Early tomorrow morning, NASA scientists will say goodbye to their Cassini spacecraft — a hardy probe the size of a school bus that has been orbiting the Saturn system for the last 13 years.
Launched in 1997, Cassini has spent a whopping 20 years in space, lasting through two mission extensions while going above and beyond what it was designed to do. But tomorrow, the probe will dive into Saturn’s atmosphere, where it will break apart and cease operating. It’s a sad time for the scientists who have worked on this mission for years, but also a triumphant one: Cassini leaves an impressive legacy of scientific discovery in its wake.
Here are some of the biggest discoveries Cassini made while it was at Saturn, some of which are so monumental that scientists think we definitely need to revisit the planetary system in the future.
Enceladus has an ocean and it may be habitable
In 2005, a strange reading from one of Cassini’s instruments changed our understanding of Enceladus forever. On February 17th of that year, the probe did its first flyby of Saturn’s icy moon, and its magnometer — which measures Saturn’s magnetic field — found a strange disturbance around the south pole of Enceladus. The magnetic field was warped in such a way that it looked as if the moon was actively emitting gases from the region.
So the team did two additional flybys with Cassini, as well as snapped pictures of the south pole to figure out what was going on. The investigation ultimately confirmed that jets of water vapor were spewing from underneath the moon’s crust. The discovery prompted NASA to completely rework the Cassini mission, adding way more flybys of Enceladus than the agency had planned. All of these follow-up observations led scientists to conclude that the moon’s jets were coming from a saltwater ocean lurking underneath the crust of Enceladus.
And that ocean may have the right ingredients to host life. Recently, the Cassini spacecraft detected hydrogen in the plumes of Enceladus, which indicates that there may be “hot spots” on the moon’s ocean floor. Known as hydrothermal vents, these features are also found on Earth and they could give rise to living organisms.
Thanks to Cassini, Enceladus has helped to change NASA’s perception of the kinds of worlds that could be habitable. Before all these discoveries, the prevailing idea was that planets had to be in the “Goldilocks Zone” — the region around a star where temperatures are warm enough for surface water to pool. But Enceladus revealed that small, icy bodies distant from the Sun can still have liquid water oceans underneath their crusts.
Unraveling the mysteries of Titan
Saturn’s moon Titan was one of the main reasons Cassini went to Saturn in the first place. NASA’s Voyager probes, which flew by Saturn in the early 1980s, showed Titan to be covered in a hazy atmosphere rich in nitrogen, and scientists were eager to learn what might be lurking underneath those clouds.
So Cassini carried with it a lander called Huygens, made by the European Space Agency. The probe landed on Titan on January 14th, 2005, becoming the first spacecraft to land on another world in the outer Solar System. During its 2.5-hour descent to the surface, Huygens gathered detailed measurements of Titan’s atmosphere and gave scientists the first up-close look at Titan’s surface, revealing dried-out river beds and lakes on the crust.
Follow-up observations from Cassini unraveled even more of Titan’s mysteries. The spacecraft’s radar instrument mapped the surface of Titan, showing lakes of liquid methane and ethane. This made Titan extra special, since it is the only other place in the Solar System we know about, besides Earth, that has liquid pooling on its surface. Many scientists have suggested that any potential life on Titan may rely on methane in the same way that life on Earth relies on water.
The moon also has a climate cycle similar to ours, forming methane clouds that rain onto the surface. And many of the chemicals found in Titan’s atmosphere are found in Earth’s atmosphere as well. In fact, scientists think Titan’s atmosphere may resemble Earth’s prehistoric one, and the moon’s famous haze has been compared to a well-known haze here on our planet. “Titan smog is like LA smog on steroids," Scott Edgington, NASA’s deputy project scientist for Cassini, said in a statement.
Mapping Saturn’s rings and giant storm
Cassini has given scientists a wealth of information about Saturn, including details about the planet’s most iconic features.
When Cassini first arrived at Saturn, it quickly started mapping the planet’s rings —huge bands of particles that stretch up to 175,000 miles wide, but are only between 30 and 300 feet thick. Through the years, the spacecraft has learned a great deal about the size and composition of the ring particles, as well as their origins. For instance, the icy particles from the plumes of Enceladus actually make up most of the materials in the outermost translucent ring, or the E Ring. Cassini also revealed that the rings aren’t actually smooth in all places; there are areas where the particles clump together, creating bumps that stick out as high as the Rocky Mountains.
Cassini also got many close-up views of Saturn’s famous hexagon — the six-sided jet stream at the planet’s north pole that spans more than twice the width of Earth. It took a while before Cassini could actually see the hexagon in visible light, since the jet stream was mostly shrouded in shadow for the first five years of the mission. That’s because Saturn takes a lot longer to orbit the Sun than Earth does, so its seasons last much longer, too. But in 2009, sunlight finally illuminated the northern hemisphere, allowing Cassini to take a peek.
The probe finally saw what was at the center of the hexagon: a hurricane very much similar to the Earth’s, but with an eye about 50 times wider than your average storm. It even found a second hurricane-like vortex swirling closer to the edge of the jet stream. Researchers later figured out that Saturn’s distance from the Sun and its atmospheric composition allow jet streams to form in geometric shapes, but the hexagon is still somewhat of a mystery: it’s unclear why the south pole doesn’t have a hexagon either, or what allows the jet stream to last so long.
However, there is some hope. Cassini got particularly close to the north pole during its final 22 orbits, known as the Grand Finale. It’s possible the data the spacecraft collected during those passes may provide further insight into this weirdly shaped storm.
Saturn is home to more than 60 moons, a few of which were discovered by Cassini. The vehicle has spotted up to six new moons at the planet, as well as potential moonlets that propel through Saturn’s rings, displacing particles above and below the bands.
Plus, Cassini has given us some incredibly detailed images of these moons. For instance, here’s the spongey moon Hyperion: the first image was taken by the Voyager 2 probe, while the second is a retouched, false-color image taken by Cassini.
The probe snapped pictures of many other moons, some of them weird looking. The moons Pan and Atlas look a bit like flying saucers (or maybe ravioli), while Prometheus looks like an awkward potato. Thanks to Cassini, we are able to marvel at these strange satellites in amazing high resolution.
And the view
Cassini is equipped with wide-angle and narrow-angle cameras, which have taken thousands of stunning photos of the Saturn system. These cameras are sensitive to visible wavelengths of light, and can also see in near-infrared and near-ultraviolet light. The cameras have been a great research tool, allowing researchers to peer through Titan’s haze, for example, or study the structure and motion of moons. But they’ve also been a phenomenal way of transporting scientists and the public to Saturn, showing the planet and its moons in ways that have never been seen before.
Plus, more to come...
Cassini may have only one day left, but it’s not done yet. On its way into Saturn today, the spacecraft will snap its last images of Titan, Enceladus, and Saturn’s rings, which will be downlinked to Earth later this evening. It will then roll a few hours before its plunge tomorrow morning, to position most if its instruments toward Saturn and its radio antenna toward Earth. That way, it’ll be able to gather data and immediately send it to our planet during its last precious minutes of operation.
So, some of the most exciting data has yet to be gathered yet. That’s because Cassini will be doing what it does best: studying Saturn up until the very end.