Remembering Cassini

Anybody who reads this blog or the Spacing Out newsletter knows that I get techno-crushes on spacecraft. I did go into some light mourning when the Ingenuity helicopter was permanently grounded on Mars and I’m still not fully over the shutdown of the Gaia Telescope.

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But of all the spacecraft that have been sent across our solar system and etched their missions into the annals of science history, I do have a favorite: my beloved, long-departed Cassini. Besides the obvious allures of the Saturn system, one of my college mentors was a Cassini team member and my first job out of grad school was working with Cassini data as a research assistant. Cassini has a very special place in my heart.

July 1 marks the anniversary of Cassini’s entry into Saturn orbit back in 2004, so it seemed as good a time as any to look back over this spacecraft’s remarkable mission—and why it has earned its place in astronomy history.

Cassini-Huygens

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Technically I should be referring to the mission as Cassini-Huygens, as it actually consisted of two spacecraft, NASA’s Cassini orbiter and the ESA’s Huygens lander. The joint mission was selected for development in the late 1980s partially to give the two space agencies a mission to work on together, allowing them to work as partners instead of competition. 

It was, in fact, this aspect of international cooperation that saved Cassini from NASA budget cuts in the early 1990s. Cutting Cassini would have also meant cutting Huygens, which would have been a bad look. It was enough to get the mission to the launchpad on October 15, 1997, when a Titan IV-B rocket lifted off carrying the several ton, 22-foot-long (6.7 m) Cassini which, in turn, had the much smaller Huygens probe nestled securely at its side.

The spacecraft was on its way, but  it ain’t easy to get to Saturn. For a number of reasons (many of which I outlined in this previous post on spacecraft trajectories), going directly there is out of the question, and Cassini-Huygens took a very long, indirect route with a number of gravity assists to get out that far. It took a trajectory nicknamed VVEJ (vee-vej): two flybys of Venus in 1998 and 1999, an Earth flyby in 1999, and a Jupiter flyby at the end of 2000. At that point it had finally accumulated enough stolen planetary momentum to make it out to Saturn’s orbit.

On July 1, 2004, Cassini-Huygens fired its engine to spill off speed and allow itself to be captured by Saturn’s gravity and pulled into an orbit around our solar system’s most famous ringed planet, the first spacecraft to do so. It had arrived.

Getting to Work

Cassini wasted no time, doing its first flyby of Saturn’s large moon Titan the very next day and discovering previously unknown Saturn moons within its first month. On Christmas Day 2004 Cassini released Huygens, which would go on to become the first (and so far only) spacecraft to land on a moon other than our own when it touched down on Titan in January 2005. Honestly, I’m not going to go into any of what Huygens found here—that’s worthy of its own post.

Cassini, now alone, continued on its primary mission, designed to last until 2008. Given the spacecraft’s health at that point, the mission was extended until 2010 and renamed the Cassini Equinox mission, since Saturn was passing through its autumnal/vernal equinox during that time. When Cassini was still going strong at the end of the Equinox mission, it got extended again through Saturn’s solstice in 2017 (and, yes, renamed the Cassini Solstice mission).

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All told Cassini spent over thirteen Earth years in orbit around Saturn—almost half of a Saturnian year. It divided that time and its attention between the planet, its ring system, and the moons it could easily explore. And, as anyone does when they spend a lot of time getting to know a place, it learned a lot.

A Treasure Trove of Discovery

Okay, so obviously I can’t go into everything Cassini discovered in thirteen years of observations, especially since those observations are still being used to make new discoveries today. But I can list a few big highlights. 

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Thanks to Cassini, we now know a lot more about Saturn’s most famous feature, its enormous ring system. Cassini mapped the intricate structure of the rings and was able to see changes happening within them, spotting periodic features resembling propellers and spokes. It also confirmed that waves in the rings can be caused by Saturn itself, giving us a view into the planet’s interior we’d otherwise lack.

Given that Cassini was able to get views of Saturn’s poles we can’t get from Earth, it was the first to discover that Saturn’s north pole is permanently ensconced in a hexagon-shaped hurricane, bound by powerful jet streams and with an eye 50 times the size of the average hurricane on Earth. Cassini also got to witness the Great Storm of 2010, when a weather event initially the size of Earth got stretched over months to cover 2 billion square miles (5 billion square km) of the planet’s northern hemisphere. And then, for reasons still not understood, it vanished within a week.

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While the planet is, obviously a source of fascination, many of Cassini’s key discoveries pertained to its ridiculous number of moons. Huygens may hold a key place in our current understanding of the Titan, but it was Cassini that was able, via radar, to map the vast extent of its methane seas, including what might qualify as the solar system’s largest lake, Kraken Mare. 

Cassini’s flight through the geysers of the moon Enceladus confirmed their origin in a subsurface ocean. While it’s difficult to say for certain, Cassini’s readings suggest that the Enceladan ocean may have all the ingredients needed to support life, putting it near the top of most lists of potentially habitable solar system places.

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Cassini flybys were able to give us our first up close images of some of the solar system’s most bizarre-looking moons, such as the two-toned Iapetus or the weirdly spongy Hyperion. One of my personal favorite discoveries is that a number of the moons in and near Saturn’s rings look hilariously like ravioli. 

And while it’s not necessarily a science achievement, Cassini did give us one of the greatest space images ever captured. Taken while Saturn was between Cassini and the Sun, the picture shows the planet and rings brilliantly backlit, showcasing the unmatched beauty of the ring system. And then, tucked just under the rings in the lower right, is a bright dot—Earth, seen from almost a billion miles away. This image has the delightful name “The Day the Earth Smiled”.

Grand Finale

Even a mission as epic and unprecedented as Cassini has to end sometime. Given the potentially habitable status of moons like Titan and Enceladus, the only safe way to dispose of Cassini was for it to burn up in Saturn’s atmosphere—but that didn’t mean it couldn’t get there in spectacular fashion! 

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In 2017, with its fuel reserves running low, Cassini began a series of maneuvers that brought it into a realm that it had heretofore steadfastly avoided due to the risks of impacts from ring particles: right between the planet and the innermost ring. Known as the Grand Finale (wonderfully described in this brilliant NASA video that definitely made me cry the first time I watched it), these final 22 orbits allowed Cassini not only to chart this tenuous part of the ring system, but gave it new opportunities to map Saturn’s gravitational and magnetic fields and take its closest images yet of the giant planet’s clouds.

On September 15^th, 2017, just one month shy of the 20^th anniversary of its launch, Cassini began its final fiery plunge into Saturn’s atmosphere. It transmitted data right up until the point where its antenna burned away, a faithful ship to the very end.

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That final signal reached Earth just before 8 am in Boston, well before the Museum opens. That didn’t stop me and about six other Museum colleagues from coming in early and putting the feed on one of the Museum’s big screens so that we could watch the end together. When the green spike on a graph that marked Cassini’s signal strength vanished, there were definitely a few tears.

The ship may be gone, its mission long ended, but the legacy remains. Cassini data still (and will for a long time coming) represent our best observations of the Saturn system and endures as the bedrock of astronomical discoveries about our ringed planet and its fascinating moons. When the Dragonfly spacecraft finally arrives in the Saturn system in 2034, it will do so flying on Cassini’s shoulders, a worthy successor to a worthy mission. I personally cannot wait.