In early 2001, I was a fifth-grader and had been obsessed with our solar system’s planets my entire life. But, thanks in part to the Internet, I came to realize our solar system is more than just the row of planets on my placemat. It is a dynamic system with asteroids, comets, and moons, a lot of moons which look nothing like the one in our sky, each with its own story to tell. I spent countless hours reading about them — the fiery sulfur volcanoes on Io, the nitrogen plumes on Triton, the mysterious ocean of water underneath Europa’s icy crust.
But what was I to make of Saturn’s moons? The best pictures of Titan showed a hazy, orange orb and that was essentially all we knew about it. Enceladus was unusually bright — could that harbor an ocean underneath its ice like Europa? I wondered. Iapetus was oddly two-faced, but the data available was inconclusive as to the nature of the yin and the yang. And were the rest of the moons hiding secrets as well?
I learned that a spacecraft called Cassini was set to orbit Saturn for four years beginning in the summer of 2004. Moreover, a separate probe would detach from the main spacecraft and land on Titan! It was official: for the next three years, no other year in my mind mattered more. The countdown to 2004 had begun.
Cassini teased us with a few distant images of Saturn, but the mission truly started when it made its closest (and only) approach to the small moon Phoebe. The pictures showed, in detail for the first time, a battered, icy moon that likely originated in the Kuiper Belt. A few weeks later, Cassini entered orbit around Saturn, (still) the farthest planet any spacecraft from Earth has orbited. Data and pictures started trickling in at a faster pace. I was buckled up and ready to go for the next four years.
January 2005 was the month of Cassini’s most ambitious maneuver: land its attached probe Huygens on the moon Titan. By this time I was in 9th grade and, given the thick haze that covered the surface, had no idea what to expect. Would Huygens splash in a hydrocarbon sea? Would it land in a desert on a much drier Titan? Would it fail entirely? During school (this was before we carried phone-computers), I had to log into the nearest computer in between classes to see if any pictures had been sent back. The very first thing I saw, on the front page of the site I checked, was an image showing what appeared to be channels:
Channels! I didn’t know yet if they were dry or filled with liquid (we now know the ones imaged above are dry), but the takeway was clear: Titan has been shaped by liquids. Later that day I saw the image Huygens took from the surface, showing what looked to be liquid-shaped pebbles on some sort of past or present shoreline.
Of course, it took another year or so of observations from Cassini itself to confirm that liquids exist on the surface today. That is actually an understatement — Titan has vast methane seas, rivers, and rain. It has dunes, mountains, and valleys. It is an exotic, beautiful world and almost everything we know about it we learned from Cassini.
Titan is not the only wonder Cassini explored. One of its greatest discoveries seemed to build up as if it were the climax of a great film. Before the spacecraft entered orbit, we already knew Enceladus was a bright moon with a relatively young surface. But in 2004, Cassini discovered that the diffuse E-ring was thickest along Enceladus’s orbit. Next, it uncovered evidence for an extremely thin atmosphere on Enceladus. To follow, closeup images of Enceladus revealed fractures on the south pole which came to be known as the tiger stripes. Thermal measurements soon revealed that these fractures were quite a bit warmer than the rest of the surface.
All of this seemed to add up to something extraordinary: that this tiny moon was geologically active. The proof — the smoking gun — came in October 2005 when Cassini sent back this:
With active water geyers confirmed, Enceladus took its place as one of the solar system’s most promising targets in the hunt for extraterrestrial life. The internal heat required to maintain such a geological phenomenon also melts its interior enough for a layer of liquid water to exist between the icy surface and the rocky core. This energetic and possibly life-sustaining environment will be prime for future exploration, and we have Cassini to thank for opening our eyes to it.
Iapetus, the yin-yang moon, was another strange place I wanted to know more about. Was the dark half coated in material accumulated from space? Or did it erupt from the interior? Or was the light material the coating? Scientists had pondered that question ever since Giovanni Domenico Cassini discovered the moon in 1671, noting how it seemed to appear and disappear as it orbited, correctly postulating that one side was dark.
An early flyby in 2005 provided some more details, but it wasn’t until September 2007 when Cassini made its closest, targeted approach to the moon. The first raw images that came back are still some of the most bizarre I have ever seen, anywhere, from any spacecraft:
Data from this targeted flyby confirmed that the dark material is indeed accumulated on the leading hemisphere and then, in a thermal runaway effect, causes the cooler, brighter ice to sublimate and deposit on the trailing side. Cassini had answered a centuries-old mystery. Then again, it also opened a new Iapetian mystery altogether, discovering a huge ridge (20 kilometers tall in places) across three-quarters of its equator. The origin of this feature will have to be answered by a future spacecraft.
Saturn’s moons were not the only ones in the spotlight. The planet itself dazzled. Cassini stunned us with images of its weird hexagonal polar storm. Perhaps the most beautiful images the spacecraft took were of the planet’s crown jewel: its rings.
Cassini was originally scheduled to orbit Saturn for four years. Fortunately, the spacecraft performed so well that its life was extended, and extended, and extended, well past its expiration date. My 5th-grader self would have never imagined that I would still be following the mission 16 years later, in the year 2017.
Yet, all things must come to end. So must Cassini.
As of this post Cassini is less than 48 hours away from being steered into a direct, fatal impact with Saturn. I am not sure how exactly I’ll react when NASA scientists report that the data feed from the spacecraft has gone silent. At that moment we will know it has vaporized under the pressure of Saturn’s atmosphere.
But in its final days, I am reflecting on how important the mission has been in my life, and for humanity in general. I consider myself part of the post-Voyager generation — that is, in spacegeek terms, a generation born after the epic Voyager spacecraft tour which first explored the outer solar system. By the time I was old enough to enjoy books, I could see photographs of all the planets from Mercury to Neptune and their moons. The initial era of exploring the major planets was over, and with better technology the next phase was to truly examine the worlds we had only, at best, glimpsed.
Cassini, as well as New Horizons at Pluto, epitomizes the excitement and wonder of this era. Moons like Enceladus, Titan, and Iapetus (and Saturn itself) had been imaged before, but it took Cassini to discover their true essence. That is what exploration for the post-Voyager generation is all about, and I feel blessed to have taken part in this journey from start to finish.