40 years of Voyaging - Part 1

I've always been interested in astronomy from an early age, and remember the glossy Sunday supplements with the first fantastic pictures of the gas giants in the solar system, transmitted back from the Voyager space craft. And they built these craft solidly: they last.

Humanity’s farthest and longest-lived spacecraft, Voyager 1 and 2, achieved 40 years of operation and exploration this August and September. Despite their vast distance, they continue to communicate with NASA daily, still probing the final frontier.

Voyager 2 launched on Aug. 20, 1977, about two weeks before the Sept. 5 launch of Voyager 1.

To get a scale of the technology, 1977 saw the Commodore PET (Personal Electronic Transactor) – a new line of home/personal computers produced in 1977 by Commodore International.

It also saw the first Apple II Home Computers. 

The Atari 2600 games console is also released. It needs connecting to a TV set.

Voyager 2, which was actually launched before Voyager 1 (I know, this gets a bit confusing) is currently at a distance of 10.713 billion miles from the Sun. It’s traveling at an estimated speed of 34,390 miles per hour. It hasn’t yet reached interstellar space.

Voyager 1, on the other hand, has. That’s due to the fact that Voyager 1 is traveling at a slightly greater speed of roughly 38,026 miles per hour. It is currently located some 12.974 billion miles from the Sun, and has successfully broken free from our star’s “bubble” called the heliosphere. Having passed that, Voyager 1 is now in interstellar space, where its fate will be determined solely by whatever it manages to run into.

The heliosphere is the bubble-like region of space dominated by the Sun, which extends far beyond the orbit of Pluto. Plasma "blown" out from the Sun, known as the solar wind, creates and maintains this bubble against the outside pressure of the interstellar medium, the hydrogen and helium gas that permeates the Milky Way Galaxy. The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly.

The Voyager spacecraft have explored the outer reaches of the heliosphere, passing through the shock and entering the heliosheath, a transitional region which is in turn bounded by the outermost edge of the heliosphere, called the heliopause. 

The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the Sun) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field.

The shock arises because solar wind particles are emitted from the Sun at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s.

As one moves far enough away from the Sun, the pressure of the solar wind drops to where it can no longer maintain supersonic flow against the pressure of the interstellar medium, at which point the solar wind slows to below its speed of sound, causing a shock wave.

So how can you have a speed of sound in space?

And what was space physicist Don Gurnett talking about when he stated at a NASA press conference in Sept. 2013 that he had heard "the sounds of interstellar space?"

Sound travels in waves like light or heat does, but unlike them, sound travels by making molecules vibrate. So, in order for sound to travel, there has to be something with molecules for it to travel through.

But space is not empty. The solar wind is a stream of particles and plasma (gas), and the interstellar medium, is a mixture of electron–proton plasma and hydrogen atoms. So linear acoustic waves can propagating through it.

Shocks occur when matter moves into a medium at a velocity that exceeds the local sound speed - a condition that is easily met in many different astrophysical contexts, just like breaking the sound barrier in air.

Strictly speaking, the plasma wave instrument does not detect sound. Instead it senses waves of electrons in the ionized gas or "plasma" that Voyager travels through. No human ear could hear these plasma waves. Nevertheless, because they occur at audio frequencies, between a few hundred and a few thousand hertz, "we can play the data through a loudspeaker and listen," says Gurnett. "The pitch and frequency tell us about the density of gas surrounding the spacecraft

References
http://bgr.com/2017/08/29/voyager-mission-space-distance/
https://en.wikipedia.org/wiki/Heliosphere
https://www.jpl.nasa.gov/news/news.php?feature=6907
https://science.nasa.gov/science-news/science-at-nasa/2013/01nov_ismsounds