This paper takes part in the series essay competition of “great science” of Recordunkown.
In ancient times, people knew that there was a more mysterious world outside the earth. However, due to the lack of scientific assistance, the ancients did not know what the world outside the earth looked like? In the eyes of the ancients, the stars at night are things outside the earth, and the world is also very beautiful and prosperous, with a large number of immortals living in it.
With the continuous development of human civilization, it embarked on the road of scientific and Technological Development hundreds of years ago. With the help of scientific power, we invented the astronomical telescope, and initially saw the world beyond the earth. However, in the eyes of the telescope, the space of the earth is dark, not as beautiful as we imagined.
By the middle of last century, with the help of science and technology, human beings finally realized their dream of flying to the sky and walked out of the earth. When we walk out of the earth, we finally see this extraterrestrial space that makes countless people look forward to. We call it the universe.
Through naked eye observation, the whole universe is vast and dark. Except for some stars that can emit light, the background of the whole universe is dark. So is the universe really dark? Of course not, what we see through the telescope is another scene, that is, the bright light everywhere in the universe, that is the light from stars.
Telescopes can see far away, but we need detectors to explore the mysteries of the universe. For people who have just stepped out of the earth, the first thing is to explore the mystery of the entire solar system, and the most mysterious and complex region of the solar system is at the edge of the solar system.
To explore the edge of the solar system, NASA launched Voyager 1 and Voyager 2 in 1977. Their goal is to advance beyond the solar system, not only to explore the mysteries of the edge of the solar system, but also to go out of the solar system and explore interstellar space.
In the view of scientists, the distribution of matter in the universe is different. In some places, if there is more material distribution, the density will be higher, while in some places, if there is less material distribution, the density will be lower. In theory, the closer to the sun, the greater the density of matter in the stellar system, and the smaller the density of matter far away from the edge of the solar system.
And the interstellar space outside the galaxy, the matter is very little, so we say the universe is a vacuum space. But this vacuum is not absolutely empty, which I believe my friends know very well. In our seemingly very empty universe, there are always some particles. Even in the cosmic cavity observed by astronomical telescopes, there are not no celestial bodies and matter at all, but they are too small for us to observe.
Theoretically speaking, interstellar space should be more open, and the density of matter is far lower than that in the solar system. Is that really the case? The answer may depend on Voyager. After decades of flight, Voyager 1 was the first to reach the edge of the solar system, crossing the top of the heliosphere in 2012. Voyager 2, on the other hand, will cross the top of the heliosphere and enter interstellar space in 2018 six years sooner or later.
After the Voyager spacecraft arrived at the edge of the solar system, it also sent a series of important data back to the earth. These data are very important for scientists to understand the matter at the edge of the solar system, and also an important reference for our preliminary understanding of interstellar space.
We talked about the concept of heliospheric zenith. Many friends may not understand what this means? Here is a brief explanation. The so-called heliospheric zenith refers to the place where the solar wind meets the interstellar medium. As we all know, the solar wind is the strong radiation from the sun, which spreads and extends around the solar system.
The areas that the solar wind can reach belong to the solar system in a narrow sense. As the solar system continues to spread to the edge, its strength is also weakening. At the weakest point, it will meet with the interstellar medium from the interstellar space. What will it look like when the solar wind and the interstellar medium do not intersect? This is something scientists really want to understand.
Back to the topic of matter density in the universe, in the solar system, the density of protons and electrons in the solar wind averages 3-10 per cubic centimeter. The farther away from the sun, the lower the density. According to previous calculations, the average electron density of the interstellar medium in the galaxy is about 0.037 electrons per cubic centimeter, and the plasma density outside the heliospheric top is about 0.002 electrons per cubic centimeter.
The above data is a result theoretically calculated by scientists. To verify this result, we need the real data detected by Voyager. When the two voyagers were passing through the top layer of the heliosphere, the detectors carried on them successfully detected the electron density of the plasma.
Voyager 1 detected 0.055 electrons / cubic centimeter at 18.3 billion km and Voyager 2 detected 0.039 electrons / cubic centimeter at 17.9 billion km. The two values are basically the same. However, after Voyager continued to fly in interstellar space for a period of time, the data detected surprised scientists.
In theory, once Voyager passes through the top of the heliosphere and enters interstellar space, the detected density of matter should be very small. After all, it is interstellar space, and the density of matter cannot be compared with that in the solar system. However, after Voyager 1 flew 2.9 billion kilometers in interstellar space, the density of interstellar medium detected was 0.13 electrons / cubic centimeter, which was much higher than when Voyager 1 crossed the heliospheric top.
After Voyager 2 flew 600 million kilometers in interstellar space, at 18.5 billion kilometers, the density of interstellar medium also increased to 0.12 electrons / cubic centimeter. The values of the two spacecraft are basically the same, which shows that the data detected by Voyager spacecraft are true and reliable. The density of matter in the interstellar space outside the solar system is really very high. What’s the matter?
For this result, scientists are puzzled. The density of matter in interstellar space is less than that in galaxies. This should be a correct conclusion. But why is extrasolar space so special? Some scientists have proposed a possibility that when the interstellar magnetic field lines cover the top of the heliosphere, the magnetic field lines will become stronger, which may form an electromagnetic ion cyclotron instability and deplete the plasma in the covered area.
There is also a view that the matter from the interstellar wind is hindered and decelerated at the top of the heliosphere, which leads to a certain extent that the matter can not diffuse out and gather in a limited range. If this is the case, voyager may be able to detect the real density of interstellar space as long as it flies a little further.
In fact, we have to think of another possibility here, that is, the space outside the top of the heliosphere may not be the real interstellar space, they also belong to the solar system, that is, the solar system in a broad sense. In the past, scientists believed that going out of the top of the heliosphere was going out of the solar system and into interstellar space. But later, scientists observed the existence of the Oort cloud, and thought that only out of the Oort cloud can we really go out of the solar system.
The Oort cloud is a nebula with a thickness of one light year, which surrounds the solar system in a narrow sense. In this nebula cluster, not only a large number of comets, asteroids and other celestial bodies exist, but also the density of matter is much higher than that of interstellar space. Therefore, Voyager spacecraft out of the heliospheric top, but detected a higher density of interstellar media, may also be able to understand.
If the density of the material detected by Voyager remains at a high level in the later flight, then it is a probable event that Voyager still stays in the solar system. And this result has an important role for us to understand the solar system, at least let us understand that the scope of the solar system is far larger than previously thought.
Of course, if the outer side of the Oort cloud is regarded as interstellar space, it will also disappoint scientists, because Voyager also shoulders an important task, that is to find alien civilization. They all carry a gold CD, recording the coordinates of the earth and human information.
Scientists expect Voyager to get out of the solar system and be very lucky to be captured by the alien civilization. Then the alien civilization will come to the solar system to communicate with human beings through the information left on the CD. Now it seems that this wish is basically hopeless. It may take at least 10000 years to cross the Oort cloud at their speed.