Goodbye Cassini

Celebrating the conclusion of a beautiful mission.

A photograph of Saturn taken by Cassini, with a rendering of the probe superimposed. (Source: NASA)

Cassini is an incredible work of human ingenuity. The bus sized space probe has been in orbit of the gas giant Saturn for over a decade and has provided priceless scientific data on Saturn and its dozens of moons.

NASA workers preparing Cassini for pre-launch testing (source: NASA)

Cassini began its life as a proposal in the 1980s, work began around 1990, and Cassini was launched in 1997. After coasting through space for 7 years and slingshotting around Venus and Jupiter, Cassini arrived at Saturn in 2004. After 13 years in Saturn’s orbit, Cassini’s mission is coming to an end.

People with a passion for space exploration are taking this opportunity to come together and celebrate the achievements of the mission.

Cassini has travelled 7.8 billion kilometers, or 4.9 billion miles, distances which are impossible to comprehend on a human scale. Cassini has 12 scientific instruments and with these, it has gathered 635GB of scientific data, which has been cited in 3948 scientific papers. At the end of the mission, Cassini will have orbited Saturn 294 times, and flown by Saturns moons 162 times. Cassini has discovered 6 new moons, and found oceans on two of Saturn’s largest moons. When Cassini enters Saturn’s atmosphere, it will be travelling at 111,637 kph, or 69,368 mph. Cassini has been a mission of immense magnitude and spectacular achievements.

For someone who doesn’t know much about Cassini, I feel like showing Cassini’s best images would be a good introduction. The pictures Cassini has taken give an impression of the huge distances travelled, and the incredible sights seen for the first time. However, choosing Cassini’s ‘best’ images for this article is an almost impossible task: Cassini has produced thousands of images which are just as beautiful as art as they are valuable as science. The New York Times has a curated collection of what they consider the 100 best images, which can be found here:

Below are some of my favourites. I cannot write anything which will do these images justice, so I’ll let them speak for themselves. All of the following images are from NASA, and are of Saturn and its moons.

I find these images awe inspiring not just for their content, but for the achievement that they represent. The Solar System has great sights for those who look, and through the great work of tens of thousands of hard working and talented people, we get a peek.

I am not knowledgeable on the human stories behind Cassini, but I have enjoyed reading these two articles about the people behind the probe:

In 1610, Galileo looked through his telescope and was the first person to see Saturn’s most distinctive feature: its rings. Centuries later, Cassini has done incredible work in studying those rings. It took photos and measurements from all angles, solved old questions, and made new discoveries.

Saturn’s rings are hundreds of thousands of kilometres wide, but only about ten meters thick from top to bottom. They’re mostly made of ice, with bits of rock, and their origins are still not understood. There’s sufficient space between the bits of debris that make up the rings that Cassini’s mission included flying through the rings. Cassini prepared by orienting itself with its high gain antenna dish first, just in case it hit any debris.

Saturn’s rings, as seen by Cassini in the visible light spectrum (source: NASA)

Cassini has provided thousands of images of Saturn’s rings, and shed more light on ring-disturbing phenomena such as ‘spokes’ and ‘propellors’. Cassini also allowed scientists to conclude that a major contributor to Saturn’s fainter outer rings are cryo-volcanic plumes from Enceladus (which I will discuss shortly).

The tiny moon Daphnis disrupting Saturn’s rings, as seen by Cassini (Source: NASA)

Saturn has two moons within its rings, which astronomers call ‘Shepherd Moons’ because their gravity corrals the ring debris. One of these moons is Daphnis, the existence of which had been hinted at before by disturbances in Saturn’s ‘A Ring’. The tiny moon — only 8km in diameter — tumbles through the ring creating a wave pattern. Cassini was able to confirm the existence of Daphnis and provide the best photographs we have of it to date.

The unusually shaped moon Pan (Source: NASA)

The other Shepherd Moon is Pan. As with Daphnis, Pan had only been theorised based on disturbances in the ring before Cassini was able to confirm its existence. Pan has accreted a layer of ring debris around its equator such that it is often described as looking like a flying saucer or ravioli. The vertical width of the moon is greater than the width of Saturn’s ring, which means that the top and bottom of Pan stick out when viewing the ring straight on.

Saturn’s hexagonal north pole (Source: NASA)

Being a ‘gas giant’, Saturn is all atmosphere with no real surface. This leads to some interesting fluid dynamics and produces some unusual effects. One of these is that conditions on Saturn create a hexagonal vortex around the north pole of the planet.

a 2013 true colour image of the hexagon on the left, and a 2017 image on the right. The colour has clearly changed from blueish to orange-brown (Source: NASA)

Several theories have been proposed to explain the hexagon, such as the interactions of jet-streams near the pole, but no explanation has yet been agreed upon.

Not only is there a strange hexagon on the north pole, but the hexagon mysteriously changes colour over time. One proposed mechanism for this is that the change is seasonal and linked to sunlight hitting the higher latitudes.

A stunning false colour image of the heart of the hexagon. False colour takes parts of the electromagnetic spectrum which we can’t see with our eyes (in this case infrared) and puts them in the visible spectrum: colours we can see. (Source: NASA)

These images couldn’t be obtained by viewing Saturn from afar, and we are fortunate to have Cassini to send us images of these curious structures.

Saturnian storm, as seen by Cassini (Source: NASA)

Cassini was also lucky enough to witness the full lifespan of a giant storm on Saturn. This storm, also called the ‘Great White Spot’ after a similar storm on Jupiter (the Great Red Spot), was almost as large as the whole planet Earth. These storms occur on the planet once every Saturnian year (the time it takes Saturn to complete one orbit of the sun, which is equal to 30 Earth years). While the cause of this phenomenon is still unknown, Cassini has provided valuable data, which will allow scientists to refine their theories.

Cassini’s work on Saturn was just half of the picture: it also studied Saturn’s dozens of moons in detail which was previously impossible. It was once assumed that any moons so far out in the solar system would be uniformly barren and icy. Cassini’s investigation of Saturn has revealed the opposite: a great variety of interesting and unusual moons — some of which may harbour life. Many articles could (and have) been written about Saturn’s moons, but this article wouldn’t be complete without a mention of some of the more interesting moons Cassini has spotted.

Enceladus (Source: NASA)

Enceladus is an icy world, and Saturn’s sixth largest moon. Its orbit is well outside of Saturn’s rings, but below its icy surface lay scientific mysteries. Unlike Earth’s moon, which is basically unmoving, inert rock, Enceladus is incredibly active. Cassini discovered that Enceladus has a liquid ocean under its surface, and its south pole is home to more than 100 geysers which frequently fire ice out into space. These plumes are thought to be caused by cryo-volcanos under the moon’s surface, and were discovered by Cassini.

Plumes of ice shooting out from Enceladus (Source: NASA)

These plumes proved tempting to scientists, and Cassini adjusted its orbit to fly through one of these plumes. This isn’t something the spacecraft was designed to do, but Cassini succeeded in collecting useful data. Cassini found that the plume contained hydrogen, carbon-dioxide, and methane, along with a few other compounds. This is a tantalising hint at the possibility of life, as on Earth bacteria called methanogens consume hydrogen and carbon-dioxide and excrete methane. The plumes of Enceladus contain all of the potential ingredients which we know can support life.

A map of Titan produced using data from Cassini’s ‘Visible and Infrared Mapping Spectrometer’, or VIMS. All the human eye would be able to see of Titan from orbit would be a hazy orange sphere. (Source: Wikimedia)

Cassini also provided great information on Saturn’s largest moon. Titan is larger than Earth’s moon, and 75% the size of Mars, but it’s prodigious size is just the beginning of its intrigue.

Titan is home to mountains, valleys, and huge lakes of liquid methane. These lakes present the potential for exotic chemical processes which are hard or impossible to reproduce on Earth, such as extreme acids and alkalis. Titan also experiences weather, and has a ‘methane cycle’ similar to Earth’s water cycle, with clouds, storms, and methane raindrops much larger than earth-raindrops falling slowly to the ground. The complex organic molecules on Titan also provide the potential for life.

Iapetus, with the pronounced equatorial ridge visible. (Source: NASA)

Cassini took images of smaller moons such as Iapetus, a moon with an unusual equatorial mountain range. This planet-circumference ridge is home to some of the highest mountains in the solar system. Iapetus is also unusual in that it is dusted with white matter with a much higher albedo than the rest of the moon.

One last moon for this article is the sponge-like, non-spherical moon Hyperion, which is notable for its lack of an axis of rotation. The Earth, and most planets and moons, rotate on an axis. In a classroom globe, this axis is represented by the rod which goes through the Earth at the North and South poles. Hyperion is spinning on multiple axes and subject to frequent, chaotic change.

Hyperion, the tumbling moon. (Source: NASA)

This video has more information about Cassini’s exploration of the Saturnian moons.

The Cassini orbiter is the much larger vehicle built by NASA. Mounted on the side is the smaller Huygens lander built by ESA. (Source:NASA, colouration mine)

Huygens is one of my favourite space probes . It hitched a ride on Cassini and landed on Saturn’s hazy moon Titan.

The probe was named after the moon’s discoverer Christiaan Huygens, and was the product of a collaboration between NASA and the European Space Agency, or ESA. Huygens was built by ESA and joined NASA’s Cassini on its trip to the Saturn system. Once the pair arrived, Huygens separated from Cassini and descended into Titan’s atmosphere. Because Huygens did not have powerful enough radio transmitters to reach Earth, Cassini relayed its signal. Despite a few glitches with Huygens’ software which were only discovered after launch, it succeded on transmitting data for 90 minutes after it landed.

Landing on Titan means that the Saturnian moon joins an elite club of spherical bodies in the Solar System which humans have soft-landed something on. Beside Earth, the list includes only 4 bodies: two planets and two moons. The first is Earth’s Moon, which is the only body which humans have stepped foot on. The others were reached by uncrewed robotic probes. The Soviet Union landed on Venus (the planet most similar to Titan) in 1970 with Venera 7, and successfully took pictures of the Venusian surface with later probes. NASA has landed multiple probes on Mars, including a handful of rovers, and is the only organisation to have successfully done so. The fourth and final body on this list is Titan, and the only attempted landing so far was Huygens, which succeeded.

A photograph of the surface of Titan, billions of miles from Earth. (Source: Wikimedia)

Landing on Titan was only possible because of the thick atmosphere (Titan is the only moon known to have such a dense atmosphere), which allowed Huygens to ‘aerobrake’ or use the atmospheric drag to slow the spacecraft. Once it was sufficiently slow, Huygens deployed multiple sets of parachutes, which are also only useful on a world with an atmosphere. These parachutes gently set the probe down on the surface, where the camera took pictures of pebbles of water-ice scattered across the surface.

This video shows images from Huygens’ cameras in real time. In the top left is the state of Hugyens’ parachutes and heat shields, and in the bottom left is its trajectory. There is miscellaneous data on the right, but the most important part of this video is the cameras: each of the cameras has a colour and a chime which go off each time a new image is taken. The result is a spectacular video of a spacecraft landing on a moon more than a billion miles from Earth.

In great NASA probe tradition, Cassini has greatly exceeded its design lifespan of 4 years at Saturn. Now, just a month shy of 20 years since launch, Cassini’s fuel is running low, and the mission is coming to an end. Before Cassini runs out of fuel to orient itself or adjust its orbit, it will fall into Saturn’s atmosphere. Crashing Cassini into the dense Saturnian atmosphere was selected as the End of Mission (EoM) proposal in 2008 out of a menu of various options in order to avoid Earthly contamination of Saturn’s potentially life-harbouring moons.

When preparing a spacecraft, especially those bound for other worlds, every possible precaution is taken to keep the vehicle sterile. This means that the vehicle is constructed in a ‘clean room’ with airlocks to keep the contamination of the world out, and the workers wear bunny-suits and face masks. Despite their best efforts, though, it is impossible to be certain that no errant bacteria or other tiny lifeforms have attached themselves to the spacecraft.

Many types of micro-organisms have been exposed to the vacuum of space and survived. This is perhaps unsurprising considering that there are many micro-organisms called ‘extremophiles’ which thrive in conditions that are unthinkably hostile to other forms of life. On Earth, these organisms flourish in extremely hot geothermal vents, acidic hot springs, or icy lakes deep underground, and so it is feasible that they could inhabit Saturn’s moons.

One proposed origin of life on Earth is that it arrived from space. This theory is called ‘panspermia’, and it suggests that life exists elsewhere and is spread to planets aboard meteors. If this is a system which exists in our universe, it is feasible that life exists on other bodies in our solar system which are amenable to it, such as Saturn’s moons Titan and Enceladus. Conversely, we want to avoid spreading life to these planets ourselves by crashing spacecraft into them if we ever want to find true extraterrestrial life.

On Tuesday Cassini had its final flyby of Titan and was slingshotted into its terminal trajectory. Cassini is now falling in to Saturn on a trajectory which will bring it into Saturn’s atmosphere. Cassini will transmit its final image on Thursday shortly after I publish this, and on Friday it will roll into its final position. This position situates Cassini with its antenna pointed toward Earth, and it will use its reaction control system to hold this position and fight the air resistance of Saturn’s atmosphere for as long as it can. Cassini will transmit information to Earth in real time for as long as it can keep this position. After it loses control, Cassini will tumble and be subjected to re-entry heating and will finally disintegrate.

When we receive Cassini’s final transmissions, it will already have broken up more than an hour ago. This is because the transmission time at the speed of light from Saturn to Earth at the moment is 83 minutes. This gives some idea of the distance at which Cassini has operated: if Cassini were to point a light at Earth and switch it on, that light — literally as fast as it is possible to travel— would still talk almost an hour and a half to reach us.

This NASA video provides a beautiful and cinematic simulation of Cassini’s Grand Finale:

Ultimately, Cassini is a tool. Humanity has made tools for millenia, but few can compare to this one. Humans have always looked up into the night sky and seen Saturn as a bright pin-prick of light. Then we built telescopes to look closer, and we found that Saturn has a beautiful ring around it. Then we built Cassini. It has allowed us to look, to measure, and to explore worlds incomprehensibly far away in immense detail. It represents the best of human endeavours: ingenuity, collaboration, and hard work. Now that its job is complete we are disposing of our tool responsibly, because we know we’ll be back.

Goodbye Cassini.

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