Voyager 1 is the only human-made object famous for breaking out of the "cosmic home" of its creators - the solar system. And at least twice. Where is he now? Technically, still in it.
The first sensational reports that the robotic probe Voyager 1, launched by NASA back in 1977 to explore Jupiter and Saturn, had left the solar system, appeared in March 2013.
The American Geophysical Union (AGU), a non-profit society dedicated to earth and space exploration, issued a press release citing sudden changes in cosmic radiation.
Just a few hours later, after a comment by NASA scientists directly working on the project that they could not assert anything like that, the AGU experts backed down. They revised the press release to indicate that the spacecraft had "entered a new space region," and admitted to trying to make the conclusions of their observations understandable to the general public.
Similar messages appeared several more times every couple of months, until six months later, NASA specialists actually confirmed all previous statements. Finally, it was officially announced that the probe entered interstellar space a year earlier - on August 25, 2012.
The media once again could not deny themselves the high-profile headlines that Voyager had left the solar system - and they were not entirely wrong. However, there are still no such bold statements in the materials of NASA - moreover, according to them, none of us will live to see the moment when this will undoubtedly become a reality.
Where does the solar system end?
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As always, this is a question of terminology - it all depends on what exactly is considered the solar system.
In the usual sense, it consists of eight planets revolving around our star (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), their satellites, the asteroid belt (between the orbits of Mars and Jupiter), many comets, as well as the Kuiper belt …
It contains mostly small bodies left over from the formation of the solar system, and several dwarf planets (including Pluto, which was demoted to this category from ordinary planets just over a decade ago). The Kuiper Belt is essentially similar to the asteroid belt, but is much larger and larger than the latter.
To imagine the scale of this part of the solar empire, it is customary to use astronomical units (AU) - one unit is equal to the approximate distance from the Earth to the Sun (about 150 million km or 93 million miles).
The last planet, Neptune, is at a distance of about 30 AU from the star. Up to the Kuiper belt - 50 AU.
Add to this a little more than 70 astronomical units - and we come to the first conventional boundary of the solar system, which Voyager crossed - the outer boundary of the heliosphere.
All of the above - the planets, the Kuiper belt and the space beyond it - are influenced by the solar wind - a continuous stream of charged particles (plasma) emanating from the solar corona.
This constant wind forms a kind of elongated bubble around our system, which "displaces" the interstellar medium and is called the heliosphere.
As they move away from the Sun, the speed of charged particles decreases as they encounter more and more opposition - the onslaught of the interstellar medium, mainly consisting of clouds of hydrogen and helium, as well as heavier elements such as carbon and dust (only about 1%).
When the solar wind slows down sharply and its speed becomes less than the speed of sound, the first boundary of the heliosphere, called the boundary of the shock wave (in English - termination shock), comes. Voyager 1 crossed it back in 2004 (its twin brother Voyager 2 did it in 2007) and thus entered an area called heliosheath, a kind of “vestibule” of the solar system. In the space of the helio-shield, the solar wind begins to interact with the interstellar medium, and their pressure on each other is balanced.
However, as we move further, the strength of the solar wind begins to weaken even more and ultimately completely yields to the external environment - this conditional external boundary is called the heliopause. Having overcome it in August 2012, Voyager 1 entered interstellar space and - if we take the limits of the most tangible influence of the solar wind as the boundaries - left the solar system.
But in fact, according to the generally accepted interpretation in the scientific community, the probe has not yet completed half the way.
How did scientists know that Voyager 1 crossed the heliopause?
Since Voyager is exploring spaces previously unexplored, figuring out exactly where it is is a daunting task.
Scientists have to rely on the data that the probe transmits to Earth using signals.
“No one has ever been in interstellar space before, so it's like traveling with incomplete guidebooks,” explained Voyager 1 project researcher Ed Stone.
When the information received from the device began to indicate a changed environment around it, scientists first started talking about the fact that Voyager was close to entering interstellar space.
The easiest way to determine whether the device has crossed the cherished limit is to measure the temperature, pressure and density of the plasma surrounding the probe. However, a device capable of making such measurements ceased to work on Voyager back in 1980.
The specialists had to focus on two other instruments: a cosmic ray detector and a plasma wave device.
While the first periodically recorded an increase in the level of cosmic rays of galactic origin (and a drop in the level of solar particles), it was the plasma wave device that managed to convince scientists of the location of the apparatus - thanks to the so-called coronal mass ejections that occur on our star.
During the shock wave following the ejection on the Sun, the device recorded the oscillations of the plasma electrons, with the help of which it was possible to determine its density.
“This wave makes the plasma seem to ring,” Stone explained. "While the plasma wave instrument allowed us to measure the frequency of this ringing, the cosmic ray detector showed where the ringing came from - from emissions on the Sun."
The higher the plasma density, the higher the oscillation frequency. Thanks to the second wave on Voyager's account, in 2013, scientists were able to find out that the probe had been flying through plasma for more than a year, the density of which was 40 times higher than previous measurements. The sounds recorded by Voyager - the sounds of the interplanetary environment - can be heard in the video below.
“The further Voyager moves, the higher the plasma density becomes,” said Ed Stone. “Is it because the interstellar medium gets denser as you move away from the heliosphere, or is it the result of the shock wave itself [from a solar flare - BBC]? We don't know yet."
The third wave, recorded in March 2014, showed insignificant changes in plasma density compared to previous ones, which confirms the location of the probe in interstellar space.
So, Voyager 1 got out of the most "densely populated" part of the solar system and is now 137 astronomical units, or 20.6 billion kilometers from Earth. You can follow him here.
So when will he finally leave the system for good? According to NASA calculations, in about 30 thousand years.
The fact is that the Sun, accumulating in itself the overwhelming part of the mass of the entire system - 99%, spreads its gravitational influence far beyond the Kuiper belt and even the heliosphere.
In about 300 years, Voyager should meet the Oort Cloud - a hypothetical (because no one has ever seen it and scientists have only a theoretical idea of it) spherical region encircling the solar system.
In it "live", being attracted to our star, mainly ice objects, consisting of water, ammonia and methane - they, according to scientists, initially formed much closer to the Sun, but then were thrown to the outskirts of the system by the gravity of the giant planets. It takes thousands of years for them to turn around us. It is believed that some of these objects manage to get back - and then we notice them in the form of comets.
Recent examples include comets C / 2012 S1 (ISON) and C / 2013 A1 (McNaught). The first broke up after passing by the Sun, the second passed near Mars and left the inner region of the system.
The hypothetical boundary of the Oort Cloud is the last boundary of the solar system - the limit of the gravitational power of our star, or Hill's sphere.
Outside the Oort Cloud, there is nothing - only light emanating from the Sun and similar stars.
In a few years, scientists will begin to gradually turn off Voyager 1's instruments. The latter is expected to shut down around 2025, after which the probe will send data to Earth for several more years before continuing on its journey in silence.
It takes about two years for sunlight traveling at the fastest speed we know to reach the limits of the Hill sphere. It takes about four years to reach the closest star to us - Proxima Centauri. Voyager, if his path ran to her, would take more than 73 thousand years.
Voyager mission
- Despite the name, Voyager 2 was launched first on August 20, 1977. Voyager 1 was launched on September 5 of the same year
- The official mission of the probes was to study Jupiter and Saturn
- The devices managed to study and take photographs of Jupiter, Saturn, Uranus and Neptune and their satellites, as well as conduct unique studies of the system of Saturn's rings and the magnetic fields of giant planets
- Voyager 1 then embarked on its "interstellar mission" and became the farthest object from Earth that a person touched. Now his task is to study the heliopause and the environment beyond the influence of the solar wind. Voyager 2 should also cross the heliopause in the coming years
“Both Voyagers have so-called Golden Records on board with audio and video recordings. They reproduced a map of pulsars with a mark of the position of the Sun in the Galaxy - in case the one who discovered it wants to find us. In addition, experts included in the recordings everything that, in their opinion, the representatives of extraterrestrial life need to know about humanity: photographs, greetings in 55 languages, including ancient Greek, Telugu and Cantonese, sounds of terrestrial nature (volcanoes and earthquakes, wind, etc. rain, birds and chimpanzees, human steps, heartbeat and laughter), as well as musical works - from Bach and Stravinsky to Chuck Berry and Blind Willie Johnson and traditional chants.
Polina Romanova