Early this morning, while I lay sleeping, the Cassini spacecraft plunged into Saturn’s upper atmosphere. It had been in space over twenty years, and at Saturn for over thirteen.
The last image transmitted by Cassini, about 15 hours before impact.
Like the Galileo spacecraft before it, mission controllers intentionally configured Cassini to deorbit when it reached the end of operational life. Once the supply of maneuvering fuel is exhausted, there’s no way to control a spacecraft’s trajectory. Cassini would, eventually, crash into one of Saturn’s moons.
This isn’t a problem in-and-of itself—we don’t have much compunction about littering the Solar System with terrestrial debris. But we wouldn’t have any control over which moon, and that is a problem.
One of Cassini’s big discoveries was that Enceladus has liquid water plumes coming from its south pole. Both Enceladus and Titan (which is loaded with organic and pre-biotic molecules) could be potential abodes for life. Given that we can’t perfectly guarantee our spacecraft are sterilized of Terran microbes, disposing of Cassini in Saturn’s atmosphere is better than risking planetary contamination. The final, ring-grazing orbits concluded with a fly-by of Titan, a distant interaction robbing Cassini of just enough mechanical energy to send it into clouds below.
I agree with that logic, but the decision is bittersweet. In many ways, Cassini was the first probe that I followed through its entire science mission. I was paying attention when Cassini arrived, when the Huygens lander touched down on Titan, when the plumes were found on Enceladus, when the hexagonal storm was spotted at Saturn’s north pole. The other real contender for that title would be the Mars Exploration Rovers, Spirit and Opportunity, but the latter is still operating. Cassini, as of this morning, is over.
This famous Chesley Bonestell painting of Saturn from the surface of Titan greatly anchored my expectations for the Huygens probe.
The joy of discovery, however, is far from finished. Unless NASA finally comes around to nuclear propulsion or outer-planet aerocapture there won’t be many more Saturn probes in my lifetime, but most of Cassini’s results still haven’t reached my eyes. Even the Huygens probe, which only operated for a few hours during descent and landing, probably has some pictures I haven’t yet laid my hands on. And that’s before we consider the data from other instruments.
Space science, you see, isn’t just about snapping the prettiest photographs—though that’s certainly a part of it. We’re gathering information to test existing models of reality, and to begin the process of building new ones. Some will be refinements to our existing ideas, and some will be new notions entirely. When a scientist in twenty years has an idea about the way gas planets or their moons behave, they’ll have a wealth of information waiting to validate it with.
Artist’s rending of Cassini at Saturn. Note the high orbital inclination.
But there’s a lot of scientific value in those beautiful images Cassini sent back to us, too. Every kilo counts, and we didn’t cart those cameras to Saturn for the hell of it. We sent them to understand the planet’s atmosphere, its rings, its moons. We’ve learned so much we didn’t even know to ask about in 1997. Those pictures give us a better perspective on the science, and on our place in the universe.
Even the picture I’ve been using a personal motif the last couple years, of Earth seen from under the rings, is a Cassini image. Space exploration lets us see the view from a height, of our planet, of ourselves, and of our universe. This mission had to end, but it didn’t have to happen in the first place. We didn’t have to expand our knowledge and perception, because there’s no cosmic law dictating humanity choose wisdom over ignorance. We chose to send Cassini-Huygens to Saturn, and I’m very glad that we did.