How bright will Comet Leonard be? – Sky & Telescope

On October 10, 2021, Comet Leonard displayed a compact coma and a dusty 4-inch tail pointing north. Now at magnitude 12 and moderately condensed, the comet is bright enough that enthusiasts can search for it before dawn.
Michel Mattiazzo

Predicting the brightness of a newly discovered comet is no easy task. Astronomers often use the power law formula to make visibility predictions, but an equation may overlook the essential unpredictability of a comet. Not only do these fragile, icy fossils experience surprise explosions and disintegrations, their brightness can vary dramatically depending on something as simple as the angle of view.

Panoramic map of comet Leonard
Comet Leonard is moving rapidly southeast in the morning sky in early December, disappearing in solar glow on the 12th. Positions are displayed every three days with stars down to magnitude 6.
Stellarium with additions by Bob King

With that in mind, let’s look at the upcoming appearance of Comet Leonard (C / 2021 A1). Senior research scientist Greg Leonard at Mount Lemmon Observatory discovered the magnitude 19 point on January 3, 2021, exactly one year before perihelion. Orbital calculations revealed that the object had passed the last 35,000 years heading towards the sun after reaching aphelion at the icy distance of about 3,500 at Comet Leonard will again pass closest to the Sun on January 3, 2022, to 0.62 at Two weeks before December 12, it makes its closest approach to Earth at 34.9 million kilometers.

Comet Kohoutek
Comet Kohoutek (C / 1973 E1) quickly lit up at a great distance, leading some astronomers to predict that it would become a spectacular sight in January 1974. It turned out that the comet was “new”, never having passed near the Sun before. Only its volatile surface ices sublimated, which led to a temporary surge that stirred unrealistic expectations.

It’s good news that Comet Leonard has already circled the block. This trip and possibly others in the distant past allowed its most volatile ice creams to vaporize. First-time comets often turn unusually bright even at great distances as the cool ice sublimates into a frenzy. This can artificially inflate their predicted brightness when approaching solar and lead to unrealistic expectations. Often, these early blasts simply die off, and a dull onset ensues. Pre-baked so to speak, Comet Leonard’s luminosity predictions may be more reliable.

Last year, comet NEOWISE (C / 2020 F3) flourished in a magnificent view, becoming the brightest comet to decorate the sky since McNaught (C / 2006 P1) in 2007. This year has seen a shortage objects to the naked eye, the reason so many of us eagerly await Comet Leonard, which could peak at size 4 or brighter in December.

Detailed map of comet Leonardo
This map traces Comet Leonard as it slowly walks south of Ursa Major near 5th magnitude star 61 Ursae Majoris (61 UMa in Finder map here) overnight through October 29. Brighter stars are labeled with magnitudes without decimal places. Details of the graph appear at the top left.
Megastar, used with permission

In mid-October, the comet shines around magnitude 12 in the southern Ursa Major and is near 30 ° elevation at the start of morning twilight for northern mid-latitude observers. It is heading slowly east, passing through Canes Venatici on November 11 and Coma Berenices later that month, while continuing to clear up. By mid-November, the comet could reach magnitude 10, putting it within range of a 6-inch telescope. By the end of the month, it should be easily seen through 50mm binoculars around 7th magnitude.

Things really heat up in December. Leonard passes about 1.5 ° west of the brilliant globular cluster M3 on December 2 and about 1 ° east on December 3, then soars about 5 ° north of Arcturus on December 6. At the start of the month, it should hover near magnitude 5.5 and become a faint object to the naked eye. The presence of Arcturus nearby will make it easier for less experienced observers to find and track the comet.

Comet Leonard on October 10, 2021
In another image taken on October 10, 2021, Comet Leonard glowed at magnitude 12.3 with a coma 3.8 ′ in diameter and a 5 ′ tail pointed in PA 340–330 °.
Michael jäger

Observers can take one last look at Leonard plunged into twilight on December 12 – and near full brightness – before he passes into the evening sky. Fortunately, the Moon will be away for the best part of its morning appearance.

Beginning in mid-December, Comet Leonard recedes while remaining stubbornly low in the southwestern sky at dusk for observers in the mid-north. Meanwhile, conditions are improving for southern hemisphere sky watchers as the comet’s solar elongation increases. Passing through Sagittarius and Microscopium, it ends the year at 6th magnitude in Piscis Austrinus.

Right now Leonard sports a small but lush dust tail. If its dust production rate increases in the coming weeks as the comet approaches the Sun and becomes more active, two special circumstances – an orbital plane crossing and a high phase angle – could increase its luminosity au- above forecasts.

Comet Leonard at the intersection of an orbital plane
When Earth crosses the comet’s orbit on December 8, we might see improved brightness and weak anti-tailing.
JPL-Horizons with additions from Bob King

Amateur astronomer Michael Mattiazzo, creator of the Southern Comets Homepage, notes that on December 8, Earth will cross the comet’s orbital plane. Northern hemisphere observers will then see Leonard’s tail and dusty trail from the front, which should shrink and lighten the tail a bit as the cometary particles pile up along its length, much like we do. let’s see the thick band of the Milky Path from our point of view in the galactic plane.

Anti-tail of comet Lulin
Comet Lulin (C / 2007 N3) shows an antique (left) on January 31, 2009, around the time the Earth crossed the comet’s orbital plane. An ion tail extends to the right.
Joseph Brimacombe

Observers should also be careful to look for an anti-tail, a narrow, spiked-like appendage pointing away from the tail. Antiquities are formed when we see larger particles of cometary dust deposited along the comet’s orbit by the edge.

Explanation of phase angle
Forward scattering by dust particles in a comet increases as the phase angle (β) increases. This can cause the brightness of the object to increase. The more the comet is aligned with the Sun, the greater the potential increase. The related diffusion angle, called Theta (θ), decreases as β increases.
Bob the king

Attractively, Comet Leonard’s phase angle (Sun-Comet-Earth angle) can play a crucial role in raising its luminosity for much of December. The larger the angle – up to a maximum of 180 ° – the more the comet is aligned with the Sun. Sunlight passing through a cloud of fine particles like comet dust is scattered towards the viewer. We see the same effect in wet breath in cold weather, contrails and cloud edges.

Examples of forward diffusion
Here are three examples of forward scattering or backlighting. From left to right: the shiny edges of the clouds covering the Sun; blows on a cold day, and Comet McNaught in daylight on January 13, 2007, when its phase angle was 145 °.
Bob King (clouds, breath) / Mark Vornhusen, CC BY-SA 3.0

In 2006, Comet McNaught’s phase angle reached 149 °, and the comet’s luminosity increased by more than two magnitudes, making it visible in daylight. The phase angle (β) of comet Leonard will be greater than 120 ° from December 9 to 22, with a maximum β of 160 ° on December 14. This could cause a large forward scatter and a subsequent increase in brightness. At its maximum phase angle, the comet will be deeply buried by evening twilight just 15 ° from the Sun, but favorable geometry could briefly improve its visibility.

Light curves of comet Leonard
This light curve graph shows three possible scenarios for Comet Leonard. The four multiple blue and black data points are observations from the Comet Observation Database (COBS) and Minor Planet Center (MPC), respectively. The gray curve is calculated based on the original Minor Planet Electronic Circular (MPEC) for the comet or the latest MPC values. The red curve is derived from recent COBS / MPC observations, while the green tip includes the effects of forward scattering.
Gideon van Buitenen /

Below are Mattiazzo’s predictions for a possible phase angle related brightness surge based on similar circumstances during McNaught’s appearance. Remember that there is no guarantee. While Leonard’s current appearance prompts optimism, we really don’t yet know how dusty the comet is or will become. And dust generation makes all the difference when it comes to the phase angle. Additionally, McNaught’s thrust coincided with perihelion when he was most active, while Leonard will still be weeks away from the closest approach.

December 10 and 20: β = 130 ° / Improvement of magnitude +1.0 up to the third magnitude
December 12 and 17: β = 145 ° / Improvement of magnitude +2 to the second magnitude
December 14th: β = 160 ° / +3.5 increase in magnitude up to the first magnitude

While we hope to see a beautiful appearance here on Earth, Venusians may witness a rare meteor shower. Leonard crosses the orbit of the planet on December 17th. Two days later, Venus passes just 4 million kilometers from Leonard’s dust trail, close enough to cross it!

Comet Leonard won’t be NEOWISE’s second coming, but I expect it to have some surprises in store. Watch for more reports and maps in the coming weeks.

Resources: weekly bright comets, C / 2021 A1 (Léonard) Twitter, visual comets and COBS.


About Johnnie Gross

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