Voyager's Journey towards Interstellar Medium

Voyager has launched 4 decades ago and every new day it is creating a new record. The duo Voyager 1 and Voyager 2 are the first spacecraft to leave the Solar System. They visited Jovian planets in the 70s and 80s. In the past decade, both the spacecraft were first to reach the interstellar medium (the void space between the stars). The magnetic field and other phenomena of the interstellar medium have shocked astronomers. The discoveries by the Voyager duo beyond the Solar System gave a new direction to the astronomers. Especially the predictions on the concept of the heliosphere. The spacecraft has offered many clues regarding the predictions of the heliosphere.

However, the Voyager duo was not the first to reach the giant planets beyond Mars. The credit goes to Pioneers 10 and 11. Pioneer 10 was launched in 1972 and Pioneer 11 in 1973. Pioneer 10 was the first spacecraft to reach Jupiter and planets beyond it. After following a shorter route than Voyager 2, the Voyager 1 reached Jupiter in 1979. Along with observing Jupiter's clouds, atmosphere and poles, Voyager 1 also discovered the moons of Jupiter including Io. The detailed observation of Io's volcanism was done by the Voyager duo.

Then in 1980, Voyager 1 reached Saturn. It was the first spacecraft to discover nitrogen atmosphere on Saturn's moon Titan. Titan is the only body after Earth having a nitrogen atmosphere in the Solar System. The credit of detailed observation of Saturn's rings also goes to the Voyager duo. Voyager 2 took a more precise route. It reached Jupiter in 1979, Saturn in 1981, Uranus in 1986 and Neptune in 1989. Voyager 2 observed the amazing geysers on Neptune's moon Triton. After that, the duo started their journey towards the interstellar medium. As described by the astronomers, the interstellar medium starts where the influence of the Sun's solar wind ends. The area within the Sun's influence is called Heliosphere and its edge is called Heliopause. Earlier astronomers thought that the heliosphere ends at Jupiter. But as the Voyager goes on moving deeper into the solar system, the limit of the heliosphere goes on increasing. The Solar winds were experienced by the Voyager duo even when they were passing by Saturn, Uranus, Neptune and even Pluto. So for many years none of the astronomers knew when will the heliosphere ends.

In 1992, the Voyager duo began experiencing strong radio waves between 2 and 3 kiloHertz. Astronomers thought that these frequencies must be the solar flares that erupted from the Sun about a year before in 1991. The possible reason was that the plasma ejected by the Sun eventually hit the Heliopause and these made electrons oscillate and generating the radio waves. 

The local interstellar medium is much denser than the outer heliopause. After knowing the density and speed of solar materials and the time it took to hit the boundary, astronomers predicted that the heliosphere must end between 116 and 177 astronomical units (1 AU is the distance between Sun and the Earth). These estimations were made by Don Gurnett. The predictions of Gurnett were much controversial to the scientific community. Gurnett said that people listened to him politely but nobody believed him. 

However, it was the first time that a spacecraft experienced such high radio frequencies. Many researchers also doubted Gurnett's explanation of radio frequencies experienced by Voyager. The reason for astronomers to not believe him was that the distance between Sun and Neptune is 30 AU and that between Sun and Pluto is about 40 AU. Thus the distance of more than 100 AU is much larger. Also from the predictions of Gurnett, it would take another 20 years for Voyager to reach the heliopause. Also, it would cost a lot for Voyager to reach the heliopause. But in the end, Gurnett's predictions were right.

The Voyager 1 reached the heliopause on 25th August 2012 at a distance of 121.6 AU, about four times the distance between Sun and the Neptune. Hence the predictions made by Gurnett two decades ago were absolutely correct. The Voyager showed many indications that it had reached heliopause. The intensity of solar particles and the solar wind decreased and the intensity of cosmic rays coming to the solar system, which were earlier blocked by the heliopause increased. However, these signs were not enough for many astronomers.

There were two problems, first, the Voyager's plasma instruments stopped working. So it might not have recorded the change in particle density when the spacecraft jumped from heliosphere to interstellar medium. Second, the magnetic field in the interstellar medium might be pointing in a different direction, so the instruments were not able to record the field. Till today there is no proof that the magnetic field inside the heliosphere must be aligned with the field in the interstellar medium. So there is a possibility that fields inside and outside the heliopause might not be aligned.

But the Sun once again helped the astronomers. In 2012, solar flares again erupted from the Sun. The next year the plasma reached the Voyager 1, oscillated the electrons and generated the radio waves. The frequency of these radio waves showed that Voyager 1 has entered a much denser medium than earlier. Thus the Voyager 1 becomes the first spacecraft to reach the interstellar medium. However, the Voyager duo has not completely exited the Sun's influence. More than a trillion small icy objects revolve around the Sun, beyond the orbits of Pluto. Hence we can say that the Voyager duo has not completely left the Solar System. The most distant icy object is believed to be about 1- 2 light-years away.

Voyager 2 reached the interstellar medium in November 2018. The spacecraft's plasma instruments were working and they detected a change in particle density. It also recorded a temperature of about 40,000 K, much higher than the average temperature of the interstellar medium. The reason behind this is that the plasma from Sun gets compressed when they reach the interstellar medium. This leads to a rise in temperature. Like Voyager 1, Voyager 2 also experienced deteriorating solar winds and an increase in cosmic rays beyond the solar system. This time the distance of Voyager 2 was 119 AU, almost the same as that of Voyager 1.

There is no fixed boundary of the heliosphere because over a span of 11 years, Sun's spot changes its position and the intensity of solar wind also changes. Thus the boundary also keeps on changing. Thus after decades of journey, the Voyager duo has now finally reached the interstellar medium. Still, as you are reading this, the Voyager duo will have created a new record. These records will take another 5 decades to break.