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Newfound Comet Leonard will blaze into view this year

The newfound comet C/2021 A1 (Leonard) will make its closest approach to Earth on Dec. 12, 2021. (Image credit: NASA/JPL)

A new comet is on its way in toward the sun, with prospects that it may become bright enough to see with the unaided eye by year's end. 

The object in question is Comet Leonard, catalogued C/2021 A1 and was discovered by astronomer Gregory J. Leonard on Jan. 3 at the Mount Lemmon Observatory, also known as the Mount Lemmon Infrared Observatory. The observatory is located on Mount Lemmon in the Santa Catalina Mountains, approximately 17 miles (28 kilometers) northeast of Tucson, Arizona. Mr. Leonard is a senior research specialist for the observatory's Lunar and Planetary Laboratory's Catalina Sky Survey

When Mr. Leonard found the comet's image, it was an exceedingly faint object of magnitude 19. That is nearly 160,000 times dimmer than the faintest stars visible to the unaided eye. Not surprisingly, when it was first sighted the comet was some 5 astronomical units from the sun (one astronomical unit, or AU, is equal to the Earth's average distance from the sun of 92.855 million miles, or 149.565 million km). So, at a distance of 5 AU, Comet Leonard was out near the orbit of Jupiter, far from the sun, but just beginning to feel the effects of its warming rays and slowly beginning the process for it to blossom into a conspicuous celestial object. 

Related: The 9 most brilliant comets ever seen 

Evolution of a "hairy star" 

Today we know comets to be made primarily of frozen gases that are heated as they approach the sun and made to glow by the sun's light. As the gases warm and expand, the solar wind — subatomic particles radiating out from the sun — blow the expanding material out into the comet's beautiful tail. 

To observers of antiquity, the tails resembled a head of flowing hair, so they called comets "hairy stars." Professional astronomers can observe anywhere from half a dozen to a dozen comets on any given night. But comets bright enough to excite those of us without big telescopes are rather unusual, perhaps appearing on an average of one or two years to every 10-15 years. 

Of course, memories are still fresh from the striking appearance last summer of Comet NEOWISE. And some are no doubt hoping that we might have a December redux with Comet Leonard.

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Coming from a long way out

When Comet Leonard was first sighted, it was about at that distance from the sun that methanol (CH3OH) and water start sublimation; in other words, going directly from a frozen state into a gas. And in some images, there was even evidence of a faint tail. 

The first calculations demonstrated that it is traveling in an exceedingly long, flattened elliptical orbit taking it out as far as 3,500 AU from the sun — 325 billion miles (523 billion km). "Out there," temperatures are just a fraction of a degree above absolute zero: minus 459.67 degrees Fahrenheit (minus 273.15 degrees Celsius) — so cold that particles stop moving. In essence, after being in a frozen state for tens of thousands of years, Comet Leonard was beginning to awake from its long slumber. 

Guarded optimism

There are a couple of reasons for being optimistic about Comet Leonard brightening up to naked-eye brightness. First is the comet's orbit itself. Its orbit demonstrates that it is not a "new" comet coming directly from the Oort cloud — an icy shell around the solar system where comets appear to originate before swoop around the sun — and experience the effects of sunlight — for the first time. 

Rather, Comet Leonard is traveling in a closed orbit and probably visited the vicinity of the sun at least once before, about 70,000 years ago. That in itself is good news. A "new" comet in a parabolic orbit — that is, a comet that has never passed near the sun before — may have its surface covered with very volatile material such as frozen carbon dioxide, nitrogen, and carbon monoxide. These ices tend to vaporize far from the sun, giving a distant comet a surge in brightness that can raise unrealistic expectations. Then as they draw closer to the sun, their rapid brightening suddenly slows and they end up falling far short of brightness expectations. 

Comet Leonard does not fall into that category.

The newfound comet C/2021 A1 (Leonard) will reach perihelion, its closest point to the sun, on Jan. 3, 2022. (Image credit: NASA/JPL)

The other reasons for expecting a bright show from this comet are its close approaches to both the Earth and the sun. On Dec. 12, it will pass within 21.7 million miles (34.9 million km) of Earth, and on Jan. 3, 2022 — exactly one year after discovery — it will pass within 57.2 million miles (92 million km) of the sun. 

When using standard power-law formulas, taking into account how bright the comet is now versus how much closer it will be by year's end (to both Earth and the sun), the current expectation is that the comet could reach as bright as fourth magnitude, making it bright enough to see without optical aid in a dark sky.

Where and when to look

During the first two weeks of December, Comet Leonard will be accessible to early risers, visible a couple of hours before sunrise, low in the east-northeast sky. It will track through the constellations Coma Berenices, Boötes and Serpens Caput. 

It should be an easy object to see with a small telescope or a pair of binoculars — and with any luck, with the unaided eye. During the latter half of December, as the comet gets closer to the sun, it will gradually get absorbed into the light of dawn and finally disappear from view.

But temper your expectations ... 

Will Comet Leonard evolve into another NEOWISE? Unfortunately, from my many years of observing comets, I would have to say no. Most comets are at their best after reaching their closest point to the sun (perihelion) and heading back out into deep space. This is when comets release their maximum amount of dust and gas and when they are intrinsically at their brightest and their tails at their longest. 

Comet Leonard will be hidden by the brilliant solar glare during this time, rapidly receding from both the sun and Earth after Jan. 3 of next year and quickly fading away. The best, I think, we can hope for is a modestly bright naked-eye comet that will be available to anyone who cares to get up before sunrise during early and mid-December mornings.

I have seen other websites stating that Comet Leonard will be a "once in a lifetime" sight. That is true, for once it rounds the sun it will be thrown out of the solar system, never to be seen again, according to EarthSky.org. Another claim suggests that it could be "the most brilliant and dramatic" comet of this year. If you compare it to the other "faint fuzzies" due to appear this year, that also is true, although to use words like "brilliant" and "dramatic" are hyperboles. 

But since all new comets are notoriously unpredictable, we can only guess just how bright Comet Leonard will get and its tail will get. We're just going to have to wait and see. Space.com will keep you posted on its development in the coming months. Stay tuned! 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook. 

Joe Rao
SPACE.COM SKYWATCHING COLUMNIST — Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's lower Hudson Valley.

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