Predicting a newly found comet’s brightness isn’t any simple enterprise. Astronomers typically use the power-law formula to make visibility forecasts, however an equation can fail to account for a comet’s important unpredictability. Not solely do these fragile, icy fossils expertise shock outbursts and disintegrations, however their luster can range radically relying on one thing so simple as viewing angle.
With this in thoughts, let’s check out the upcoming apparition of Comet Leonard (C/2021 A1). Senior analysis specialist Greg Leonard at Mt. Lemmon Observatory found the Nineteenth-magnitude speck on January 3, 2021, precisely one year earlier than perihelion. Orbital calculations revealed that the thing had spent the final 35,000 years wending its way sunward after reaching aphelion on the chilling distance of round 3,500 a.u. Comet Leonard will cross nearest the Sun once more on January 3, 2022, at 0.62 a.u. Two weeks earlier than that on December twelfth, it makes its closest strategy to Earth at 34.9 million kilometers.
It’s excellent news that Comet Leonard’s been across the block earlier than. That journey and maybe others within the distant previous allowed its most unstable ices to vaporize. First-time comets typically turn into unusually vivid even at nice distances as recent ice sublimates in a frenzy. This can artificially inflate their predicted brightness throughout solar strategy and result in unrealistic expectations. Often, these early outbursts merely fizzle out, and a lackluster apparition follows. Pre-baked because it had been, Comet Leonard’s brightness predictions could also be extra dependable.
Last year, Comet NEOWISE (C/2020 F3) blossomed into a powerful sight, changing into the brightest comet to brighten the sky since McNaught (C/2006 P1) in 2007. This year has seen a dearth of naked-eye objects, the explanation so many people are wanting ahead to Comet Leonard, which can prime out at magnitude 4 or brighter come December.
In mid-October, the comet glows round magnitude 12 in southern Ursa Major and stands virtually 30° excessive firstly of morning twilight for observers at mid-northern latitudes. It slowly heads east, crossing into Canes Venatici on November eleventh and Coma Berenices later that month, whereas persevering with to brighten. By mid-November the comet could attain magnitude 10, placing it inside vary of a 6-inch telescope. At month’s finish it needs to be simply seen in 50-mm binoculars round seventh magnitude.
Things actually warmth up in December. Leonard passes about 1.5° west of the intense globular cluster M3 on December 2nd and about 1° to its east on the third, then dashes some 5° north of Arcturus on the sixth. Early within the month it ought to hover close to magnitude 5.5 and turn into a faint naked-eye object. Having Arcturus close by will make it simple for much less skilled observers to search out and observe the comet.
Observers could get their final peek at Leonard steeped in morning twilight on December twelfth — and close to peak brightness — earlier than it transitions into the night sky. Fortunately, the Moon might be absent all through one of the best a part of its morning apparition.
From mid-December onward, Comet Leonard fades whereas remaining stubbornly low within the southwestern sky at nightfall for mid-northern observers. Meanwhile, circumstances enhance for Southern Hemisphere skywatchers because the comet’s solar elongation will increase. Powering throughout Sagittarius and Microscopium, it ends the year at sixth magnitude in Piscis Austrinus.
Right now, Leonard sports activities a small however lush mud tail. If its mud manufacturing fee climbs within the coming weeks because the comet approaches the Sun and turns into extra lively, two particular circumstances — an orbital plane crossing and a excessive section angle — could enhance its brightness above predictions.
Amateur astronomer Michael Mattiazzo, creator of the Southern Comets Homepage, notes that on December eighth Earth will cross the orbital plane of the comet. Northern Hemisphere observers will then view the tail and dusty path of Leonard edge-on, which ought to trigger the tail to slender and brighten a bit because the cometary particles stack up throughout its size, a lot the identical way we see the thick band of the Milky Way from our viewpoint inside the galactic plane.
Observers must also be eager to search for an antitail, a slender, spike-like appendage pointing within the route reverse that of the tail. Antitails kind after we view bigger cometary mud particles deposited alongside the comet’s orbit edge-on.
Tantalizingly, Comet Leonard’s phase angle (Sun-comet-Earth angle) could play an important position in elevating its brightness throughout a lot of December. The bigger the angle — as much as a most of 180° — the extra practically in line the comet is with the Sun. Sunlight shining by way of a cloud of positive particles like comet mud is scattered ahead towards the observer. We see the identical impact in steamy breath on a chilly day, contrails, and cloud edges.
In 2006, Comet McNaught’s phase angle reached 149°, and the comet swelled in brightness by greater than two magnitudes, rendering it seen in daylight. Comet Leonard’s section angle (β) might be larger than 120° from December ninth to twenty second, with a most β of 160° on December 14th. This may lead to vital ahead scattering and a subsequent surge in brightness. At its peak section angle the comet might be burrowed deep in night twilight simply 15° from the Sun, however favorable geometry could briefly enhance its visibility.
Below are Mattiazzo’s predictions for a doable phase-angle-related brightness surge primarily based on related circumstances throughout McNaught’s apparition. Remember, there aren’t any ensures. While Leonard’s present look provides trigger for optimism, we actually do not know but how dusty the comet is or will turn into. And mud manufacturing makes all of the distinction with regards to section angle. Further, McNaught’s surge coincided with perihelion when it was most lively, whereas Leonard will nonetheless be a pair weeks shy of closest strategy.
December tenth and twentieth: β = 130° / +1.0-magnitude enhancement to 3rd magnitude
December twelfth and seventeenth: β = 145° / +2-magnitude enhancement to second magnitude
December 14th: β = 160° / +3.5-magnitude enhance to first magnitude
While we hope to see a positive apparition right here on Earth, Venusians could witness a uncommon meteor bathe. Leonard crosses the planet’s orbit on December seventeenth. Two days later, Venus passes simply 4 million kilometers from Leonard’s mud path, shut sufficient to plow by way of it!
Comet Leonard will not be the second coming of NEOWISE, however I anticipate it has surprises in retailer. Watch for extra experiences and maps within the weeks forward.