But just in 20 17, scientists discovered an asteroid at the nearest location 23 million kilometers away from the earth. Its properties and orbital parameters make scientists immediately feel that it is different, unlike any asteroid we have found in the solar system before.
Scientists believe that this is the first time that humans have discovered interstellar objects in the solar system. It originated outside the solar system and is crossing the solar system, so it is named "Oumuamua", which means: an emissary from the distant past.
Due to the peculiar nature of Omo Mo, a few scientists speculate that it is not a natural celestial body, but an alien spacecraft, but this conjecture has not been widely accepted by scientists.
Scientists have been studying the mystery of its life experience. Recently, a group of researchers put forward a new theory, which may solve the mystery of Omo Mo.
Omo Mo's strangeness
The above picture shows the orbit of Omo Mo, with a very large eccentricity, even passing through the orbit of Mercury. Omo Mo's speed is 26 km/s, and it can't have originated from the Kuiper Belt or Oort Cloud in the solar system.
Because the small celestial bodies here can't reach such a high speed even if they reach the interior of the solar system through the acceleration of planetary gravity, when scientists discovered Omo, they agreed that an interstellar celestial body had entered the solar system.
Some scientists have found that the shape of Omomomo is different from that of the asteroid we found before. Although asteroids in the past were not perfect spheres, their long and short axis ratios were not high.
The ratio of long axis to short axis of Omomomomo can reach 6: 1, and the width of narrow side is about 100 m, so Omomomomo looks like a cigar from the side.
We found some asteroids in the inner solar system in the past. When they are close to the sun, the volatile substances (water ice) in them will evaporate in large quantities, forming coma or tail.
However, OMOMOMO did not detect the release of volatile gas (water ice), so scientists speculated that it was an extremely "dry" celestial body.
Moreover, the brightness of Omo Mo also changes greatly, and the brightness can change 15 times in a period of 3.6 hours.
In fact, the above characteristics are not particularly strange. We can all find a theory to explain it. Take the brightness change as an example. Omo may be a periodically rolling object, and its shape may be a slender cigar or a rolling flat object.
The albedo of each face is different, so its brightness can change periodically.
As for the shape of Omo and the emission of volatile gases, it is not a big deal. After all, all kinds of celestial debris may be produced in the universe. As long as the time is long enough, it is very reasonable to completely consume volatile gases.
There is only one thing that scientists can't explain, that is, the orbital parameters of Omomo don't match the orbit predicted by gravity theory. It seems to have a very small but negligible acceleration: 5 microns/second? .
In the past, asteroids we found in the solar system all had extra acceleration during flight. This is because there are volatile gases in asteroids, and water, carbon dioxide, carbon monoxide and dust will be ejected during flight. These things will provide a weak force to the asteroid and produce acceleration.
However, we have not found the phenomenon that these volatile substances are sprayed on Omo Mo, so we can't explain the source of its acceleration according to past experience.
Coupled with the strange shape of Omo, people speculate that it may be an alien spacecraft, and the acceleration is caused by the solar wind hitting the sails.
Ommo's scientific explanation
20 19 points out that the volatile substances on the surface of Omo are not water ice that we often see on other asteroids, and there are no carbon monoxide and carbon dioxide in it, but 6% of its surface is covered with hydrogen ice.
Therefore, our observation equipment did not detect the familiar gases in the past, and thought that Omo had no volatile substances. The existence of hydrogen ice increased the volatilization of Omo after it entered the solar system, which provided it with acceleration.
But the problem is that hydrogen ice evaporates very quickly. Even if the celestial body is many times larger than Momo, the hydrogen ice in it will disappear completely within 100 million years, and Momo comes from interstellar space. It takes a long time to form from one star and fly to another star system, so according to the size of Omo, its hydrogen ice has already evaporated.
So hydrogen-rich ice is not a good explanation.
But this explanation also provides a train of thought for scientists. If it's not hydrogen ice, could it be other gases?
Recently, scientists suggested that there might be nitrogen ice on Omo. The existence of nitrogen ice can not only explain the change of Omomo brightness, because it can reflect two-thirds of sunlight;
Moreover, the evaporation rate of nitrogen ice is slower than that of hydrogen ice, and 6% nitrogen ice on Omo-sized celestial bodies can exist for at least 600 million years.
So this gives Omo, an interstellar object, time to enter the solar system with nitrogen ice without evaporating on the way.
So will there be an asteroid with nitrogen ice in interstellar space?
Absolutely. Among celestial bodies outside our solar system, such as Pluto and Triton, they have been born for 4.5 billion years, but there are still thousands of meters of nitrogen ice on them.
When the solar system was first born, its nitrogen ice thickness could reach tens of kilometers. It is believed that there are many dwarf planets similar to Pluto in the Kuiper Belt of the early solar system. When they collide with each other, they will produce a lot of debris, two thirds of which are rich in water ice and one third in nitrogen ice.
Many of them still exist in the Kuiper Belt, and there are still some small celestial bodies rich in nitrogen ice in the Oort cloud of the solar system.
There are still many fragments that will be thrown out of the star system into interstellar space and wander in the Milky Way. If there are 400 billion stars in our galaxy, they can produce at least 10 25 large and small fragments into interstellar space during their evolution.
Scientists predict that 4% of interstellar asteroids are celestial bodies rich in nitrogen and ice like Omo. In the future, we can confirm this statement by studying asteroids in the Kuiper belt of the outer solar system. If there are asteroids outside our solar system that are rich in nitrogen ice but have no water ice, then the strange acceleration of Omo can be explained.