The human mind can have a hard time conceptualizing this: a cosmic cloud so colossal that it encircles the sun and eight planets while stretching trillions of miles deep into space.

The spherical shell known as the Oort cloud is invisible for all practical purposes. The particles that make it up are so thinly dispersed and so far away 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 is also theoretical. Astronomers conclude that the Oort cloud is there 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 actually a gigantic reservoir that can hold billions of icy celestial objects.

Two of those bodies will pass by 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, its closest approach to Earth – just watch the western sky shortly after sunset. As the days pass, the comet becomes fainter and moves to a higher part of 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 by the end of October. The comet will pass closest to Earth on October 24 – look low in the eastern sky just before sunrise. Then, after orbiting the sun, the comet may reappear in the western night sky around Halloween. However, it may completely or partially disintegrate, as sometimes happens when comets pass by the Sun – and this comet will come within 1.6 million kilometers of our star.

As a planetary astronomer, I am especially curious about the Oort Cloud and the icy bodies that inhabit it. The Cloud’s inhabitants may be one reason why life emerged on Earth; These icy bodies, which crashed onto our planet centuries ago, may have provided at least some of the water that all life needs. At the same time, these same objects pose an ever-present threat to the Earth’s survival – and to our survival.

Billions of comets

As an Oort cloud object makes its way into the inner solar system, its ice evaporates. That process produces a tail of debris that becomes visible as a comet.

Some of these bodies, known as long-period comets, have orbits of hundreds, thousands or even millions of years, such as Tsuchinshan-ATLAS. This is different from the so-called short-period comets, which do not visit the Oort cloud and have relatively fast orbits. Halley’s Comet, which blazes a path through the solar system and orbits the sun approximately every 76 years, is one of them.

The 20th-century Dutch astronomer Jan Oort, intrigued by the long-period comets, wrote an article about them in 1950. He noted that about twenty of the comets had an average distance from the Sun exceeding 10,000 astronomical units. This was amazing; just one AU is the distance from the Earth to the Sun, which is about 150 million kilometers. Multiply 93 million by 10,000 and you’ll see that these comets come from more than a trillion miles away. Moreover, Oort suggested, they were not necessarily the outermost objects of the cloud.

An illustration of the solar system and the Oort cloud. The numbers in the graph 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

Nearly 75 years after Oort’s article, astronomers still cannot directly image this part of space. But they do estimate that the Oort Cloud is 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 great distances, making only short and quick visits close to the Sun as they come from all directions. Oort speculated that the cloud contained 100 billion of these icy objects. That could be as numerous as the number of stars in our Milky Way.

How did they get there? Oort suggested, and modern simulations have confirmed, that these icy bodies could have initially formed near Jupiter, the solar system’s largest planet. Perhaps the orbits around the sun of these objects were disrupted by Jupiter – similar to how NASA spacecraft en route to destinations from Saturn to Pluto have typically swung past the giant planet to speed up their outward journeys.

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

Threats to Earth

Long-period comets pose a particular potential danger to Earth. Because they are so far from our sun, their orbits are easily changed by the gravity of other stars. That means scientists have no idea when or where one will appear until it suddenly happens. By then it is usually closer than Jupiter and moving quickly, at tens of thousands of kilometers per hour. The fictional comet that doomed Earth in the movie “Don’t Look Up” did indeed emerge from the Oort Cloud.

New Oort cloud comets are constantly being discovered, approximately ten per year in recent years. The chance of any of them colliding with Earth is extremely low. 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 fending off these small bodies. But that mission was developed after years of research into its purpose. An Oort cloud comet may not offer that much time – perhaps only 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 originate in the Oort cloud, the statement still applies; 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 basic properties, including their size and composition. With this goal in mind, my colleagues and I are working to characterize new long-period comets. The largest known, Bernardinelli-Bernstein, discovered only three years ago, has a diameter of about 120 kilometers. Most known comets are much smaller, from one to several kilometers, and some smaller ones are too faint to see. But newer telescopes help. In particular, the Rubin Observatory’s decade-long Legacy Survey of Space and Time, due to launch in 2025, could double the list of known Oort cloud comets, which now 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. With a launch planned for 2029, the probe will park in space until a suitable target emerges from the Oort Cloud. Studying one of these ancient and pristine objects could give scientists clues about the origins of the solar system.

As for the comets now near Earth, it’s fine to look up. Unlike the comet in the DiCaprio movie, these two won’t crash into Earth. The closest Tsuchinshan ATLAS will get to us is about 70 million kilometers; C/2024 S1 (ATLAS), approximately 80 million miles (130 million kilometers). Sounds like a long way, but in space it’s almost an accident.

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