The Anticipated Phenomenon of Nishimura: An Expert’s Insight into the Spectacular Green Comet Illuminating the Sky

Among the celestial wonders in our Solar System, few rival the splendor of the occasional appearance of great comets gracing our skies. If you’ve recently perused social media, you’ve likely encountered reports announcing the presence of one such celestial traveler currently illuminating our heavens: C/2023 P1 (Nishimura). As I pen these words, comet Nishimura embarks on its first visit to our neighborhood in over four centuries. Hideo Nishimura, a Japanese astronomer, first laid eyes on this comet on August 12. Subsequently, astronomers unearthed pre-discovery images of the comet, dating back to January, enabling them to chart its trajectory.

Swiftly, they realized that Nishimura would venture closer to the Sun than Mercury’s orbit this month. Given the comet’s luminosity at its discovery, there was optimism it might attain naked-eye visibility. So, will it bestow upon us a magnificent celestial spectacle? Regrettably, Nishimura’s trajectory will keep it in close proximity to the Sun in our earthly perspective. While it is indeed bright enough to be seen with the unaided eye under the cloak of dark skies, at best, it will linger near the horizon just after sunset, nearly engulfed by the Sun’s radiance.

Nevertheless, astronomers worldwide are abuzz with excitement. Even a challenging-to-discern naked-eye comet merits observation. As the renowned science writer and astronomer David H. Levy once quipped, “Comets are like cats: they have tails, and they do precisely what they want.” There exists a slim chance that Nishimura might unexpectedly intensify in brightness. If it does, we could witness something extraordinary in the coming weeks. If not, there’s always the promise of another opportunity next year, but more on that later.

The Recipe for an Illuminated Comet

When comets dwell far from the Sun, within the frigid expanse of space, they essentially constitute dirty snowballs composed of ice, dust, and rocks, remnants from the Solar System’s inception. As a comet draws nearer to the Sun, its surface begins to heat up. The surface ices become warm and “sublime,” transforming into gas and bursting outward from the comet’s exterior. This emanating gas carries along dust and debris, enveloping the nucleus in a gossamer shroud of gas and dust known as a “coma.”

Subsequently, the solar wind sweeps this gas and dust away from the Sun, sculpting the comet’s distinctive tails, always pointing away from our radiant star. The comet we behold is, in essence, sunlight reflected off the gas and dust within the coma and tails, with the nucleus itself concealed from view. A comet’s brightness, therefore, hinges on three primary factors: the size of its nucleus (larger nuclei typically yield greater activity and brightness), its proximity to the Sun (closer approach results in heightened activity and brightness), and its proximity to Earth (closer proximity to us equates to enhanced apparent brightness).

What About Nishimura?

This brings us to comet Nishimura. It appears that Nishimura isn’t particularly large, as we would have detected it sooner if it were. Nor is it particularly close to Earth. Nevertheless, it is poised to pass relatively near the Sun, anticipated to be exceptionally active around its perihelion, the closest point to the Sun in its orbit. In an ideal scenario, where Nishimura would be observable against a dark night sky, the comet would undoubtedly present a striking display. Alas, even at its zenith, Nishimura will remain proximate to the Sun in our perspective.

Moreover, the comet’s position and Earth’s alignment present suboptimal conditions for observation. Nishimura will remain in close proximity to the Sun as it recedes from us, veiled within the Sun’s luminous aura.

A Brief Window to Glimpse Nishimura From Australia

Nishimura will soon emerge above the western horizon shortly after sunset, though barely so. The most favorable opportunity to observe it from Australia arrives in the week spanning September 20 to 27, when the comet’s head will descend around one hour after sunset. Its farthest point from the Sun in the evening sky occurs on September 23. As twilight yields to darkness, Nishimura will hover in close proximity to the western horizon, teetering on the brink of setting. Consequently, it is likely to be obscured by the Sun’s brilliance.

However, remember that comets possess an unpredictable quality akin to cats. Some comets disintegrate when closest to the Sun, often leading to substantial brightening. While Nishimura, with its orbital period of approximately 430 years, is a seasoned visitor with likely numerous encounters with the Sun, diminishing the likelihood of fragmentation, the tail may still remain visible as the sky darkens. Earlier observations placed the comet’s tail at roughly six degrees in length before it vanished in the glare for northern hemisphere viewers, and this tail is likely to extend as the comet draws nearer to the Sun.

If fortune favors you, you may spot the tail proudly extending above the horizon as the night sky deepens.

The Next Great Comet

Should Nishimura fail to deliver the grand spectacle you anticipated, there is a prospect that another comet could grace our skies with a truly spectacular performance next year. Comet C/2023 A3 (Tsuchinshan-ATLAS) was discovered at the outset of this year and currently resides in the vicinity of Jupiter, nearly as distant from the Sun as the gas giant. Over the ensuing 12 months, it will steadily approach the Sun, culminating in its closest proximity in late September 2024. Tsuchinshan-ATLAS shows promise, and if it adheres to expectations, it could offer a breathtaking spectacle. But always bear in mind, comets, like cats, have a whimsical disposition!

Written by Jonti Horner, Professor of Astrophysics at the University of Southern Queensland.
Adapted from an article originally published in The Conversation.

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