Now is the time to start tracking Comet C/2021 A1 Leonard, as it starts its dawn dive sunward.

The days following New Year’s 2021 saw a comet discovery with potential. On the night of January 3rd, exactly one year to the day prior to perihelion, astronomer Gregory J. Leonard working at the Mount Lemmon Observatory near Tucson Arizona discovered the first long-period comet of the year, C/2021 A1 Leonard. Shining at magnitude +19 and 5 Astronomical Units (AU) distant (about the distance of Jupiter from the Sun) at the time of discovery, early indications hinted that comet A1 Leonard might prove to be something special, come the end of 2021.

On an 80,000 year orbit inbound, Comet A1 Leonard is due for ejection from the solar system after its perihelion passage early next year. The comet reached aphelion 3,500 AU distant in the Oort Cloud about 35,000 years ago. The upcoming perihelion pass on January 3rd, 2022 will be 0.62 AU from the Sun, interior to the orbit of Venus.

The orbit of Comet A1 Leonard, set for its closest passage near Earth.

Keep in mind, A1 Leonard may be a seasoned long-term visitor to the inner solar system with a potential to over-perform as it nears the Sun. Current predictions have the comet reaching +4th magnitude, and it may flirt with naked eye brightness. The comet passes just 0.233 AU (21.7 million miles or 34.9 million kilometers) from the Earth on December 12th, another plus.

Often, dynamically new comets exhibit a burst of activity as they approach the inner solar system for the first time, heat up, and sublimate their outer layers. This can also fake out observers initially, leading to overly optimistic predictions. We all remember the Great Comet that wasn’t during the 1973 passage of C/1973 E1 Kohoutek. A comet such as A1 Leonard has survived a perihelion passage before without disintegrating… though this one will be its last.

Getting brighter… Comet A1 Leonard from November 13th. Credit and copyright: Hisayoshi Kato.

Currently, A1 Leonard is located high at dawn for northern hemisphere observers in the constellation Canes Venatici, and begins the plunge sunward during the first half of the month. We actually cross the plane of the comet’s orbit on December 8th, and the comet may exhibit a sharp ‘anti-tail’ spike sunward around this time.

Another intriguing effect may also come into play during the December apparition of Comet A1 Leonard. The the Sun-Earth-Comet phase-angle for the tail will sit at greater than 120 degrees from December 9th to December 22nd, and reach a maximum of 160 degrees on December 14th just after it passes closest to the Earth on December 12th. This sets up conditions ideal for a possible surge in brightness, adding the brightness of the tail to the coma of the comet itself.

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