Comet Leonard's discoverer explains how the unusual object was found

Astrophotographer Chris Schur captured this stunning photo of Comet Leonard on Dec. 4, 2021 from Payson, Arizona using a 10-inch Newtonian telescope and 60-minute camera exposure.
Astrophotographer Chris Schur captured this stunning photo of Comet Leonard on Dec. 4, 2021 from Payson, Arizona using a 10-inch Newtonian telescope and 60-minute camera exposure. (Image credit: Courtesy of Chris Schur)

Discovered nearly a year ago, Comet Leonard is on its final tour of Earth's neighborhood, lighting up the night sky for viewers this holiday season. 

Comet Leonard — the brightest and most anticipated comet of the year — was discovered by Gregory Leonard, a senior research specialist at the University of Arizona's Lunar and Planetary Laboratory, in January 2021. The comet was spotted, somewhat accidentally, using the Catalina Sky Survey's 1.5-meter (60-inch) telescope at the Mount Lemmon Infrared Observatory, located in the Santa Catalina Mountains in Arizona.

The comet, formally known as C/2021 A1, has since been making its way to the inner solar system, passing near Earth on Dec. 12, Venus on Dec. 17, and now en route towards the sun, expected to reach perihelion on Jan. 3. Here's our telescopes and binoculars guide to see Comet Leonard, and if you're hoping to photograph the comet check out best cameras for astrophotography and best lenses for astrophotography guides.

Related: Comet Leonard will light up the sky this month — here's how to see it

However, this showing is expected to be the comet's last — at least in our solar system (it won't be near Earth again for another 80,000 years). sat down with Leonard to talk about how the comet was first spotted and what skywatchers can expect to see in the night sky during the last leg of the comet's solar system tour. This interview has been edited for length and clarity. How did you discover Comet C/2021 A1? 

Gregory Leonard: I am an astronomer with the Catalina Sky Survey, a NASA-funded project based out of the University of Arizona, and we are directed to discover and track near-Earth asteroids — the kind of asteroids whose orbits can bring them close to the Earth and potentially impact the Earth. I discovered Comet C/2021 A1 Leonard — also known to the world as Comet Leonard — on the morning of Jan. 3, 2021. It was a serendipitous, or incidental, discovery in one of our standard survey fields, looking for near-Earth asteroids. 

We do occasionally stumble into unknown comets, and that's exactly what Comet Leonard was. I saw the object not as a point or star-like object like most asteroids would appear to us, but this one had the telltale fuzzy coma that comets have — and the coma is that thin, tenuous atmosphere that forms around the nucleus of a comet when it gets close enough to the sun to excite and sublimate, or boil off, the ices that the comet is made of. In addition to seeing that thin coma, or that fuzziness around the comet, I also detected a little stubby tail, and that of course is another telltale sign that it's likely a comet. 

Astronomer Gregory Leonard at the Catalina Sky Survey observing station.

Astronomer Gregory Leonard at the Catalina Sky Survey observing station. (Image credit: Catalina Sky Survey) You mentioned that the comet's discovery was serendipitous or unexpected. Can you explain why?

Leonard: This comet was my 10th comet discovery, and since that time I've discovered three more, so there are actually 13 Comet Leonards out there. Now, I don't name these comets. They automatically assume the name of the discoverer, which in all these cases was me over the last six years. They are unexpected [because] we are looking for near-Earth asteroids and it's only on occasion that we stumble into what appears to be a comet, and then we report those to the Minor Planet Center at Harvard, Massachusetts, which is the clearinghouse for all asteroid and comet observations. 

What was unexpected about this comet was its orbit. Unlike most other comets that are discovered throughout the year, this comet is on an orbit that brings it relatively close to the sun and the Earth — close enough for it to potentially be seen from the backyard by casual observers. What is the comet's origin and composition?

Leonard: It's origin is likely from a distant sphere of comets called the Oort Cloud, which is a vast reservoir of millions, or perhaps billions, of comets. The Oort Cloud is really far [from the sun]. It's at least 5,000 astronomical units — an astronomical unit is the sun-Earth distance — and may go out as far as 100,000 astronomical units. This comet likely came from the inner Oort Cloud, 3,700 astronomical units away and it's what we call a long-period comet. The comet has been inbound, on this leg, for the last 40,000 years. [However], if we trace that back, it's really been on an 80,000-year orbit. The last time it would've come [into] the inner solar system would have been 80,000 years ago. 

What it's made of is still to be determined. There are most likely telescopes around the planet that have been looking at the spectra from the comet, and I think we'll see some papers and research come out in 2022 talking a little bit more about exactly what [the comet] is made of. I'm sure we'll see some of the usual comet suspects, and that would be water ice, carbon dioxide, carbon monoxide, some ammonia and maybe some other exotic compounds as well. What's interesting, too, is that when I discovered the comet, it was 450 million miles away — about the distance that Jupiter orbits the sun — and the fact that I spotted a tail is interesting [because] water ice will not sublimate at that distance, in that cold region of space. So it's some other compound that would likely have been boiling away and sublimating, perhaps a carbon dioxide or carbon monoxide. I think this comet will be observed after its perihelion — its close passage by the sun on Jan. 3 — as far out as it can be observed by telescopes, and we're going to get a better idea about what it's composed of. You said you discovered the comet on Jan. 3, and now its closest approach is slated for Jan. 3, 2022. Is that just a coincidence? 

Leonard: It is. Jan. 3 of this year, 2021, was the actual discovery. So it was discovered exactly one year before perihelion — that is strictly coincidental, but I like coincidence. 

Comet Leonard shines bright in this image from the European Space Agency's Near-Earth Object Coordination Centre  using the Calar Alto Schmidt telescope in Spain. It was created by stacking 90 5-second exposure images of the comet taken on Dec. 7, 2021 on top of each other.  (Image credit: ESA/NEOCC) Since its discovery, what have we learned from this comet, either about comets in general or about the early solar system? 

Leonard: Once we understand better the exact composition of this comet, it will give us some clues and hopefully patch in some holes of knowledge about the conditions that existed at the time of the formation of our solar system. Comets are very, very pristine and they carry this information with them. These were basically unaltered, unaffected, primitive bodies leftover from the formation of all the planets. So this is why they are really exciting for astronomers and researchers to scrutinize to better understand where we came from. Comets, in addition to the near-Earth asteroids that my project looks for, impacted the Earth billions of years ago and likely brought some amount of water, contributing to our oceans and freshwater resources. So, when we look at comets, we're really looking at pieces of our own history; our own formation of our climate, and, in fact, maybe even some of the constituents of what we ourselves are made of. How did Comet Leonard make its way into the inner solar system? 

Leonard: The comets that reside in the Oort Cloud are held in a gravitational balance between the feeble tug of the sun's gravity and the entire gravitational tug of our [Milky Way] galaxy. With the slightest bit of gravitational nudging, or disturbance in the galaxy, these comets can either sort of fall out of the Oort Cloud or they can begin their long journey inwards to our sun — and that's exactly what happened in the case of this comet. That's how many long-period comets get their beginning: They're gravitationally nudged or perturbed and the sun wins the battle. So they start cascading inwards toward the sun, taking thousands and thousands of years to do so. 

In this case, we have to say hello and goodbye to Comet Leonard. This is its final passage [in our solar system]. It has an escape velocity, moving at 44 miles per second, or 70 kilometers per second, which is sufficient for it to be flung out from our solar system forever. Just like it takes rockets a certain minimal velocity to escape the gravity of Earth, the solar system also has an escape velocity. Comet Leonard has that velocity, so once it passes close to the sun on Jan. 3, it will, from that point forward, be moving away and will leave our solar system and travel millions of years perhaps to stumble into some other stellar system a long time from now. The comet's most recent planetary flyby was at Venus on Dec. 17. What impact do you think the comet had on the planet?

Leonard: That remains to be seen. One thing that we will be looking for is the potential for Venus to pass through the dust stream left by the comet. The comet is going to go past Venus's orbit and then Venus is going to pass through that area. It's possible then that Venus may experience a meteor shower from the dust particles left in the wake of Comet Leonard's passing. It's possible with the geometry of Earth that that could be visible with telescopes from Earth. [Also] the Japanese Space Agency, JAXA, has an orbiting spacecraft around Venus right now. It's called Akatsuki … and it's on a Venus climate mapping mission. However, there may be an opportunity for some of the sensors on Akatsuki to look for potential meteors entering the upper Venusian atmosphere. Oh, to be an observer on the night side of Venus, as Comet Leonard cruised by, it would have been breathtaking to see. It passed about 2.6 million miles [4.2 million kilometers] from Venus on Dec. 17 and Dec. 18, which sounds like a long way — and for us on Earth, it is — however, that's really just a cosmic whisker. So it just missed almost clobbering Venus by a couple of days. … [The comet's] Earth flyby was about 22 or 23 million miles [35 or 37 million kilometers], so it was 10 times closer to Venus in its flyby there. What can we expect as the comet heads now towards the sun? 

Leonard: Well, I think the only thing that's consistent about comets is their utter unpredictability. A wise comet hunter and discoverer once said comets are like cats: both have tails and both do precisely what they want — and that is true. 

It's likely that [Comet Leonard] is going to dim. And that's not so much because it's getting close to the sun, [but because] the Earth is now really receding. Both objects are receding from one another. So typically, when a comet approaches the sun, we tend to see a brightening. If the Earth wasn't moving away from [the comet], we would likely see just that. But we're likely to see it dim only because we're moving away. However, there's always a chance that as it nudges closer to the sun, that there could be some increased activity, maybe some outgassing or some jetting — some outburst that occurs that will brighten the comet's appearance from Earth. And what causes those outbursts?

Leonard: As a comet approaches the sun, you'll get the ices sort of violently sublimating and boiling away. If there's a pocket of material that's suddenly released, which releases pressure, that could be enough to kind of pull the comet apart — they are very fragile objects. Comets have a really low density. The density of water is one gram per cubic centimeter, and the density of a comet is a third of that … so these are very fragile objects and any forces at all, be they gravitational or related to the volatile release from these ices blowing away, is enough to expose more material, and have that material boil away. This could be one cause for a brightening — for an outburst of the comet. 

Another recent effect … is a phenomenon known as forward scattering. The comet was very close in angle to the sun and the sunlight shining through, or essentially backlighting, all the dust in the tail has been known to enable comets to brighten considerably by one, two or even three magnitudes. So, it can temporarily brighten for 12 or 24 hours. So we won't see the comet again because of its escape velocity. Where do you think it's going to go? 

Leonard: I haven't seen any projections as to which direction it's flying off into our galaxy. It'll be fun to see what potential star system it could bump into many, many years from now. Suffice it to say that this is the final visit [to our solar system]. But there still is an opportunity for folks that really want to see it. What I would recommend is to get away from the bright lights of your town or city, be armed with a pair of binoculars — the larger, the better — or a small telescope, if you have one. It's really beyond naked-eye visibility at this point. … And, if you don't see it, just know that we've had a wonderful cosmic visitor for this winter season.

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Samantha Mathewson
Contributing Writer

Samantha Mathewson joined as an intern in the summer of 2016. She received a B.A. in Journalism and Environmental Science at the University of New Haven, in Connecticut. Previously, her work has been published in Nature World News. When not writing or reading about science, Samantha enjoys traveling to new places and taking photos! You can follow her on Twitter @Sam_Ashley13.