Description
The Oort Cloud is a part of the Solar System. But it is a hypothetical part. Science does not have sufficient evidence, and even less so, there are no direct observations of it. But there are enough reasons for its existence.
The idea that there are countless relatively small icy bodies on the outermost edges of our planetary system was first put forward by the Estonian astronomer Ernst Öpik (in 1932). This idea was developed two decades later by the Dutch astrophysicist Jan Oort. The latter's name is now firmly associated with this hypothesis, which explains well where new comets regularly appear from in the inner part of the solar system.
The paradox was that comets are destroyed by the Sun quite quickly — a few approaches are enough. And even if the periods of cometary revolutions are long by human standards — thousands or even hundreds of thousands of years, but during the existence of the Solar System (4.6 billion years) not a single one would have remained alive. This means that somewhere there is a cometary reservoir — far from the Sun — in which, like in a refrigerator, potential cometary nuclei are carefully stored for the time being.
Analysis of the orbits of long-period comets shows that the concentration of these reserves most likely has two range levels — the inner Oort Cloud (from 2 to 20 thousand AU), and the outer Oort Cloud (from 50 to 100 thousand AU).
1 Astronomical Unit is the average distance from the Earth to the Sun. It is a convenient measure of length for the inner part of the solar system. But when talking about the entire solar system, it noticeably becomes overgrown with zeros.
The inner Oort Cloud is more like a "Donut", and scientists call this shape a "Torus". This structure gravitates toward the plane of the ecliptic, relatively not moving away from it. But the outer Oort Cloud is a spheroidal array of billions of icebergs slowly drifting around the Sun. Their flight is so leisurely that in the depths of their eternal sleep they may not realize that they are satellites of the Sun, because one revolution around it takes millions, tens of millions of years. It may seem to them that they are flying in a straight line or resting in the center of the Universe. And the Sun from such a distance is no brighter than other distant stars. How can you guess that this is your star?
At such a distance, the Sun cannot warm these blocks of ice, and certainly cannot melt them. It is not even able to cover them with the shield of its Heliosphere, and the icy celestial bodies in the Oort Cloud are constantly subjected to the action of galactic radiation, which is a combination of radiation from all the stars in the Galaxy, as well as more distant quasars — active nuclei of other galaxies. And even this cannot awaken them.
But they do wake up sometimes. This happens when another star passes close to the Sun, literally invading the Oort Cloud. The centuries-old balance in it is disrupted. Chaos reigns. Some objects leave the Cloud forever, carried away by a star wandering around the galaxy, or are simply thrown out into interstellar space. But another part rushes into the inner regions of the Solar System, and becomes new comets, which are now destined to regularly approach the Sun until their final destruction.
It is not at all necessary that only a random visitor star can send us new comets, awakening ice blocks in the Oort cloud. These blocks, although rarely, can interact with each other — come closer, collide, influence each other with their own (albeit very weak) gravity, and thus change each other's orbits.
And the same can be done by the still undiscovered planets of the Solar System — no one guarantees that there are none left in it. But such influences can give us one or two new comets per millennium. A passing star can arrange a whole cometfall, the consequences of which are catastrophic. It has been discovered that episodes of mass extinction of living organisms on Earth quite accurately correlate with close approaches of the Sun and other stars, which astronomers have already managed to calculate — both in the past and in the direction of the future. It turns out that such meetings are ahead of us.
In about a million and 200 thousand years, the star Gliese 710, which is now invisible to the naked eye even in the mountains (it has only 8th magnitude and is currently located 64 light years away), will pass extremely close to the Sun — 1/6 of a light year. But this is deep inside the Oort Cloud — both the outer and inner. Translated into more understandable metric units, the distance to Gliese 710 will be only 10 thousand astronomical units. In the earth's sky, the visitor star will become the brightest. And only the Sun, the Moon and Venus will compete with it. But the most impressive spectacle will be a swarm of comets heading towards the Sun, each of which can carry a mortal danger to all living things — if it collides with the Earth.
It would seem that a million years is not so soon. But everything happens sooner or later. A million years ago, the Earth was already inhabited by people. I would like to believe that in another million years, people on Earth will not die out, and will even prosper — unlike today's state of affairs — without wars and economic crises. It would be a shame to lose such a future because of some comet. Therefore, we need to study the Oort Cloud today. And it would be good to start by confirming or refuting its existence.
It is possible that more sharp-sighted telescopes of the future will be able to discern relatively small but very numerous objects of the Oort Cloud. As for spacecraft, the fastest of the Earth's trailblazers leaving the Solar System, the Voyager 1 probe, has covered 166 astronomical units to date, which is only 1/12 of the distance to the beginning of the inner Oort Cloud. But it should be borne in mind that the trajectory of Voyager 1 is close to perpendicular to the plane of the ecliptic, and in its flight this device will most likely not get into the inner Oort Cloud. And it will fly 25 times further to the beginning of the outer one. It is possible that faster explorer ships of the future will be able to reach the farthest outskirts of the Solar System much faster than modern space probes.
