NASA's Astronomy Picture of the Day for November 11, 2024, showcases an unexpected phenomenon in comet C/2023 A3 (Tsuchinshan-ATLAS), dubbed the 'comet of the century': a dark band in the middle of its long, bright tail.
This unusual effect has sparked discussion among astronomers. Possible explanations for the darker band include a plume of darker dust, overlapping sections of the bright tail creating an optical illusion, or a shadow cast by a portion of the coma (the cloud of dust and gas surrounding the comet's nucleus) on dust particles.
The image, captured on October 14 from Texas, also displays the comet's lengthy bright tail and a thin anti-tail, which appears to point in the direction of the comet's trajectory.
A comet's tail typically consists of ions—gases ionized by ultraviolet photons blown by solar wind, which appear bluish—and dust and ice, which take on a white hue.
The anti-tail is a rarer optical phenomenon, resulting from our perspective on Earth. This slender light streak is formed by dust left behind by the comet as it completes its orbit around the Sun.
The 'comet of the century' reached its closest approach to Earth in mid-October, remaining visible in the night sky for nearly a month. The celestial body has since moved away and is exiting our Solar System.