Q&A: Look up! What it takes to discover a comet

For more than 1,000 years, the 2-mile-wide chunk of rock and ice lurked in the depths of space, hurtling towards the dim, distant sun. All that time, nobody knew it existed – until January 3. That night, University of Arizona astronomer Carson Fuls was trying to stay warm inside the observation room in an observatory dome dusted with snow atop Mount Lemmon just north of Tucson.

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As director of the Catalina Sky Survey program of the U of A Lunar and Planetary Lab, Fuls was sifting through telescope data when he noticed a dot of light moving against the backdrop of stars. Hundreds of such moving dots show up on any given observing night, so he didn't think much of it at first. Follow-up observations revealed that he had discovered not an ordinary asteroid, but a comet – a leftover piece from the formation of the solar system 4.6 billion years ago. Fast forward to late October, when comet "C/2025 A6 (Lemmon)" became visible to the unaided eye. We spoke with Fuls about what it is like to discover a comet nobody will ever see again in their lifetimes.

What is the Catalina Sky Survey?

We're in the business of discovery, throughout the solar system, specifically near-Earth asteroids. Our survey has found about a quarter of all objects in the solar system – main belt asteroids, comets, Kuiper Belt Objects, centaurs (small objects that blur the line between asteroids and comets), Hildas, Trojans (two specific types of objects around Jupiter), all sorts of things. We have discovered about half of all known near-Earth asteroids, and we still find about half of the roughly 3,000 NEO's that are discovered each year.

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How do you discover asteroids and comets? 

We're not out there in the dome with the telescope and an eyepiece. I'm behind a computer that processes the images as they come in. We take four images of the exact same spot in the sky, seven minutes apart. The telescope moves every 30 seconds. The background stars don't move. But asteroids, comets, other things in the solar system, move slightly, and we use our big rack of computers to pick out any points of light that change between each exposure. The computer then presents those to the observer that may have any potential moving objects. Next, we have to figure out which of the many potential moving objects are real, and which ones are image artifacts or noise. Our observers only have so much time in a night, so they'll look at the top 20 candidates out of thousands. And then the citizen science part takes the next 20 for every set of images we do and validates those. We have refined our system over 20 years, and we're still finding tons of stuff. We cover the entire sky visible from the northern hemisphere in less than a month.

What does a typical workday or, rather, work night, look like?

We have a team of 17, and we work shifts – three days on the mountain, three days off. Our beds are behind our 'warm room,' where we process the observations. So you wake up and – there's your office. When I'm running the 60-inch survey telescope, the one that discovered comet Lemmon, I wake up in the afternoon, have my coffee and as the sun goes down, we begin observing – all night long until the sun comes up again. We get those potential candidates from the telescope, we look at them, we think about them, we check against known objects, and most of them are not real. For most of the night, I'll be saying 'no, no, no,' but a decent number of them are going to be real. When they are, we send them in to the Minor Planet Center immediately. In the summertime, nights are pretty short. In the winter, we are working 12-14 hours, every night. It's a lot, but it's worth it, because on a good night, if you're running that telescope, you could find 15, 20, maybe even 30 new Near-Earth Objects. It's an incredible amount. Our unofficial motto is "relentless discovery." I like to say that from the last photon hitting our telescope to submitting for new discoveries it takes no longer than about 10 minutes. That's how we get our discovery numbers. We are on it.

When did you first realize you had found a new comet?

I see the images as they come in from the telescope, we identify moving objects, we hand it off to the observer who's there to make the final determination. I was looking and there was another object I saw that looked real. It was a real moving object, had a decent chance of being a Near Earth Object, and it didn't match any known asteroids or comets. So I reported it to the Minor Planet Center, like I did all the other objects that night, and I didn't think anything else of it. You know, it was followed up by our dedicated follow up telescope and others around the world, and I just kind of went on with my life. It wasn't until a while later that people who were doing follow up observations with different telescopes said, 'I'm starting to see a coma, you know, a fuzzy cloud around the this object, and then later a tail, so that, oh, this, this is not an asteroid, it's actually a comet.' So that, that's kind of how I found out about it. So far, I have 10 comets with my name on them.

What is special about comet Lemmon?

Unlike more common, short-period comets such as the famous Halley's Comet, which comes around every 70 years or so, comet Lemmon comes from the Oort Cloud, the outer edge of the solar system. It's a long-period comet that comes by only every 1,000 years or so. It's got a nice green color around the coma, or the comet's tail, that's due to the presence of carbon in the comet's nucleus. Like others from further out, this comet has more carbon monoxide and carbon dioxide compared to the short-period comets, which contain more water ice. Its unique composition makes for beautiful pictures.

How are comets named? 

If you discovered a moving object, and right then you could say, 'Oh, I see a coma, I see a tail, this is a comet!' recognizing it as a comet and submitted it as such to the Minor Planet Center, it will be named after you, the discoverer. This one I initially reported as an asteroid, because it was just a moving dot, and only later, others figured out that this is a comet. It bears the name of Sara Lemmon, the 19^th-century botanist after whom the mountain was named, because that's where the telescope is located. Sara has about 70 comets named after her.

What can comets tell us about the early solar system, or even Earth itself?

Earth is a water world. And for a while, folks were testing the hypothesis that water was brought to Earth by comets. But actually, when we analyze the water present in comets, we see that it contains much more deuterium, a natural variation hydrogen, than water on Earth. Think of it like a fingerprint of water. So our water probably didn't come from comets. More likely, it was water locked in carbonaceous, near-Earth asteroids such as Bennu, from which the U of A-led OSIRIS-REx mission collected a sample. 

Comet Lemmon quickly became a favorite among sky watchers. What was it like for you to see the images and social media posts of an object that you discovered? 

Honestly, I love it, because our telescopes are optimized for our discovery mission, not for taking pretty pictures. So we really depend on other individuals like comet hunters and astrophotographers to get these beautiful pictures. Without them, we wouldn't know how amazing this comet looks, and to us, that's really special.

How can people see the comet?

When I was on the mountain last week, I got to see it with the naked eye for myself. It looks like a little gray fuzzy ball, but through a regular pair of binoculars, it's great, you can kind of see the green color, you can see the tail going off. It is moving towards the west. If you go outside in the evening, after the sun has fairly well set, and you find the Big Dipper, the handle is pointing towards the comet. Most importantly, you have to find a very dark spot, with as little light pollution as possible.