Forty-two years ago, NASA released the Voyager twin spacecraft on a voyage that would make them travel to interstellar space.
Today, Voyager 1 and 2 are both somewhere near the outer edge of the solar system, transmitting valuable information about their environment to Earth scientists.
In 2012, these scientists used data from Voyager 1 tools to confirm that it entered the interstellar space on August 25 that year. Voyager 2 appears to have accomplished the same feat on November 5, 2018. This means that both Voyagers have passed heliopause, the limit to which the Sun's magnetic field extends. Heliopause encompasses all the planets and part of the Kuiper Belt where Pluto resides.
Now, after further analysis, scientists have made some intriguing findings about heliopause.
The Reach of the Sun [1
Just as the Earth's magnetic field protects us from radiation from outer space, so does the magnetic field of the Sun protect us from cosmic radiation from interstellar space. Cosmic radiation particles are hundreds of times more energetic than particles in the solar wind. The sun's magnetic field forms a protective bubble that blocks most of this cosmic radiation.
The outermost edge of this protective bubble is called the heliopause, the boundary where the Sun's magnetic field and the interstellar / galactic magnetic field meet. In this way, heliopause separates the hot solar plasma from the relatively cooler interstellar plasma.
While Voyager 1 and 2 were flying out of space, they were transmitting data that indicated to scientists the exact location of the heliopause. Voyager 1 found heliopause at 122 AU and Voyager 2 found at 119 AU. Thus, heliopause is located about three times the average Sun-Pluto distance.
However, heliopause does not cover the entire solar system. The average orbit of many dwarf planets and small bodies in the Kuiper Belt takes them beyond heliopause. Even beyond the Kuiper Belt lies the Oort Cloud, which is where most of the comets that visit the Sun come from.
Because the Kuiper Belt is part of the Solar System, the Voyagers have not yet left the Solar System. However, they have certainly entered the interstellar space, where particles in space have different energy, distribution, velocity, etc., compared to the interior of the heliopause.
The newly discovered discoveries (published here and here) relate to this region of space
Voyager 1 and 2 entered the interstellar space on different trajectories. Voyager 1 moves north of the plane in which the planets orbit the Sun, while Voyager 2 moves south of it. As such, the instruments of both spacecraft have experienced different parts of heliopause and interstellar space at different times.
Scientists track the magnetic field strength in the probe environment changed before and after the Voyagers underwent heliopause with a magnetometer on board. They found that the galactic magnetic field was much stronger than the solar one.
But the field strength did not increase dramatically. Just before closing the heliopause, Voyager 2 measured a 3x increase in the power of the solar magnetic field. After the intersection of heliopause, the local magnetic field is further enhanced when the galactic magnetic field is revealed.
Scientists have found that the region of magnetic transition to heliopause is 0.7 AU (Sun-Venus distance), confirming earlier predictions (this and that) that such a magnetic barrier exists. This is the result of interactions between the solar and galactic magnetic fields during heliopause.
However, scientists do not find such a magnetic barrier in the data of Voyager 1, whose interstellar transition is smooth. But it experiences a wider heliopause than Voyager 2 and measures a weaker galactic magnetic field.
Therefore, scientists now have reason to believe that heliopause is heterogeneous.
Voyagers also recorded changes in the amount of charged particles as they approached interstellar space. As both vessels crossed the heliopause, there was a sharp decrease in the number of charged particles from the solar wind. At the same time, the amount of cosmic radiation increased sharply, respectively, by 20% and 30%, respectively, according to Voyager 1 and 2.
Again, the change was not abrupt. Voyager 2 indicated that a significant portion of the solar wind particles had "leaked out" into the interstellar space. They were conducted along the lines of the magnetic field. After about half the distance between the Sun and the Earth, the amount of charged particles decreases and then stagnates. Scientists are now puzzled as to why Voyager 1 tools cannot see a gradual decline outward instead of a steep decline.
More strikingly, before Voyager 1 went through heliopause, scientists discovered two "pocket" regions in the Sun's balloon where the galactic magnetic field was moving, carrying with it energetic cosmic radiation. Voyager 2 didn't see anything like that.
So, while Voyager 2 saw a single but multilayered heliopause, Voyager 1 saw a non-uniform structure.
These measurements perfectly complement the measurements of the magnetic field and reinforce the idea that heliopause is constantly changing under the influence of complex interactions with the magnetic field of the Milky Way. This is no different than how the solar wind continuously shapes the Earth's magnetic field.
Scientists used the PWS on board Voyagers to measure electron density in plasma before and after helium intersection. Both units found that the electron density was low before heliopause and 60 times higher after that.
The scientists predicted this increase, but were surprised that the transition to a higher density was not abrupt. They discovered an area between heliopause and the interstellar space that hosts an intermediate electron density. This transitional region extends for quite a large 10 AU – the Sun-Saturn distance.
In fact, scientists saw signs of just such a transitional layer 25 years ago, when both Voyagers remotely examined interstellar plasma beyond heliopause. With direct measurements, scientists are now confident that there is a large transition region between heliopause and interstellar space.
Hot interstellar plasma
Unlike all other observations, only Voyager 2 takes direct plasma measurements. Voyager 1's primary plasma instrument failed in 1980, three years after launch, forcing it to rely on indirect measurements from the PWS instrument.
Voyager 2 saw that at 1.5 AU (Sun-Mars distance) before heliopause, the plasma density doubled and the temperature rose. This contrasts sharply with the indirect measurement of Voyager 1, whereby the plasma density decreases gradually by about 6 AU until the heliopause arrives.
Scientists were also surprised that the interstellar space was much hotter than expected. Voyager 2 registers temperatures in the range of 30,000 ° C to 50,000 ° C. Scientific models suggest a relatively cooler environment of 15,000-30,000 ° C.
Scientists believe the interaction between the two magnetic fields is solar and galactic. – Compresses the surrounding plasma and heats it. This could also explain the higher plasma temperature of Voyager 2, measured just before heliopause.
Armed with data from Voyager, scientists now know that heliopause changes in shape and size at different times and places as the solar and galactic
Voyagers are expected to keep in touch with Earth 2025, after which the small nuclear generators on board will begin to fade. Until then, scientists hope to accurately characterize the properties of the interstellar space.
The New Horizons, the first spacecraft to visit Pluto and the object of the Kuiper Belt (Ultima Thule), will undergo heliopause sometime after 2038. If its nuclear power source does not leak, it may help improving what we know about the gateway to interstellar space. Space agencies have not planned any other interstellar missions in the near future.
In fact, the Voyagers probes were part of a NASA experiment that continued to be productive for four decades after launch. The world has acquired a whole generation of scientists during this time.
Studying the interaction of solar and galactic magnetic fields is useful to understand how stars influence their environment.
Voyager's observations give us a first look at what the outermost part of the protective bubble formed by the magnetic field of our Sun is like an interstellar space beyond. This is the beginning of the human project to map and characterize our almost unexplored interstellar neighborhood and to set the first milestone for future interstellar missions.
Jathan Mehta is a science writer and a former scientist at MoonI Mission TeamIndus. He has a background in astrophysics research and is passionate about space advocacy, scientific communication and open source. His portfolio is in jatan.space and he is on Twitter @ nezvesark .