Oort Cloud of comets. Artwork showing the Oort Cloud (orange), a postulated cloud of comet nuclei surrounding the Solar System. It is thought to extend over a light year from the Sun (centre), up to a distance of about 100000 astronomical units (AU). One AU is the distance from the Earth to the Sun, some 150 million kilometres. Near the Solar System planets it has the form of a disc in the plane of the planets' orbits (ecliptic). Further out, it forms a spherical shell. The existence of the Oort Cloud is inferred from the orbits of long-period comets. These tend to orbit the Sun on elongated paths (some seen here) with their most distant point around 50000 AU from the Sun.

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Oort Cloud of comets. Artwork showing the Oort Cloud (orange), a postulated cloud of comet nuclei surrounding the Solar System. It is thought to extend over a light year from the Sun (centre), up to a distance of about 100000 astronomical units (AU). One AU is the distance from the Earth to the Sun, some 150 million kilometres. Near the Solar System planets it has the form of a disc in the plane of the planets' orbits (ecliptic). Further out, it forms a spherical shell. The existence of the Oort Cloud is inferred from the orbits of long-period comets. These tend to orbit the Sun on elongated paths (some seen here) with their most distant point around 50000 AU from the Sun.

Photo by: MARK GARLICK/SCIENCE PHOTO LIBRARY

MARK GARLICK/SCIENCE PHOTO LIBRARY

Take a Trip Through the Oort Cloud, It’s Fun

The most distant spacecraft launched by humans, Voyager 1, is currently 155 times farther away from the Sun than the Earth is. That’s almost four times more distant than Pluto, and the Voyager 1 craft has been traveling at over 38,000 mph for over four decades.

June 02, 2022

Indeed, Voyager 1 is so far away that it’s technically in interstellar space – it’s beyond the boundary where the charged particles that constantly rain from the Sun mix and mingle with the interstellar medium.

We’re not even getting started.

Beyond the planets, beyond the icy members of the Kuiper belt, beyond our more far-flung space probes, lies the very last remnant of the solar system: the Oort cloud. The Oort cloud contains billions of tiny fragments, each a mixture of ice and rock. These fragments, each no bigger than a few hundred meters across, are so far away from us that we’ve never directly observed any of them.

Instead, we can only guess as to the existence of the Oort cloud. The Dutch astronomer Jan Oort first made the conclusion in 1950 after studying the origins of long-period comets. These comets come from seemingly anywhere, and appear only once, making a single trip around the Sun before racing back off into the dark. He suspected that these comets had a home, a reservoir surrounding the solar system – the Oort cloud.

On Oct. 20, the Kepler spacecraft joined the fleet of NASA science assets that observed distant Oort Cloud native Comet Siding Spring as it passed through K2's Campaign 2 field-of-view on its long journey around the sun. The data collected by K2 will add to the study of the comet, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago.

On Oct. 20, the Kepler spacecraft joined the fleet of NASA science assets that observed distant Oort Cloud native Comet Siding Spring as it passed through K2's Campaign 2 field-of-view on its long journey around the sun. The data collected by K2 will add to the study of the comet, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago.

Photo by: NASA Ames/W Stenzel; SETI Institute/D Caldwell

NASA Ames/W Stenzel; SETI Institute/D Caldwell

On Oct. 20, the Kepler spacecraft joined the fleet of NASA science assets that observed distant Oort Cloud native Comet Siding Spring as it passed through K2's Campaign 2 field-of-view on its long journey around the sun. The data collected by K2 will add to the study of the comet, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago.

Astronomers believe that the inner edge of the Oort cloud sits about 1,000 AU (astronomical units, the distance between the Sun and the Earth) away. That means that Voyager 1 won’t reach it for another two centuries. Once it gets there, there won’t be any flashing lights or welcome banners. Despite hosting billions of individual comets, the volumes here are so tremendous that Voyager 1 will pass within ten million kilometers of one…if it’s lucky.

And the outer edge? Astronomers aren’t exactly sure where the Oort cloud cuts off, but it’s probably close to one or two light-years away, up to half the distance to our nearest neighbor star, Proxima Centauri. That means it will take Voyager tens of thousands of years before it finally passes through.

The Oort cloud represents the last true remnants of the formation of the solar system. When our system formed, it contained trillions of tiny bits of rock and ice that never got to form a planet. But as the planets formed, they stirred up the debris and flung them into the outermost edges of the solar system.

Even though the members of the Oort cloud live in interstellar space, they are still gravitationally connected to the Sun. But only weakly – even the tiniest of nudges can send them either scattering loose into the depths of space or plunging down into the inner solar system, where we recognize them as a comet.

It’s lonely out there.

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Paul M. Sutter

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

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