Credit – NASA/JPL-Caltech. Source.
Voyager Interstellar Mission
The primary mission of the Voyager space probes was completed in 1989, with the close flyby of Neptune by Voyager 2. The mission was extended as Voyager Interstellar Mission (VIM) the objective of which is to extend the exploration of the solar system beyond the neighborhood of the outer planets of the Sun’s sphere of influence and possibly, beyond.
The Voyager probes are now on a mission to characterize the outer solar system environment and search for the heliopause boundary, the outer limits of the Sun’s magnetic field, and the outward flow of the solar wind.
Phases in the Interstellar Mission
This interstellar mission can be considered to be composed of three phases;
- The termination shock
- Heliosheath exploration
- Interstellar exploration
In the region before the termination shock, the plasma particles present are those from the expanding supersonic solar wind. At the region in the termination shock, the supersonic solar wind is held back from further expanding by the interstellar wind.
Here the solar wind slows from supersonic to subsonic speeds and large changes in plasma flow direction and magnetic field direction occur. The heliosheath is the outer layer of the heliosphere. It still contains particles from the solar wind and is dominated by the Sun’s magnetic field.
Voyager 1 and 2: Leaving the Solar System
Voyager 1 crossed the termination shock in December 2004 at a distance of 94 AU from the Sun and Voyager 2 crossed the termination shock in August 2007 at a distance of 84 AU. The region beyond the termination shock is called the heliopause.
Voyager 1 crossed the heliopause on 25 August 2012 and entered interstellar space at a distance of 122 AU from the Sun, making it the first man-made object to go beyond the solar system. Voyager 2 crossed the heliopause on 5 November 2018.
The surprising remarkable discovery by Voyagers
Recently, astronomers reported a significant increase in the density in the space beyond the Solar System which was detected by the Voyager 1 and 2 space probes. This shows that the density gradient is a large-scale feature of the VLISM (Very Local Interstellar Medium) in the general direction of the heliospheric noise. The space inside the heliopause is the heliosphere, and the space outside it is the VLISM.
But the heliosphere isn’t a round sphere. As Voyager 2 moves farther and farther from the Sun, it is finding that the density of the space is increasing. When Voyager 1 entered interstellar space in 2012 it detected a similar density gradient in a different region. New data from Voyager 2 confirmed Voyager 1’s detection but that the increase in density may be a large-scale feature of the very local interstellar medium (VLIM).
Credit – NASA. Source.
The Solar Wind and the Plasma Density
Space is not completely vacuum. The density of matter is extremely low, but it still exists. In the Solar System, the solar wind is found to have an average proton and electron density of 3 to 10 particles/cc but it reduces as we move away from the Sun.
The mean electron density in the interstellar medium in the Milky Way is about 0.037 particles/cc and the plasma density in the outer heliosphere is about 0.002 electrons/cc. As the Voyager probes crossed beyond the heliopause, their Plasma Wave Science instruments detected the electron density of the plasma through plasma oscillations.
Voyager 1 detected a plasma density of 0.055 electrons/cc on 23 October 2013 at 122.6 AU. Voyager 2 detected a plasma density of 0.039 electrons/cc on 30 January 2019 at a distance of 119.7 AU which is very close to the Voyager 1 measurement. After traveling another distance of about 20 AU through space, Voyager 1 reported an increased plasma density to about 0.13 electrons/cc.
In June 2019, Voyager 2 found a density to be about 0.12 electrons/cc at a distance of 124.2 AU. One theory says that the interstellar magnetic field lines become stronger as they drape over the heliopause which could generate an electromagnetic ion cyclotron instability that depletes the plasma from the draping region.
Voyager 2 in fact detected a stronger magnetic field than expected when it crossed the heliopause. Another theory says that the material blown by the interstellar wind slows as it reaches the heliopause, causing a sort of traffic jam.
This could be detected by the outer Solar System space probes such as the New Horizons space probe. In 2018, it detected a faint UV glow resulting from the buildup of neutral hydrogen at the heliopause. Both theories could be true. Again, it is up to our favorite Voyager probes to confirm the theories through its discoveries in the regions so far away from us, that it is outside our Solar System.
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