How is our solar system different from interstellar space, and what separates the two? Author Jim Bell explains in this clip from THE FARTHEST and in an excerpt from his 2015 book, THE INTERSTELLAR AGE.
Although astronomers and planetary scientists don't yet know exactly how far toward the nearest start the sun's gravitational influence extends (it's probably somewhere near a half to two-thirds of the way), they have been expecting over the past decade or so that far-flung spacecraft like Voyager should soon be able to find the edge of the sun's nongravitational influence on the solar system. The sun produces energy by the conversion of four hydrogen atoms into one helium atom deep in its interior, at super-high pressures and at temperatures of millions of degrees. The conversion releases a tiny bit of energy, in the form of photons and other subatomic particles like protons and electrons, that bounce around inside the sun and eventually make their way out. The sunlight—photons—that warms our faces on a sunny afternoon was created, on average, deep inside the sun, maybe 50,000 years ago or more. The stream of protons and electrons coming off the sun every second creates a flow of charged particles called the solar wind. The solar wind creates a giant spherical "bubble" around the sun in interstellar space, known as the heliosphere. The heliosphere extends far beyond the orbit of Neptune, until it becomes so diffuse and weak that it merges into the background of rarefied hydrogen and helium gas that permeates the space between the stars—the interstellar medium. The sun and every other star reside inside their own such cocoons, blowing bubbles in the interstellar medium from their own solar, or stellar, winds. Like all bubbles, there must be an edge, a boundary between inside and outside of that bubble. Inside the bubble is the solar wind, outside the bubble is the interstellar wind. Finding that edge, then, and going beyond it, provides a way to explore truly interstellar space.