Away from the protection of the sun, the edge of the solar system looks extremely cold, empty and dark. For a long time, the vast space between the solar system and neighboring stars has also been regarded as terrible and vast nothingness. Until recently, humans could only peek into these spaces from a distance. Astronomers are not interested in this either. They prefer to project telescopes onto luminous objects, such as nearby stars, galaxies and nebulae.
However, in the past few years, two spaceships built and launched in the 1970s have returned from this strange and peculiar interstellar space to scenes that humans have never seen before. As the first two man-made objects to leave the solar system, they are exploring unknown areas billions of kilometers away from the earth, and so far no other spacecraft have flown. The two spacecraft also revealed a chaotic and turbulent activity area outside the boundary of the solar system.
Michelle bannister is an astronomer at the University of Canterbury, New Zealand, who mainly studies the outer regions of the solar system. "When you look at different parts of the electromagnetic spectrum, that space is completely different from the darkness we perceive with the naked eye," she said. "You will see that the magnetic fields are fighting with each other, pushing each other and contacting each other. You can imagine the pool under Niagara Falls. "
What is tumbling here is not water, but turbulence generated by the collision between the solar wind and the so-called "interstellar medium". The solar wind is a powerful charged particle or plasma stream, which is emitted from the sun in all directions. Various forms of matter and radiation existing in interstellar space are called "interstellar medium".
In the past century, scientists have described the composition of interstellar medium to us, which is largely due to the observation ability of radio telescopes and X-ray telescopes. They revealed that the interstellar medium is composed of extremely dispersed ionized hydrogen atoms, dust and cosmic rays, and there are also dense molecular gas clouds, which are considered to be the birthplace of new stars.
However, the exact nature of the interstellar medium outside the solar system has been a mystery to a great extent, mainly because the sun, all eight planets and the distant disk-shaped Kuiper belt are all contained in the huge bubble formed by the solar wind, which is called the "heliosphere". When the sun and its surrounding planets pass through the Milky Way quickly, the solar circle is like an invisible shield, which can buffer the interstellar medium and keep out most harmful cosmic rays and other substances.
However, the protective characteristics of the solar circle also make it more difficult for scientists to study the situation outside its scope, and even it is difficult to determine the size and shape of the solar circle from the inside.
Elena Provo Nikova, a postdoctoral researcher at the Applied Physics Laboratory of Johns Hopkins University in the United States, said: "It's like you are in your own home, and you want to know what this home is like, so you have to look outside to really judge." "The only way to know something is to stay away from the sun, then look back and take pictures from outside the sun circle."
Voyager 1 crossed the solar system in a more direct way and entered interstellar space in 20 12. Voyager 2 joined it in 20 18. At present, they are about 20.9 billion kilometers away from the Earth and 654.38+077 billion kilometers away from the Earth, respectively, and they are constantly drifting outward, going deeper and deeper into the outer space of the solar system and sending back more data.
These two old detectors reveal the boundary between the solar circle and the interstellar medium, providing us with new clues about how the solar system formed and why there is life on earth. In fact, the edge of the solar system has no clear boundaries, but is full of rolling magnetic fields, colliding stellar winds, high-energy particle storms and vortex radiation.
In 20 14, the solar activity surged, which led to the violent solar wind sweeping through space. This shock wave swept through Mercury and Venus at a speed of 800 kilometers per second. Two days later, it wrapped the earth through a distance of 65.438+0.5 billion kilometers. Fortunately, the earth's magnetic field protects us from its powerful destructive radiation.
One day later, the solar wind blew over Mars, continued to cross the asteroid belt and headed for the distant gas giants-Jupiter, Saturn and Uranus; More than two months later, the solar wind reached Neptune, and its orbit was nearly 4.5 billion kilometers away from the sun. After more than six months, the solar wind finally reached a distance of more than 30 billion kilometers from the sun/KLOC-0, and encountered a "terminal shock wave" and suddenly slowed down. Here, the solar magnetic field that drives the solar wind becomes very weak, and the interstellar medium can push it.
The speed of the solar wind coming out of the terminal shock wave is less than half of that before, just as a hurricane weakens into a tropical storm. At the end of 20 15, this solar wind caught up with the irregular Voyager 2, which was about the size of a car. Sensors on Voyager 2 detected a surge in plasma, a technology that has been used for 40 years and is powered by slowly decaying plutonium batteries.
Voyager 2 was still in the solar system when the solar wind caught up with it. After more than a year, this dying solar wind finally caught up with Voyager 1, and it entered interstellar space in 20 12.
The two detectors chose different routes, one of which is located 30 degrees above the solar plane and the other is located at a corresponding position below the solar plane. The solar wind broke out in different areas at different times, which provides useful clues for studying the properties of the heliopause (also known as the solar wind top).
The data show that the width of this turbulent boundary can reach millions of kilometers, covering billions of square kilometers on the surface of the solar circle. The solar circle is also surprisingly large, which indicates that the density of interstellar medium in this part of the galaxy is lower than the original estimate. The sun cuts a path in interstellar space, just like a ship sailing in the water, producing an "bow shock wave" and forming a wake behind it. This wake may have a (or more than one) tail similar to a comet. Both voyager spacecraft took off from the "nose" of the heliosphere, so they didn't provide any information about the comet's tail.
Not only the solar wind and interstellar wind will produce turbulence in the boundary area, but also the particles seem to exchange charge and momentum. As a result, some interstellar medium is transformed into solar wind, which actually increases the outward thrust of bubbles.
Although the solar wind can provide interesting data, it seems to have little effect on the overall size and shape of bubbles in the solar circle, which is somewhat surprising. It seems that what happens outside the solar circle is much more important than what happens inside the solar circle. The intensity of solar wind can increase or decrease with time, and has no obvious effect on bubbles. However, if this bubble enters a region of the galaxy where the density of interstellar wind is higher or lower, it will contract or increase.
Of course, there are still many unanswered questions, such as whether bubbles protecting our solar circle are common in the universe. Provo Nikova said that the more we know about the solar circle, the more clearly we can know whether we are lonely in the universe. She said: "Research in our own star system will tell us the conditions for the development of life in other star systems."
The development of life on earth is largely because the solar circle blocks the interstellar medium. At the same time, the solar wind also prevents the bombardment of deadly radiation and high-energy particles (such as cosmic rays) from outer space. Cosmic rays are composed of protons and nuclei and flow in space at a speed close to the speed of light. When catastrophic cosmic events such as the explosion of stars and the collapse of galaxies into black holes occur, cosmic rays will be produced. Outside the solar system, these high-speed subatomic particles keep pouring in, powerful enough to cause fatal radiation damage on a less protected planet.
"Voyager's detection clearly shows that 90% of cosmic rays are filtered by the sun," said Jamie Rankin, a researcher in solar physics at Princeton University. "Without the protection of the solar wind, I don't know if we can survive." Rankin was also the first person to write a doctoral thesis based on the interstellar data of Voyager.
Three other NASA probes will soon join Voyager and enter interstellar space. Although two of the detectors have run out of energy and stopped sending back data. Of course, on the huge boundary of the solar system, these tiny detectors can only provide limited information even if they are available. Fortunately, we can make more extensive observations closer to the earth.
Interstellar boundary explorer (IBEX) of NASA is a microsatellite that has been orbiting the earth since 2008, and its detection target is "high-energy neutral atoms" on the border of cross star. IBEX also drew a three-dimensional map of the interaction around the boundary of the solar circle.
Jamie Rankin said: "You can think of the map of IBEX as some kind of Doppler radar, and Voyager is like a ground weather station." Using data from Voyager, IBEX and other sources, she analyzed the smaller power generation in the solar wind. At present, I am writing a paper, mainly discussing the bigger outbreak starting from 20 14. There is already evidence that the solar circle is shrinking when the traveler 1 crosses the border; But when Voyager 2 crossed the border, the solar circle began to expand again.
"This is a very dynamic boundary," Rankin said. "This discovery was captured by IBEX and drawn into a three-dimensional map, so that we can track the local reaction of Voyager at the same time, which is simply amazing."
IBEX reveals the dynamic changes of the boundary of the solar circle. In the first year of operation, it found a huge high-energy atomic band winding through the boundary, and the boundary changed with time. These characteristics only appeared and disappeared within 6 months. This belt is located at the front end of the solar circle, where solar wind particles are reflected back to the solar system by the interstellar magnetic field.
However, there is another turning point in Voyager's story. Although they have left the solar circle, they are still within the influence of the sun. For example, in other star systems, sunlight can be seen by the naked eye. The gravity of the sun also goes far beyond the solar circle, blocking the distant and sparse spherical clouds composed of ice, dust and space debris. This is Oort Cloud.
Although Oort Cloud floats in distant interstellar space, there are still many celestial bodies orbiting the sun. The orbits of some comets can extend all the way to the Oort Cloud, but it is generally considered that the area of 3000 to10.5 trillion kilometers is too far for the detectors launched from the earth.
These distant celestial bodies have hardly changed since the formation of the solar system. They may have answered many unanswered questions, including how planets formed and the possibility of life in the universe. With the emergence of every new wave of data, new mysteries and problems also arise.
Provo Nikova said that part or all of the solar circle may be covered by a layer of hydrogen, and its influence is unknown. In addition, the solar circle seems to be tilting towards the interstellar clouds left over from ancient cosmic events. It is impossible to predict how the particles and dust in this interstellar cloud will affect the boundary of the solar circle and the people living in it.
"It can change the size and shape of the solar circle," said Provo Nikova. "It may have different temperatures, different magnetic fields, different ionization and all completely different parameters. This is very exciting because this is an area that will produce many discoveries, and we know very little about this interaction between the sun and the Milky Way. "
No matter what happens, two brave Voyager probes will be the pioneers of our solar system. When exploring strange and unknown areas in space, they will reveal more mysteries and bring more puzzles. (Ren Tian)