloud.
Realm of the giants
Cassini has sent us astonishing information about the most visually awe-inspiring planet in our Solar System, and has taught us important lessons not only about Saturn, but about our whole Solar System — lessons that we can apply to the many alien solar systems we are discovering out in our Galaxy. Gas giant planets like Jupiter and Saturn are the first to form in a young solar system, and being so large, they influence the formation of the other planets that form later on in the system, planets like Earth.
By studying Saturn and what it is made of, we can learn about how it formed. What material did it form from? How close to the Sun did it form? How long did it take? The data from Cassini help to answer these questions, and help us understand the processes that shape our planetary system and others like it.
Saturn’s moons like Titan and Enceladus also give us insights into the chemistry of early Earth, before the emergence of life. Observing the weather on Saturn and Titan helps us to better understand our own weather. Being so far from the Sun, Saturn and Titan receive only 1/100th the amount of light from the Sun that the Earth does, and by studying such an extreme environment we gain a better understanding of how to model and predict the weather back home.
However not every mystery of Saturn was solved by Cassini. One that remains is a storm at Saturn’s north pole. The clouds around the storm have formed into the shape a hexagon, several times larger than the diameter of the Earth. The hexagon was originally discovered by the Voyager missions in the 80s, and Cassini extensively studied the fluid processes that form and maintain the odd feature, but the data are still inconclusive.
Smile, you’re on Cassini Camera
On 19th July 2013, Cassini was scheduled to take a picture of Saturn and its entire ring and moon system back-lit by the Sun. Although Cassini had done this twice before, NASA realised that this presented an excellent opportunity for the public to get involved. NASA and other teams spread the word and told the public to go outside, look up, and smile at the time the picture was scheduled to be taken. The image, titled The Day the Earth Smiled, was released soon after, with Earth appearing in frame as a small, pale-blue dot.
Cassini’s Grand Finale
All good things must come to an end. After nearly 20 years in space and studying Saturn and its rings and moons for more than a decade, Cassini is running out of fuel. When it does, NASA will be unable to control the orbit of the craft. The worst case scenario when that happens is that Cassini crashes into one of Saturn’s moons, contaminating any potential alien life with Earth microbe stow-aways. To avoid that possibility, Cassini will be directed to plunge into Saturn’s atmosphere where it will safely burn-up on entry, keeping the moons pristine for future missions of exploration.
In preparation for its final moments, Cassini has been making a series orbits skimming low over Saturn, inside the rings. This manoeuvre is risky because it greatly increases the chances that Cassini could collide with a stray piece of material from the rings, possibly crippling or destroying the craft. But with the end in sight, it was decided that the amazing close-up views of Saturn’s atmosphere and the inner rings that Cassini would obtain were worth the risk. The Saturn-skimming orbits, culminating in the fiery plunge into the atmosphere, are Cassini’s final mission, it’s Grand Finale.
Cassini is a staggering technical achievement. A robotic probe, able to operate for nearly 20 years in the extreme environment of interplanetary space, carrying a suite of scientific instruments designed to explore one of the most captivating planets in our Solar System. Cassini has taught us a huge amount, not just about Saturn, but about other planets, including our own. But the mission has also opened up many new and exciting questions. What is going on under Saturn’s clouds? What is the cause of the hexagon at the north pole? Is there life on Enceladus?
The answers to those questions will have to wait for the next mission.
Images courtesy of NASA and JPL