Interstellar space is the vast expanse between star systems, beginning where the sun’s influence ends and stretching across trillions of kilometers to the next star. It’s not perfectly empty. This space contains a thin soup of gas, dust, and high-energy particles collectively known as the interstellar medium, all threaded through by a weak galactic magnetic field. The boundary where our solar system ends and interstellar space begins lies roughly 120 to 180 times the distance from Earth to the sun.
Where the Solar System Ends
The sun constantly blows a stream of charged particles outward in all directions, called the solar wind. This wind inflates a giant bubble around the sun known as the heliosphere, and everything inside that bubble is technically still part of the sun’s domain. The outer edge of the bubble, where the solar wind can no longer push back against the pressure of interstellar gas, is called the heliopause. Cross that boundary and you’re in interstellar space.
NASA estimated the heliopause sits between 117 and 177 AU from the sun (one AU is the Earth-to-sun distance, about 150 million kilometers). That estimate came from tracking how long solar outbursts took to reach the boundary: roughly 400 days. Voyager 1 crossed the heliopause in 2012 at about 121 AU; Voyager 2 followed in 2018 at around 119 AU. The crossing wasn’t marked by any dramatic visual change. Instead, instruments detected a sharp drop in solar wind particles and a surge in galactic cosmic rays, the telltale signature of leaving the sun’s protective bubble.
What Fills Interstellar Space
Interstellar space looks empty but isn’t. About 99% of the material between stars is gas, primarily hydrogen and helium. The remaining 1% consists of tiny solid particles, silicate and carbon-based dust grains, each less than one ten-thousandth of a millimeter across. A cubic kilometer of interstellar space holds only a few hundred to a few thousand of these grains.
The gas is extraordinarily thin. On average, our galaxy contains roughly one hydrogen atom per cubic centimeter. For comparison, the air in the room you’re sitting in holds about 10 billion billion atoms in that same volume. Even the densest interstellar clouds, which can reach 10,000 molecules per cubic centimeter, are a better vacuum than anything we can create in a laboratory on Earth. The lowest-density regions drop to 0.1 atoms per cubic centimeter or less.
Despite being sparse, this material matters. Interstellar gas and dust are the raw ingredients for new stars and planets. Gravity slowly pulls the denser clouds together over millions of years until they collapse and ignite new stellar systems.
Cosmic Rays and Radiation
Inside the heliosphere, the sun’s magnetic field deflects most galactic cosmic rays, which are high-energy particles (mainly protons) accelerated by distant supernova explosions and other violent events. Once you cross into interstellar space, that shielding disappears. The intensity of hydrogen cosmic rays in interstellar space is about 15 times higher than what’s measured near Earth during the sun’s quietest periods.
This radiation environment is one of the biggest practical challenges for any future interstellar mission. Without the heliosphere’s natural shield, spacecraft and any hypothetical crew would face a constant bombardment of these energetic particles.
The Galactic Magnetic Field
A weak magnetic field permeates interstellar space, with a strength of a few microgauss. That’s roughly a million times weaker than a refrigerator magnet, but across interstellar distances it plays a significant role. It shapes how cosmic rays travel, influences the structure of gas clouds, and affects how the heliosphere itself is compressed and deformed.
NASA’s IBEX mission (Interstellar Boundary Explorer) helped map this field from Earth orbit by detecting a narrow ribbon of high-energy neutral atoms streaming back into the solar system. These particles originate as solar wind protons that escape the heliosphere, interact with the interstellar magnetic field over a period of three to six years, and then fly back toward the sun. By analyzing where and how these particles return, scientists can infer the direction and strength of the magnetic field just beyond the sun’s reach.
The Local Interstellar Cloud
The solar system is currently drifting through a specific patch of interstellar space called the Local Interstellar Cloud. This cloud is partially ionized, meaning some of its atoms have been stripped of electrons, likely by ultraviolet radiation from nearby hot stars. It has an electron density of about 0.07 particles per cubic centimeter, which is thin even by interstellar standards.
The cloud appears to be moving away from the Scorpius-Centaurus stellar association, a nearby region of young, massive stars whose powerful winds and supernova explosions likely shaped it. The solar system won’t remain inside this cloud forever. At some point, tens of thousands of years from now, we’ll drift into a different region of interstellar space with potentially different density and temperature characteristics.
The Scale of Interstellar Distance
Understanding interstellar space means reckoning with distances that are difficult to grasp. The nearest star to the sun, Proxima Centauri, is 4.25 light-years away. In more tangible units, that’s about 40 trillion kilometers, or roughly 268,770 AU.
Voyager 1, the fastest and most distant human-made object, is currently about 172 AU from Earth, traveling at 17.3 kilometers per second. At that speed, reaching Proxima Centauri would take over 73,000 years. Voyager 1 has been flying since 1977 and has barely scratched the inner edge of interstellar space. The overwhelming majority of the journey between stars lies ahead, filled with that same sparse, cold, magnetically threaded medium.
What Voyager Is Measuring Now
Both Voyager spacecraft are still operating in interstellar space, though they’re gradually shutting down instruments to conserve their dwindling power supplies. Voyager 1 sits about 172 AU from Earth, with three instruments still active: a magnetometer, a plasma wave detector, and a low-energy charged particle detector. Its cosmic ray instrument was turned off in early 2025 to save power. Voyager 2, at about 143 AU, has a similar set of instruments still running, including its cosmic ray detector and magnetometer.
These instruments continue to send back data about the magnetic field strength, plasma density, and particle environment of interstellar space. Each measurement refines our understanding of conditions between the stars. No other human-made objects have reached this region, and none currently under construction will get there soon, making the Voyagers’ remaining operational years uniquely valuable.