It may be difficult for the human mind to imagine: a colossal cloud so colossal that it surrounds the sun and eight planets while stretching trillions of kilometers into space.

The spherical envelope known as the Oort cloud is virtually invisible. The individual particles are so thinly distributed and so far from the light of any star, including the Sun, that astronomers simply cannot see the cloud, even though it envelops us like a blanket.

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It's also theoretical. Astronomers conclude that the Oort cloud exists because it is the only logical explanation for the arrival of a certain class of comets that sporadically visit our solar system. It turns out that the cloud is essentially a huge reservoir that can contain billions of icy celestial bodies.

Two of these bodies will fly past Earth in the days leading up to Halloween. Tsuchinshan ATLAS, also known as Comet C/2023 A3, will be at its brightest and likely visible to the naked eye for a week or two after October 12, the day it is closest to Earth – just look in the western sky just after sunset. As the days pass, the comet becomes fainter and moves higher in the sky.

A view of comet Tsuchinshan-ATLAS from the International Space Station.

The second comet, C/2024 S1 (ATLAS), just discovered on September 27, should be visible around the end of October. The comet will make its closest approach to Earth on October 24th – look deep into the eastern sky just before sunrise. Then, after orbiting the sun, the comet could reappear in the western night sky around Halloween. However, it is possible that all or part of it will disintegrate, as sometimes happens when comets pass by the sun – and this one will come within a million miles (1.6 million kilometers) of our star.

As a planetary astronomer, I am particularly curious about the Oort Cloud and the icy bodies that live within it. The inhabitants of the cloud could be a reason for the emergence of life on Earth; When these icy bodies crashed onto our planet eons ago, they may have provided at least some of the water that all life requires. At the same time, these same objects pose an ever-present threat to the continued existence of the Earth – and our survival.

billion comets

When an Oort cloud object finds its way into the inner solar system, its ice evaporates. This process creates a tail of debris that becomes visible as a comet.

Some of these bodies, called long-period comets, have orbits of hundreds, thousands or even millions of years, like Tsuchinshan ATLAS. This is different from the so-called short-period comets, which do not visit the Oort Cloud and have comparatively fast orbits. One of these is Halley's Comet, which orbits the solar system and orbits the sun approximately every 76 years.

The Dutch astronomer Jan Oort was fascinated by long-period comets in the 20th century and wrote an essay about them in 1950. He found that about 20 of the comets had an average distance from the Sun that was more than 10,000 astronomical units. That was amazing; Just one AU is the Earth's distance from the Sun, which is about 93 million miles. Multiply 93 million by 10,000 and you'll find that these comets come from over a trillion miles away. Furthermore, Oort says, they weren't necessarily the outermost objects in the cloud.

An illustration of the solar system and the Oort cloud. The numbers in the graphic represent AUs, or astronomical units. Note the location of Voyager 2, which will take another 30,000 years to fly out of the cloud. Image provided by NASA

Almost 75 years after Oort's paper, astronomers still cannot directly image this part of space. However, they estimate that the Oort Cloud extends up to 10 trillion miles from the Sun, which is almost halfway to Proxima Centauri, the next closest star.

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The long-period comets spend most of their time at these vast distances, making only short and quick visits near the Sun as they arrive from all directions. Oort speculated that the cloud contained 100 billion of these icy objects. This could be as numerous as the number of stars in our galaxy.

How did they get there? Oort suspected and modern simulations have confirmed that these icy bodies may have originally formed near Jupiter, the solar system's largest planet. It's possible that these objects' orbits around the sun were disrupted by Jupiter – similar to how NASA spacecraft bound for destinations from Saturn to Pluto typically swam past the giant planet to speed up their outward journeys.

Some of these objects would have permanently left the solar system and become interstellar objects. But others would have ended up with orbits like those of long-period comets.

Threats to Earth

Long-period comets pose a particular danger to the Earth. Because they are so far away from our sun, their orbits are easily changed by the gravity of other stars. This means that scientists have no idea when or where any of these animals will appear until they suddenly do. At this point it is usually closer than Jupiter and moving quickly, at speeds of tens of thousands of miles per hour. In fact, the fictional comet that doomed Earth in the movie “Don't Look Up” came from the Oort cloud.

New comets are constantly being discovered from the Oort Cloud, in recent years about a dozen per year. The chance that one of them will collide with Earth is extremely small. But it is possible. The recent success of NASA's DART mission, which altered the orbit of a small asteroid, demonstrates a plausible approach to defending against these small bodies. But this mission was developed after years of studying its goal. A comet from the Oort cloud may not provide that much time – perhaps just months, weeks or even days.

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Or no time at all. “Oumuamua, the strange little object that visited our solar system in 2017, was discovered not before but after its closest approach to Earth. Although 'Oumuamua is an interstellar object and does not come from the Oort cloud, the suggestion still holds; One of these objects could sneak up on us and the Earth would be defenseless.

One way to prepare for these objects is to better understand their fundamental properties, including their size and composition. To this end, my colleagues and I are working on characterizing new long-period comets. The largest known star, Bernardinelli Amber, was discovered just three years ago and is about 120 kilometers across. Most known comets are much smaller, from one to a few miles, and some smaller ones are too faint for us to see. But newer telescopes are helping. In particular, the Rubin Observatory's decade-long Legacy Survey of Space and Time, starting in 2025, could double the list of known Oort cloud comets, which currently stands at about 4,500.

The unpredictability of these objects makes them a challenging target for spacecraft, but the European Space Agency is preparing a mission to do just that: Comet Interceptor. Launch is scheduled for 2029 and the probe will park in space until a suitable target emerges from the Oort cloud. Studying one of these ancient and pristine objects could provide scientists with clues about the origins of the solar system.

As for the comets that are now near Earth, it's okay to look up. Unlike the comet in the DiCaprio film, these two will not crash into Earth. The closest Tsuchinshan ATLAS that will reach us is about 44 million miles (70 million kilometers); C/2024 S1 (ATLAS), about 80 million miles (130 million kilometers). Sounds like a long shot, but in space it's a near miss.

This article is republished from The Conversation under a Creative Commons license. Read the original article.