The Cassini spacecraft’s 20 years in space has been a marathon performance. It’s orbited Saturn more than 200 times. Along the way, it’s taken hundreds of thousands of images of the giant planet, its splashy rings and its many moons. On September 15, though, Cassini will use its last burst of fuel to plunge into the sixth planet from the sun. With awe and nostalgia, scientists and space enthusiasts the world over will watch it disappear.
“It’s hard not to anthropomorphize the spacecraft,” says Matthew Tiscareno. He works at the SETI Institute in Mountain View, Calif. And this astronomer has been working on Cassini since it entered Saturn’s orbit in 2004. “We’ve been riding on its back for these 13 years. And it’s done everything we’ve asked," he says. "I think it’s the most spectacularly successful mission that NASA has ever run.” (NASA is short for the National Aeronautics and Space Administration.)
Cassini was designed to train its 12 scientific instruments on the Saturn system for just four years. NASA, however, extended the mission twice. Even with the extra time, Cassini’s 13-year run is less than half of one year on Saturn. There, a year lasts 29 Earth years.
After all this time, we’ve witnessed only the transitions to Saturnian spring and summer. That’s the equivalent of January to June on Earth's Northern Hemisphere. And yet we’ve seen so much.
Cassini has revealed massive churning storms that rage for decades. It’s allowed the study of rings that may be the best laboratory for learning how planets form. And it’s unveiled details of some of Saturn’s more than 60 moons. Two of those natural satellites — Titan (TY-tun) and Enceladus (En-SEL-uh-dus) — surprised Cassini scientists by having many of the right ingredients for life. The craft has revamped our picture of Saturn and its celestial family.
Saturn’s potentially habitable moons are the reason Cassini must meet a dramatic end. The mission team at NASA decided it was safer to crash the craft into Saturn itself than to risk the craft wandering off and brushing up against Enceladus or Titan. If it crashed there, it might spread earthly germs to any nascent ecosystems on those moons.
But the craft will be busy until the very end. Since April, Cassini has been making weekly dives into the possibly rubble-strewn region between Saturn and its rings. This is a zone the team hadn’t dared explore before. Plus, the craft will collect data during its last hurtle into the gas giant’s atmosphere. Those final measurements should help solve some of the most basic mysteries about the planet, including when it got its iconic rings.
“Cassini data,” says team member Ralph Lorenz, are “going to keep us busy for decades.” He is a planetary scientist at Johns Hopkins University Applied Physics Lab in Laurel, Md.
In the eye of Saturn’s hexagon swirl, cloud speeds can reach 150 meters per second (340 miles per hour). The storm is shown here in false color from 2012. It has probably been there for decades, if not centuries. Saturn has no mountains or oceans to interrupt the storm.
Saturn’s tiny moon Daphnis orbits within the Keeler Gap in Saturn’s outer A ring. That gap is 42 kilometers (26 miles) wide. These 10 images were each taken about 90 seconds apart. The sequence shows Daphnis’ gravitational pull perturbing the particles at the gap’s edge. The moon is only 8 kilometers (5 miles) across. Still, its gravitational pull is enough to raise ripples in the rings around it. These waves were first noticed in 2009. That’s around the time of Saturn’s spring equinox. It’s when the planet’s spring begins, so the angle of sunlight made the waves stand out more. Daphnis has a ridge around its equator. It’s probably made of fine particles the moon has gathered from the rings.
Saturn’s rings are “arguably the flattest structure known to man,” says Tiscareno. Over a span of hundreds of thousands of kilometers, their vertical thickness typically varies by only about 10 meters (33 feet). But Cassini snapped these structures, as tall as 2.5 kilometers (1.6 miles), in 2009, when sunlight struck the rings at a perfect angle to cast long shadows.
Cassini caught a family portrait of five of Saturn’s moons in this image from July 2011. At far left is Janus, which is 179 km (111 mi) across. Then, to its right, comes Pandora, which is 81 km (50 mi) across and nestled in the rings. Enceladus (504 km, or 313 mi, across) appears next over. It’s half lit above the rings in the center of the image. Finally, on the far right are Mimas (396 km, or 246 mi, across), and Rhea (1,528 km, or 949 mi, across).
Cassini mission scientist Ralph Lorenz has this false color image of Ligeia Mare (LIE-jhee-uh MARR-ay) hanging in his office. It’s a large sea on Saturn’s moon Titan. Cassini’s radar peered through the moon’s thick orange haze to reveal an Earthlike surface with seas, rivers and clouds filled with the liquid hydrocarbons ethane and methane. This moon could possess the ingredients for life.
“Titan has been doing prebiotic chemistry experiments for us for a huge amount of time,” says team member Elizabeth Turtle. By that she means its chemical reactions could createthe ingredients needed for life to exist. She also is a planetary scientist at the Applied Physics Lab. She, Lorenz and others are working on a proposed mission called Dragonfly. It would land drones on the moon to sample its surface.
Titan is the only place in the solar system — other than Earth — known to host long-lived liquid lakes and streams. But on Titan, the liquid is mostly methane and ethane. The video below combines radar images of Titan from 2004 to 2013 as Cassini flies over its two largest seas, Kraken Mare and Legia Mare. Where the lakes look dark, the liquid is exceptionally still and flat as a mirror.
By guiding tiny particles around themselves, small moons embedded in Saturn’s rings create the propeller-like features seen here. Scientists have followed these objects for more than a decade. They’ve even named the larger ones after pioneers of aviation. These images, taken February 21, 2017, show two views of Santos-Dumont, named for a Brazilian-French aviator. “This is the only time in the history of astronomy that we’ve tracked the orbit of an object that is orbiting in a disk,” says Tiscareno of the SETI Institute. Studying the propellers can help reveal how planets that form in the disk of gas and dust around a young star grow.
One of the biggest surprises of the Cassini mission was that the icy moon Enceladus is spewing its guts into Saturn’s rings. These jets from the moon’s south pole come from a subsurface ocean. That ocean may have the right chemistry for life. The jets also supply icy material to one of Saturn’s rings.
This false color image from 2005 shows the reach of the spectacular plumes on the moon Enceladus. Later sampling by Cassini revealed that the plumes contain molecular hydrogen, ammonia and a variety of organic compounds. All are signs that this moon might be habitable. NASA is considering a mission to go back and sample the plumes.
Cassini spotted Saturn's shimmering aurora dancing near its south pole in July 2017. The bright spots shooting across this video from the bottom left are due to charged particles hitting the detector. Behind the spots, you can see the aurora's ghostly glow. These light shows are created when charged particles from the sun strike the planet’s atmosphere and make its gas glow.
Saturn's outer ring is called the F ring. It’s sculpted by tiny moons that pass by. The ring’s dust and ice particles are tugged by the moons’ gravity. These images from Cassini, taken between 2006 and 2008, show various disturbances in the F ring.
This iconic Cassini image is known as “The Day the Earth Smiled.” On July 19, 2013, Cassini turned back toward its planet of origin and shot a picture with Saturn’s rings and Earth and its moon all in the same frame. It was the third time Earth was imaged from the outer solar system. But this was the first time humankind got a heads-up, so people could look up and smile or wave for the camera.
ammonia A colorless gas with a nasty smell. Ammonia is a compound made from the elements nitrogen and hydrogen. It is used to make food and applied to farm fields as a fertilizer. Secreted by the kidneys, ammonia gives urine its characteristic odor. The chemical also occurs in the atmosphere and throughout the universe.
anthropomorphize (adj. anthropomorphic) To attribute human traits or behaviors to some animal or object.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atmosphere The envelope of gases surrounding Earth or another planet.
aurora A light display in the sky caused when incoming energetic particles from the sun collide with gas molecules in a planet’s upper atmosphere. The best known of these is Earth’s aurora borealis, or northern lights. On some outer gas planets, like Jupiter and Saturn, the combination of a fast rate of rotation and strong magnetic field leads to high electrical currents in the upper atmosphere, above the planets’ poles. This, too, can cause auroral “light” shows in their upper atmosphere.
Cassini A space probe sent by NASA to explore the planet Saturn. Cassini was launched from Earth in 1997. It reached Saturn in late 2004. The craft included a variety of instruments meant to study Saturn’s moons, rings, magnetic field and atmosphere.
celestial (in astronomy) Of or relating to the sky, or outer space.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
drone A remote-controlled, pilotless aircraft or missile.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
Enceladus The sixth largest of Saturn’s more than 50 moons. Enceladus is bright white and covered with a thick shell of ice. Deep beneath that ice sits what appears to be a global ocean of salty liquid water. Enceladus is a round sphere, 500 kilometers (310 miles) across. It is a little less than one-third the width of Earth's moon.
equator An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.
equinox The day on Earth (or another planet) when the period of sunlight and darkness are roughly equal.
false color One or more hues that have been applied to a normally black-and-white image to highlight regions of significance.
fuel Any material that will release energy during a controlled chemical or nuclear reaction. Fossil fuels (coal, natural gas and petroleum) are a common type that liberate their energy through chemical reactions that take place when heated (usually to the point of burning).
gas giant A giant planet that is made mostly of the gases helium and hydrogen. Jupiter and Saturn are gas giants.
germ Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
habitable A place suitable for humans or other living things to comfortably dwell.
haze Fine liquid or solid particles scattered through the atmosphere that make it hard to see. Haze can be caused by harmful substances such as air pollutants or by water vapor.
hexagon A geometric shape that has six equal sides. It takes its name from the Greek word for six.
hydrocarbon Any of a range of large molecules containing chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
icon (adj. iconic) Something that represents another thing, often as an ideal version of it.
methane A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
nostalgia (adj. nostalgic) A sad, wistful or affectionate feeling about something or somebody that you miss from the past.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
organic (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms.
particle A minute amount of something.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
plume (in geology) Fluids (air, water or magma typically) that move, largely intact, in a feather-like shape over long distances.
prebiotic An adjective that describes something that existed prior to living things. (in nutrition) A food or nutrient that promotes the growth of beneficial microbes in the gut.
radar A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
Saturn The sixth planet out from the sun in our solar system. One of the four gas giants, this planet takes 10.7 hours to rotate (completing a day) and 29 Earth years to complete one orbit of the sun. It has at least 53 known moons and 9 more candidates awaiting confirmation. But what most distinguishes this planet is the broad and flat plane of seven rings that orbit it.
SETI An abbreviation for search for extraterrestrial intelligence, meaning life on other worlds.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
terrain The land in a particular area and whatever covers it. The term might refer to anything from a smooth, flat and dry landscape to a mountainous region covered with boulders, bogs and forest cover.
titan The term for any gigantic being. The term comes from Greek mythology. The six sons and six daughters of the Greek gods Uranus and Gaea were known as titans. (in astronomy) A large moon of Saturn.
transition The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another.
vertical A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.