The heliosphere is a bubble through which the Sun’s magnetic field and wind is propagated. The solar plasma, which is a mixture of charged particles and atoms, does not mix with interstellar medium, because of their different nature when it comes to their densities, temperatures and origin. The heliosphere extends from the Sun over three times the distance of the Sun to Pluto , and shields the whole system from interstellar radiation.
The Voyager mission was initially designed to last 5 years and study Jupiter, Saturn and their moons and the spacecrafts were launched in 1977. After reaching their goal, they were reprogrammed to reach distances beyond the solar system.
Voyager 1 crossed the heliosphere border in August 2012 and Voyager 2 in November 2018. Interestingly, Voyager 1 was launched later than its twin but due to gravity assist mechanisms used, it reached its goal first and is till date the fastest object in space made by humans.
Voyager 1 provided valuable information on the boundary of the heliosphere. Some expectations were confirmed, with cosmic radiation levels increasing rapidly, while the particles characteristic to the sun were no longer detectable. However, some observations were quite surprising, such as the discovery of a magnetic field between the heliosphere and interstellar space not changing its direction as markedly as expected. The spacecraft’s plasma detector was destroyed early in the mission and the data could be incomplete.
Voyager 2's working instrument obtained more detailed data regarding the densities, temperatures and magnetic fields, leading to better understanding of the nature of the heliosphere’s borders and the interstellar space. It indicated that the heliosphere is not a static, rigid bubble. It seems to move and “breathe” depending on the sun’s activity.
The spacecraft’s data also confirmed that the magnetic fields on the boundary are aligned and made it seem more like a phenomenon rather than a surprising coincidence. The authors of the published papers state that the reason for that alignment is not yet understood. The Voyager missions provided incredibly valuable data after travelling over 10 billion miles from Earth but there is still a lot more needed to understand the nature of the heliosphere.
Unfortunately, the power supply of Voyager mission is only predicted to last for less than 10 years and it is unclear whether any further similar missions are planned.