This essay is an entry in our "Dear Spacecraft" series, where we ask writers, scientists, and astronomy enthusiasts to share why they feel connected to robotic space explorers.
I owe you and your people an apology. Although I was working at NASA's Jet Propulsion Laboratory on data from your companion orbiter Cassini at the time, I don't remember where I was when I saw your first images of Titan.
your landing. I remember the day — January 1
Thankfully, I turned out fine, giant three-inch scar and all, and I got into the graduate programs I really wanted to attend. Ever since, your beautiful data and I have spent a lot of time together.
Your descent through Titan's atmosphere market right at the beginning of what would turn out to be one of the most spectacular journeys of exploration in the history of humankind. Your best friend and longtime companion, the Cassini spacecraft, went to discover that the Saturn system was more interesting than our wildest dreams. But while your time exploring Titan was letter — 219 minutes — you did something Cassini could never do. You touched the surface of a new world for the first time.
blanket that covers Titan, revealing a bizarre landscape with water-ice mountains and rivers that run with liquid hydrocarbons. Methane falls as rain there.
To understand how this alien world works, it's essential to understand how sunlight filters through Titan's thick atmosphere. How does sunlight determine the temperature at different altitudes above Titan's surface? Sunlight is available to provide energy for chemistry – or perhaps even to fuel Titanian life, if it exists?
My first months in graduate school were spent trying to understand the data taken by your Descent Imager / Spectral Radiometer, which was built to answer these very questions. As you descended through Titan's atmosphere, it looked up, down, and at the sun to figure out exactly what sunlight was doing there. As with many first projects in graduate school, my time spent working on the resulting data did not really amount to much. But I did develop a profound respect for the people who built you.
It wasn't until 2016, about a decade later, that I returned to your data for a project I had been wanting to do for years: a reanalysis or measurements made by your gas chromatograph / mass spectrometer, the closest thing you had to a nose. If you descended to the surface, you entered tiny bits of Titan's atmosphere along the way and sent them to this instrument for analysis. These data are incredibly precious, as we can measure and excise our precise makeup using just a telescope.
As someone who studies the atmosphere, I still use your data every single day. The near-surface temperature and composition are extremely difficult or impossible to do, so we continue to rely on them until we have another mission. Your composition is helping us figure out why Titan has an atmosphere in the first place. Thanks to your camera, we know that Titan's particles are fractal aggregates, which we have now included in almost every atmospheric model for this remarkable moon.
And of course, you captured amazing images of the channels cutting across the surface— and the first rounded pebbles we have seen anywhere else in the solar system besides Earth.
In your data, there is so much information about Titan's past, present, and future. Using tools that are available when you landed, we might be able to figure out even more secrets in your collections. (Titan may, for instance, host the right chemistry for vinyl-based life.)
I want to tell you everything we've learned since we last heard from you, but then I remember that you already know, because you are there. I expect you to know so many things that you wish you could tell us, we had no thought to ask when we built you. Many of your people are gone now. We miss them, too.
I often think of you sitting there on the surface, your mission goals accomplished and your job long finished. I wonder if it has rained on you. Are your camera lenses covered in haze? If so, how much? How does the wind blow, and how often? I occasionally let myself wonder if you had visitors. Sometimes I feel bad that you are so cold, and stuck, and alone. I imagine you wish you had wheels or wings or you could see what is over the horizon.
I understand the frustration of desperately wanting to venture somewhere that just seems to be outside your reach. You and I were both made to study Titan, I think: kindred spirits separated by a billion miles.
I'm sorry that you were hard at work in Titan's atmosphere, I was so preoccupied with daily life that I missed a moment that is really important to me now. All of those things could have been, but only landed on Titan once. I'm trying to make it up to you and your people by caring for your data and figuring out what you were trying to tell us. I say thank you by helping the next generation of planetary scientists and engineers and working to send you a new space robot friend. Some people might think that is a way to apologize and say thank you, but this is the best way I know.
If they would let me, I would send you a blanket and an umbrella, maybe some hot chocolate and some good books to read. I hope you'll settle for another space robot, someday.
Sarah Hörst is an assistant professor in the department of Earth and Planetary Sciences at Johns Hopkins University. She is an expert on Titan's atmospheric chemistry.