Follow me around our solar system - Kuiper Belt

2020-05-31 23:04


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A donut-shaped region of icy bodies beyond the orbit of Neptune. There may be millions of these icy objects, collectively referred to as Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNOs), in this distant region of our solar system.


In Brief


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Similar to the asteroid belt, the Kuiper Belt is a region of leftovers from the solar system's early history. Like asteroid belt, it has also been shaped by a giant planet, although it's more of a thick disk (like a donut) than a thin belt.


The Kuiper Belt shouldn't be confused with the Oort Cloud, which is a much more distant region of icy, comet-like bodies that surrounds the solar system, including the Kuiper Belt. Both the Oort Cloud and the Kuiper Belt are thought to be sources of comets.


The Kuiper Belt is truly a frontier in space, it's a place we're still just beginning to explore and our understanding is still evolving.


Where is the Kuiper Belt?


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The inner edge of the Kuiper Belt begins at the orbit of Neptune, at about 30 AU from the Sun. (1 AU, or astronomical unit, is the distance from Earth to the Sun.)


The inner, main region of the Kuiper belt ends to around 50 AU from the Sun. Overlapping the outer edge of the main part of the Kuiper Belt is a second region called the scattered disk, which continues outward to nearly 1,000 AU, with some bodies on orbits that go even farther beyond.



How was the Kuiper Belt Created?


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Astronomers think the icy objects of the Kuiper Belt are remnants left over from the formation of the solar system. Similar to the relationship between the main asteroid belt and Jupiter, it's a region of objects that might have come together to form a planet had Neptune not been there. Instead, Neptune's gravity stirred up this region of space so much that the small, icy objects there weren't able to coalesce into a large planet.


Size and Distance


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The Kuiper Belt is one of the largest structures in our solar system, others being the Oort Cloud, the heliosphere and the magnetosphere of Jupiter. Its overall shape is like a puffed-up disk, or donut. Its inner edge begins at the orbit of Neptune, at about 30 AU from the Sun. (1 AU, or astronomical unit, is the distance from Earth to the Sun.) The inner, main region of the Kuiper belt ends to around 50 AU from the Sun. Overlapping the outer edge of the main part of the Kuiper Belt is a second region called the scattered disk, which continues outward to nearly 1,000 AU, with some bodies on orbits that go even farther beyond.


So far, over 2,000 trans-Neptunian objects have been cataloged by observers, representing only a tiny fraction of the total number of objects scientists think are out there. In fact, astronomers estimate there are hundreds of thousands of objects in the region that are larger than 60 miles (100 kilometers) wide or larger. However, the total mass of all the material in the Kuiper Belt is estimated to be no more than about 10 percent of the mass of Earth.



Formation/Origins


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Astronomers think the icy objects of the Kuiper Belt are remnants left over from the formation of the solar system. Similar to the relationship between the main asteroid belt and Jupiter, it's a region of objects that might have come together to form a planet had Neptune not been there. Instead, Neptune's gravity stirred up this region of space so much that the small, icy objects there weren't able to coalesce into a large planet.

The amount of material in the Kuiper Belt today might be just a small fraction of what was originally there. According to one well-supported theory, the shifting orbits of the four giant planets (Jupiter, Saturn, Uranus and Neptune) could have caused most of the original material, likely 7 to 10 times the mass of Earth, to be lost.

The basic idea is that early in the solar system's history, Uranus and Neptune were forced to orbit farther from the Sun due shifts in the orbits of Jupiter and Saturn. As they drifted farther outward, they passed through the dense disk of small, icy bodies left over after the giant planets formed. Neptune's orbit was the farthest out, and its gravity bent the paths of countless icy bodies inward toward the other giants. Jupiter ultimately slingshotted most of these icy bodies either into extremely distant orbits (to form the Oort Cloud) or out of the solar system entirely. As Neptune tossed icy objects sunward, this caused its own orbit to drift even farther out, and its gravitational influence forced remaining icy objects into the range of locations where we find them in the Kuiper Belt.

Today the Kuiper Belt is slowly eroding itself away. Objects that remain there occasionally collide, producing smaller objects fragmented by the collision, sometimes comets and also dust that's blown out of the solar system by the solar wind.



Structure and Characteristics


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The small dot moving slightly off to the left of center in this image is newly-discovered Kuiper Belt object 2004 DW.


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The first Kuiper Belt Object "1992 QB1" was discovered in 1992 by astronomers David Jewitt and Janet Luui. Credit: European Southern Observatory



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Kuiper Belt Object Arrokoth (2014 MU69)


The Kuiper Belt represents an enormous, donut-shaped volume of space in the outer solar system. While there are many icy bodies in this region that we broadly refer to as Kuiper Belt Objects (KBOs) or trans-Neptunian objects (TNOs), they're fairly diverse in size, shape and color. And importantly, they're not evenly distributed through space, once astronomers started discovering them in the early 1990s, one of the early surprises was that KBOs could be grouped according to the shapes and sizes of their orbits. This led scientists to understand that there are several distinct groupings, or populations, of these objects whose orbits provide clues about their history. Which category an object belongs to has a lot to do with how it has interacted with the gravity of Neptune over time.


Most of the objects in the Kuiper Belt are found in the main part of the belt itself or in the scattered disk



Scattered Disk


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The scattered disk is a region that stretches far beyond the main part of the Kuiper Belt, and is home to objects that have been scattered by Neptune into orbits that are highly elliptical and highly inclined to the plane of the planets. Many scattered disk objects have orbits that are tilted by tens of degrees. Some venture hundreds of AU from the Sun and high above the plane of the planets at the farthest point in their orbits, before falling back to a closest point near the orbit of Neptune. The orbits of many objects in the scattered disk are still slowly evolving, with objects here being lost over time, compared to the classical Kuiper Belt, where orbits are more stable.


The scattered disk gives the donut-shaped classical Kuiper Belt a much wider and thicker extent. Some astronomers talk about the two as separate regions, although their boundaries overlap and they are linked together in a number of ways. (In particular, the objects in both regions are thought to have ended up there as a result of the migration of Neptune from its original, closer orbit to where it is now.)


Eris is an example of an object in the scattered disk (in fact, it's the largest known member of this population).



Pluto's Place in the Kuiper Belt


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Pluto was the first Kuiper Belt object to be discovered, in 1930, at a time before astronomers had reason to expect a large population of icy worlds beyond Neptune. Today it's known as the "King of the Kuiper Belt", it's the largest object in the region, even though another object similar in size, called Eris, has a slightly higher mass. Pluto's orbit is said to be in resonance with the orbit of Neptune, meaning Pluto's orbit is in a stable, repeating pattern with Neptune's. For every three orbits completed by Neptune, Pluto makes two orbits. In this situation, Pluto never comes close enough to Neptune to be affected much by its gravity. In fact, even though its orbit crosses Neptune's orbit, Pluto gets physically closer to Uranus than it ever does to Neptune.


Kuiper Belt Moons and Binaries


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A fairly large number of KBOs either have moons -- that is, significantly smaller bodies that orbit them -- or are binary objects. Binaries are pairs of objects that are relatively similar in size or mass that orbit around a point (a shared center of mass) that lies between them. Some binaries actually touch, creating a sort of peanut shape, creating what's known as a contact binary.


Pluto, Eris, Haumea and Quaoar are all Kuiper Belt objects that have moons. Telescope observations suggest the target of the NASA New Horizons spacecraft's 2019 flyby, known as 2014 MU69, may be a contact binary.


One thing that makes binary KBOs particularly interesting is that most of them may be extremely ancient, or primordial, objects that have been altered little since their formation. The various ideas for how these pairs form require a lot more objects than the present day Kuiper Belt appears to contain. One leading idea is that binaries may result from low-speed collisions between KBOs, which would allow them to survive the impact and stick together due to their mutual gravity. Such collisions were likely much more common billions of years ago, when most KBOs were on similar orbits that were more circular and close to the plane of the planets (called the ecliptic). Today such collisions are much rarer. They also tend to be destructive, since lots of KBOs are on now orbits that are tilted or elliptical, meaning they crash into each other with greater force and break apart.



Relationship to Comets


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The Kuiper Belt is a source of comets, but not the only source. Today the Kuiper Belt is thought to be very slowly eroding itself away. Objects there occasionally collide, with the collisional fragments producing smaller KBOs (some of which may become comets), as well as dust that's blown out of the solar system by the solar wind. Pieces produced by colliding KBOs can be pushed by Neptune's gravity into orbits that send them sunward, where Jupiter further corrals them into short loops lasting 20 years or less. These are called short-period Jupiter-family comets. Given their frequent trips into the inner solar system, most tend to exhaust their volatile ices fairly quickly and eventually become dormant, or dead, comets with little or no detectable activity. Researchers have found that some near-Earth asteroids are actually burned-out comets, and most of them would have started out in the Kuiper Belt. Many comets crash into the Sun or the planets. Those that have close encounters with Jupiter tend to be ripped apart or tossed out of the solar system entirely.


The other source of comets is the Oort Cloud, where most long-period comets on highly tilted orbits come from.




以上我们说的是生命的进化,而科技的进化与自然进化相似,主要的科技变革也是从一个信息组织层次过渡到另一个层次的通道。比如科技的起点从语言起步,文字的出现是一个重要转折从工业文化到现在。



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