Comet C/2023 A3 (Tsuchinshan-ATLAS) appears in the western sky shortly after sunset above rock formations in the Red Rock Canyon National Conservation Area on October 13, 2024 in Las Vegas, Nevada. Credit: Ethan Miller

This October, two fascinating comets will grace the night skies, offering a rare cosmic display. These celestial visitors, originating from the distant Oort Cloud, are set to provide an exciting spectacle leading up to Halloween.

The Oort Cloud, an immense, theoretical shell of icy bodies surrounding the Sun, stretches trillions of miles into space. Though invisible to astronomers, the cloud is inferred from the periodic arrival of comets that originate far beyond our solar system. This cloud acts as a cosmic reservoir, possibly containing billions of icy objects.

Two Comets to Watch

The first comet, Tsuchinshan-ATLAS (C/2023 A3), is set to be its brightest just after October 12, 2024. Look for it in the western sky shortly after sunset as it slowly fades and climbs higher in the sky. With clear conditions, it should be visible to the naked eye for a week or two.

The second comet, C/2024 S1 (ATLAS), discovered recently in September, will be closest to Earth around October 24. It should be visible low in the eastern sky just before sunrise, and, after swinging around the Sun, may reappear in the western sky around Halloween. There’s a chance it could disintegrate as it passes near the Sun, but if it remains intact, it will be a spooky cosmic sight.

Comets from the Oort Cloud

These comets are examples of long-period comets, objects whose orbits can span thousands or even millions of years. Unlike short-period comets, like Halley’s Comet, which revisits our solar system frequently, Oort Cloud comets are rare visitors.

The Oort Cloud, first hypothesized by Dutch astronomer Jan Oort in 1950, extends up to 10 trillion miles from the Sun, far beyond the planets. Astronomers estimate that it may hold up to 100 billion icy bodies, many of which originated near Jupiter and were flung outward due to the planet's immense gravity. Some escaped the solar system, while others remained in long orbits, occasionally returning to the inner solar system as comets.


An illustration of the solar system and the Oort Cloud. The numbers on the graph depict AUs, or astronomical units. Note the location of Voyager 2, which will take another 30,000 years to fly out of the Cloud. Credit: NASA

The Threat of Long-Period Comets

While these comets offer stunning views, they also pose a potential danger to Earth. Long-period comets are unpredictable, with their orbits easily altered by nearby stars, and they can appear with little warning. Moving at tens of thousands of miles per hour, these objects can present a significant impact threat. However, the odds of a direct collision with Earth are low, and ongoing advancements in astronomy, such as NASA’s DART mission and ESA’s upcoming Comet Interceptor project, are working toward better understanding and possibly mitigating these threats.

For now, though, these Halloween comets will safely pass by, giving skywatchers a chance to marvel at the beauty of the universe. The closest Tsuchinshan-ATLAS will come to Earth is about 44 million miles, while C/2024 S1 (ATLAS) will stay roughly 80 million miles away—close by cosmic standards, but no cause for concern.

Though comets from the Oort Cloud present a potential threat to Earth, the chances of a direct collision are extremely low. NASA's recent DART mission demonstrated that it is possible to deflect a small asteroid, but comets from the Oort Cloud travel much faster and offer less warning time.

As we prepare to observe the comets passing by this month, it's important to remember that they are not a danger to Earth. Comet Tsuchinshan-ATLAS will pass no closer than 44 million miles from our planet, and C/2024 S1 will be about 80 million miles away. While these distances are vast, in the scale of space, they are considered near misses.

For now, we can enjoy the spectacle these comets provide, knowing that they offer valuable insights into the origins of our solar system and the ongoing mysteries of the Oort Cloud. As the European Space Agency prepares its Comet Interceptor mission for 2029, we may soon have an opportunity to study one of these ancient visitors up close, revealing more about the frozen building blocks that have shaped our cosmic neighborhood.

So, as Halloween approaches, take a moment to look up at the night sky and witness these rare visitors from the outermost reaches of our solar system. While they won't crash into Earth like in the movie Don’t Look Up, their fleeting presence reminds us of the cosmic forces that continue to shape our existence.

So, grab your binoculars or simply step outside after sunset and enjoy this celestial show, courtesy of our ancient cosmic neighbors from the Oort Cloud.