The Brightest Comet In 27 Years

Many residents of New York City witnessed a phenomenon during the sunset on October 12 that seemed surreal—a bright comet soaring across the sky. Onlookers gathered in local parks and beaches to view this rare occurrence in the city’s sky.

This cosmological anomaly was Comet C/2023 A3 (Tsuchinshan-ATLAS), also known as Comet A3 or the “Halloween comet.” It was discovered on February 22, 2023. The name follows a comet-naming standard established by the International Astronomical Union; “Comet C/” refers to a non-periodic comet—a comet that has never been observed before. These comets have an orbital period—the time it takes to return to the inner solar system—greater than 200 years, and Comet A3’s orbital period is 80,000 years. “2023” refers to the year it was discovered, “A” refers to the half-month it was discovered—the first half of January is designated as “A” while the second half is designated as “B,” and so on. As the third comet discovered that month, Comet A3 ends with a “3.” Finally, “Tsuchinshan-ATLAS” is named after the Purple Mountain Observatory in China that discovered it and the Asteroid Terrestrial-Impact Last Alert System (ATLAS). 

Comets are bodies of ice and dust that orbit in an ellipse around the Sun. Some comets were formed 4.5 billion years ago from the ice-covered dust and gas that constitute protoplanetary disks—collections of hot gas and debris that later form developing stars. Over time, the clumps of dust and gas congregate into a larger body due to the gravitational attraction of smaller masses to larger masses. The high-speed collisions of these clumps of icy rock cause the body to loosely fuse together into the comet’s nucleus. The comet’s motion is driven by the star’s strong gravitational pull as a result of the star’s enormous mass relative to that of the comet. As a comet passes through the inner solar system, its proximity to the Sun causes the comet’s ice to sublimate into gas, forming a cloud around the nucleus. This formation leaves behind a trail of dust and gas that we observe as the comet’s tails, which can extend for millions of kilometers. If comets venture too close to the Sun, they immediately disintegrate due to the Sun’s radiation and gravity. Comet A3 didn’t suffer this fate and was able to pass by Earth after reaching its perihelion—the closest point in a comet’s transit to the Sun—on September 27, 2024. As such, comets are often detected by powerful telescopes in observatories like ATLAS’s in Hawaii. ATLAS’s telescopes can capture a view of the sky that is 100 times larger than the view of a single full moon, and they can detect objects, including other near-Earth objects and comets, at about 384,000 kilometers away from the Earth.

Due to Comet A3’s close proximity to the Earth, it was the brightest comet to pass Earth in 27 years since Comet Hale-Bopp in 1997. Comet A3 has an apparent magnitude of -4.9 while Hale Bopp’s apparent magnitude was -1.0. Apparent magnitude refers to how bright a celestial object appears to be, and the lower the magnitude’s value, the brighter the object appears to be. For reference, the apparent magnitude of the Sun is -27, a full moon’s is -13, and the star Polaris, or the “North Star,” has an apparent magnitude of two. Comet A3 came within approximately 70 million kilometers—the closest point on the comet’s transit by Earth—on October 12, making that day optimal for clearly viewing it. Since the comet flew close to the horizon approximately 45 minutes after sunset, those who had an unobstructed view of a dark sky were easily able to spot the comet. Due to its extremely long orbit, Comet A3 likely originated from the Oort Cloud, a vast collection of icy debris extending from the Kuiper Belt, which is a region of icy bodies that is further away from the Sun than Neptune is. 

Although comets are not easy to spot, there are some strategies to maximize your chances of spotting one in the sky at the appropriate time. First, research the exact time that the comet will pass through the skies above your location. Take note of the altitude and direction of the comet’s trajectory to determine an optimal viewing location. Usually, comets are most visible within one hour before sunrise and within one hour after sunset due to the comet’s proximity to the Sun in the sky. The reflection of sunlight off of the comet ice contributes to the comet’s distinct brightness in the sky. However, during those time frames, comets fly close to the horizon, which emphasizes the need for an unobstructed view of the horizon. Make sure that you view comets in a dark location with minimal light pollution from events like a bright or full moon. Clear skies are also necessary, as clouds and precipitation reduce the visibility of the comet. Since it is difficult to view comets with only the naked eye, you may need telescopes or binoculars. To photograph the comet, a high-speed camera with a shutter speed between one to five seconds is recommended in order to capture details of the comet’s nucleus. 

If you missed Comet A3 in October 2024, you have another chance—C/2024 G3 (ATLAS). Comet G3 is predicted to be visible during the middle of January 2025. Aside from being a cosmological spectacle, comets like the Rosetta Comet carry traces of organic molecules found in Earth’s organisms. These molecules include the amino acid glycine, found in protein molecules, and phosphorus, found in DNAe, an enzyme involved in the synthesis of our genetic code. Comets can help scientists uncover the processes that bring life to planets, and they are aesthetic phenomena. Although many variables like weather and comet trajectory are uncertain, hopefully, more people can witness comets and perhaps delve deeper into other functions that comets serve in our universe.