Call it A3

Now comes the last of the five comets we listed in January as likely to be observable in 2024.

In this space view, the ecliptic plane is represented by a grid of lines on it at intervals of 1 AU (astronomical unit, the Sun-Earth distance, about 150 million kilometers or 93 million miles). The viewpoint is 6 AU from the Sun at a latitude of 15° north of the plane.

The comet has a more than usually cumbersome designation, enclosing an acronym of the even more than usually forced type: C/2023 A3 Tsuchinshan-ATLAS. It means that it was the third comet discovery recorded in the first half of January 2023 (on Jan. 9), and was independently discovered by the Purple Mountain Observatory in China and the Astronomical Terrestrial-impact Last Alert System’s robotic telescope in South Africa. And C/ means that it has not been shown to be a periodic comet (which would get a designation with P).

It has fallen from a vast distance in the outer solar system’s Oort Cloud, in a near-parabolic orbit (eccentricity given as 0.999992) whose steep inclination (139°) means that it is retrograde (counter to the direction of the Earth and other planets). It was in the morning sky (that is, west of the Sun) till opposition on Apr. 17, and since then in the evening sky. It descended southward through the ecliptic plane on June 18, then across the equator into the southern hemisphere of the Earth’s sky on Aug. 5, sliding southwestward from the southern extension of constellation Leo into the small constellation Sextans. On Aug. 20 crossed to the inner side of Earth’s orbit; that is, its distance from the Sun became less than 1 AU. On Sep. 5 it passed 11° south of the Sun, as seen from Earth.

It will on Sep. 27 arrive at perihelion (nearest to the Sun), 0.39 AU – the same as the average orbit of Mercury. And on Oct. 12 it will be nearest to Earth, 0.47 AU.

It will cross back northward through the equator on Oct. 14; cross the 1 AU distance outward on Nov. 4. At the end of 2024 it will be 1.97 AU from the Sun, 2.73 AU from Earth, 32° out in the evening sky.

What about this comet’s brightness? This in astronomy is expressed in the logarithmic magnitude scale, in which the brightest stars are -1 or 0 or 1 and the limit for unaided-eye visibility is 5 in perfect sky conditions. Predictions are notoriously unreliable for comets’ brightness, and for the tails they may form, because of their “behavior” as they approach the Sun.

Early observations of A3 suggested that it could eventually reach mag. 0.4, or even brighter. Then, that it might be disintegrating and could disappear. Then it recovered, and could even reach 2 – about the brightness of the Pole Star. But a comet, with its brightness spread over an area, is less easy to discern than a star of the same magnitude.

Here is the pre-dawn scene for an American on the date of this comet’s perihelion.

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