Illustration by ESA/C. Carreau
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An illustration shows the landing site of the Huygens probe on Titan, Saturn's largest moon. On January 14, 2005, Huygens completed the farthest landing on another world ever attempted.

Illustration by ESA/C. Carreau

Dear Huygens: When you landed on an alien moon, you changed my life

Fourteen years ago today, the spacecraft completed the farthest landing on another world—forever shaping the work of one scientist.

This essay is an entry in our "Dear Spacecraft" series, where we ask writers, scientists, and astronomy enthusiasts to share why they feel personally connected to robotic space explorers.

Dear Huygens,

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.

Truth be told, I paid very little attention to your landing. I remember the day—January 14, 2005—quite vividly, but not because of your accomplishments. At the time, I was dealing with a health problem that would result in surgery a few weeks later, and while trapped in typical Los Angeles traffic leaving the doctor, I found out I’d basically failed the physics GRE, dampening my hopes of going to graduate school and becoming a scientist. I cried a lot that day. None of my tears were from space-exploration joy.

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Thankfully, my leg 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 marked just 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 on to discover that the Saturn system was more interesting than our wildest dreams. But while your time exploring Titan was brief—219 minutes—you did something Cassini could never do. You touched the surface of a new world for the first time. You told us things about Titan that we could only know by going there.

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On October 15, 1997, a Titan IVB/Centaur carried the Cassini orbiter and its attached Huygens probe to Saturn. More than seven years later, Huygens made the most distant landing ever attempted, touching down on the surface of Saturn's moon Titan.

Thanks to you and Cassini, we have peered through the hazy 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? What kinds of sunlight are 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 of measurements made by your Gas Chromatograph/Mass Spectrometer, the closest thing you had to a nose. As you descended toward the surface, you ingested tiny bits of Titan’s atmosphere along the way and sent them into this instrument for analysis. These data are incredibly precious, as we cannot measure an atmosphere’s 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 measurements you made are extremely difficult or impossible to do remotely, so we’ll continue to rely on them until we have another mission. Your composition measurements are helping us figure out why Titan even has an atmosphere in the first place. Thanks to your camera, we know that Titan’s haze particles are fractal aggregates, which we’ve 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 had 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 weren’t available when you landed, we might one day be able to figure out even more secrets hiding in your collections. (Titan may, for instance, host the right chemistry for vinyl-based life.)

I keep wanting 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 know so many things that you wish you could tell us, answers to questions we had not even 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 strongly does the wind blow, and how often? I occasionally let myself wonder if you’ve 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 rotors so that you could see what is over the horizon.

I understand the frustration of desperately wanting to venture somewhere that seems just 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 while 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 waited, but you 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’m saying thank you by helping train 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 weird 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.

Love always,


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.