A hairy star of enormous beauty, with a bright tail capable of “lighting up” the sky. The passage of the comet C/2023 A3 TsuchinshanATLAS could turn into an incredible space show. The celestial body discovered in 2023 is the first to deserve the title of “great comet”, comparable to that of Hale-Bopp 27 years ago. Obviously in the last decades several comets have crossed our sky, some even visible to the naked eye, but C/2023 A3 TsuchinshanATLAS could really deserve the title of “comet of the century”, obviously if it manages to survive the ordeal. On September 27, the star will in fact pass at the point of its orbit closest to the Sun, the Perihelion, and if it does not disintegrate, in the following days it will approach the Earth brighter than ever, with the peak expected for October 9.
Spectacular Comets: The Precedents
After Hale-Bopp, there were several comets that attracted attention. In the summer of 2020, comet C/2020 F3 Neowise proved to be very interesting, making itself easily visible to the naked eye even from cities, interrupting a long fast that had actually lasted since 1997, the annus mirabilis of comet Hale-Bopp. In 2009, lucky southern observers witnessed the passage of the great comet C/2006 P1 McNaught, which was visible in broad daylight even at our latitudes. Comet C/2023 A3 did not escape this fate of hopes and predictions. The first reported observations date back to February 22, 2023 and were made with a telescope installed in South Africa, part of the ATLAS project (Asteroid Terrestrial-impact Last Alert System. Later, the Minor Planet Center linked this new star to observations previously reported by the Purple Mountain Observatory, China, on January 9. Therefore, in accordance with the rules of the astronomical community that govern these circumstances, upon the official announcement of its discovery, the comet was named C/2023 A3 Tsuchinshan-ATLAS (Tsuchinshan is a transliteration of the Mandarin name of the Chinese observatory): third non-periodic comet discovered in the first half of January 2023.
The passage of C/2023 A3 Tsuchinshan-ATLAS
The enthusiasm for this celestial body was almost immediate. From the beginning, in fact, it was noted that the comet could have become interesting shortly after its passage at perihelion (minimum distance from the Sun, expected for September 27, 2024): in early October, in fact, its position with respect to the Sun and the Earth could have made it particularly bright, if it had turned out to be a dust-rich comet. All of this, obviously, with the obligatory caution that is always required with comets: history, even recent history, is dotted with plumed stars that have greatly disappointed the most optimistic predictions. What fueled astronomers’ enthusiasm at the time in the case of C/2023 A3 was a precise geometric condition that would have occurred right at the beginning of October 2024. By then, the comet would have been almost exactly on the Earth-Sun line, a condition capable of producing a physical effect known as forward scattering of sunlight by its dust.
This effect is well known to everyone. A layer of fog at dawn or dusk appears much more evident if you observe it in the direction of the low Sun than if you look laterally. This is because the suspended water droplets create the phenomenon of forward scattering of sunlight towards the observer. An iconic image of this effect is the one obtained by the Cassini probe while the globe of Saturn was eclipsing the Sun: this alignment condition allowed the emphasized view of the most elusive external rings, which contain particles capable of triggering this forward scattering.
Similarly, the dust present in the comet preferentially reflects sunlight forward, so if a comet passes between us and the Sun, its brightness can be increased by tens, hundreds or thousands of times. Once it was noticed that comet C/2023 A3 would have been in that exact geometric condition (that is, observed from Earth, almost in the line of sight of the Sun), people began to imagine the potential resulting spectacle. Therefore, since its discovery, Tsuchinshan-ATLAS has been constantly monitored, documenting its developments, its increases in brightness and its setbacks, with the consequent, predictable changing moods of the astronomical community. It was easily observed from Earth until the beginning of last summer, before the conjunction with the Sun, spied for a while with the prying eyes of space probes, and then returned visible in our sky about ten days ago, from the southern hemisphere.
When will it be visible?
But when will it be possible to admire the comet? As explained by astrophysicist Gianluca Masi, this morning, Tuesday 24 September, the Virtual Telescope Project managed to photograph the celestial body from Italy: “Overall, the comet has shown itself to be in excellent health and to be developing well, thus preparing for the imminent turning point around the Sun (perihelion) on 27 September. Above all, it has shown itself to be a very dusty comet, that is, rich in those particles essential to hope for the forward scattering mentioned above. In short, taking some risks that are right to take, we can say that comet C/2023 A3 Tsuchinshan-ATLAS has what it takes to truly delight our eyes, with a maximum brightness of magnitude -4 expected, based on the data currently available, similar to the brightness of the planet Venus”.
But it’s not all red and flowers. The peak of brightness due to forward scattering is expected for October 9, when the comet will be prospectively very close to the Sun. Perhaps the dusty tail will be visible due to the physical phenomenon mentioned several times, but in any case this would happen in the bright sky of the evening twilight. In the days immediately following, the comet will gradually improve its visibility from Earth, but the brightness will rapidly decrease, since the conditions for “forward” scattering will quickly disappear. In any case, the comet will be low on the horizon, in the residual light of twilight: conditions far from ideal: what we see, therefore, will greatly depend on the behavior of the comet.
Appointment on October 9th
A look at the visibility conditions for the general public, limited to the postperihelion phase for our latitudes, the only one of interest. After September 27, the date of the halfway point around our star (it will arrive at about 60 million km from it), the comet will be in conjunction with the Sun on October 9, when the expected effect of forward scattering will be at its maximum. On October 8, observation could already be attempted close to dawn, while from day 9 the comet will be seen just after sunset. On October 12, C/2023 A3 will be at its minimum distance from Earth, equal to about 71 million km.
From then on, day after day, C/2023 A3 will be better positioned in the sky, although less and less favored by the increase in brightness due to scattering. It will pass through the constellations of Virgo, Serpens and Ophiuchus. Around October 15th it should be possible to obtain the best view between the brightness of the comet and its position in the sky. The Moon will be visible in the first quarter (October 10th), but at the beginning it will interfere little compared to the invasiveness of the evening twilight. It will be full on October 17th and then it will start to make itself felt, while the brightness of the comet will be somewhat reduced, but hopefully still interesting. It is advisable to choose an observation site as free as possible from light pollution and that offers the western horizon free from important obstacles (hills, trees, buildings). The observation can be conducted with the naked eye, better with binoculars. For those who are unable to observe the comet with the naked eye, the Virtual Telescope Project is preparing the schedule of the various live streaming sessions with which it will share the appearance of this interesting comet. The first appointment is scheduled for October 9, at 18:45: as always, participation is free and available on the Virtual Telescope Project website.