Our solar system is a dynamic environment filled with planets, moons, asteroids, and comets, all orbiting our Sun. This familiar region, known as interplanetary space, is dominated by the Sun’s influence. Beyond this immediate neighborhood lies interstellar space, the vast realm between star systems. Scientists have long sought to understand where our solar system ends and this immense cosmic ocean begins. The transition from the Sun’s domain to the broader galactic environment involves several distinct boundaries, each marking a step further into the interstellar medium.
The Solar System’s Protective Bubble
Our solar system is enveloped within a vast magnetic bubble called the heliosphere. This bubble is generated by the solar wind, a continuous outflow of charged particles and magnetic fields emanating from the Sun. The heliosphere acts as a shield, extending far beyond the orbits of the planets, protecting our solar system from cosmic rays. Without this protective barrier, Earth would be exposed to much higher levels of radiation. The heliosphere’s size and shape are not static, fluctuating with changes in the solar wind and the surrounding interstellar medium, creating a cavity within the galactic environment.
Journey to the Edge: Markers of Interstellar Space
The edge of the heliosphere is not a single, sharp boundary but a series of distinct regions where the solar wind interacts with the interstellar medium. The first major transition is the termination shock, where the supersonic solar wind abruptly slows to subsonic speeds due to the pressure exerted by the interstellar medium. Voyager 1 crossed this boundary in December 2004 at about 94 astronomical units (AU) from the Sun, and Voyager 2 followed in August 2007 at 84 AU.
Beyond the termination shock lies the heliosheath, a broad region where the solar wind is compressed, heated, and made turbulent. The heliosheath acts as a buffer zone, preparing for the final transition into interstellar space. Its thickness is estimated to be between 10 and 100 AU.
The heliopause marks the outermost boundary of the heliosphere. Here, the outward pressure of the solar wind is balanced by the inward pressure of the interstellar medium, and the Sun’s magnetic field and particles no longer dominate the surroundings. Scientists define this as the true beginning of interstellar space.
The True Interstellar Medium
Beyond the heliopause lies the true interstellar medium (ISM). This vast expanse is not empty, though its density is remarkably low compared to what we experience on Earth. The ISM is primarily composed of gas, making up about 99% of its mass, with the remaining 1% consisting of dust and ice particles.
The ISM gas is about 91% hydrogen and 8.9% helium, with trace amounts of heavier elements. Its density in the lowest regions can be as sparse as 0.1 atoms per cubic centimeter, significantly less dense than Earth’s atmosphere. Temperatures in the ISM vary widely, with some regions as cold as 10 Kelvin, while others can be much hotter. This environment is also permeated by cosmic rays, high-energy particles.
Our First Steps Into the Beyond
Humanity has taken its first direct measurements of interstellar space thanks to the Voyager probes. Voyager 1, launched in 1977, became the first human-made object to cross the heliopause on August 25, 2012, at approximately 122 AU from the Sun. Its entry was confirmed by a sudden increase in cosmic rays and a decrease in heliosphere particles.
Although Voyager 1’s plasma instrument had ceased working in 1980, its plasma wave subsystem detected oscillations indicating denser plasma, supporting its new location. Voyager 2, launched shortly after its twin, crossed the heliopause on November 5, 2018, at about 119.7 AU from the Sun. Data from Voyager 2’s functioning plasma science instrument provided direct measurements of the plasma density change, confirming its entry into the colder, denser interstellar medium. These twin spacecraft continue to transmit data, offering unique insights beyond the Sun’s influence.