By David Warner 
Our solar system feels huge to us. It is home to our planet, Earth, and all the other planets we know, like Mars, Jupiter, and Saturn. All these planets, along with many asteroids and comets, circle our star, the Sun. For all of human history, this has been our neighborhood. We have sent robotic probes to the farthest planets, giving us amazing pictures of icy moons and giant gas worlds.
But our solar system is not everything. It is just one small spot in a much, much bigger place called the Milky Way galaxy. The solar system has an end, a boundary where our Sun’s influence fades away and the vast, dark space between the stars begins. This region is known as interstellar space.
Humans have not yet traveled there, but we have sent two messengers, the Voyager 1 and Voyager 2 spacecraft, which have crossed this final frontier. They are sending back clues about what it is like on the outside. So, what really happens when a spacecraft, or even a person, finally leaves our home system?
When most people think of the solar system, they think of the eight planets. But the planets are just the beginning. After we pass the orbit of the last official planet, Neptune, we enter a new, dark, and cold region of space. This area is called the Kuiper Belt. It is a huge, flat ring of icy objects, almost like a giant asteroid belt, but far larger and made mostly of ice instead of rock. This region is full of what we call “icy bodies” or “planetesimals,” which are leftover building blocks from when the solar system first formed billions of years ago.
The Kuiper Belt is incredibly wide. It starts at around 30 Astronomical Units (AU) from the Sun and stretches out to about 50 AU. An AU, or Astronomical Unit, is the distance from the Earth to the Sun. So, the Kuiper Belt is 30 to 50 times farther from the Sun than we are. This area is home to trillions of objects, including several “dwarf planets.” The most famous resident of the Kuiper Belt is Pluto. For a long time, Pluto was called our ninth planet, but in 2006, scientists realized it was just the first and largest of many similar icy worlds found in this distant belt. Traveling through this area would be a long, dark journey, passing countless frozen worlds that have been in a deep freeze since the dawn of our solar system.
Even as we travel past the Kuiper Belt, we are still very much inside a “bubble” created by our Sun. This bubble is called the heliosphere. It is not a physical object, but a giant, invisible shield that surrounds our entire solar system, including all the planets and the Kuiper Belt. This shield is formed by something called the solar wind. The Sun is constantly blowing a stream of hot, charged particles in all directions at more than a million miles per hour. This solar wind travels out, pushing against the “stuff” that fills the space between the stars.
This “stuff” between the stars is called the interstellar medium. It is a very thin mix of gas, dust, and cosmic rays that exists in the galaxy. The heliosphere is the area where our Sun’s solar wind is stronger than the push from the interstellar medium. You can think of it like a boat moving through the water. The boat (our solar system) creates a wake, or a bubble, around itself as it moves through the water (the interstellar medium). This protective bubble is very important for life on Earth. It shields us from many of the most dangerous, high-energy galactic cosmic rays that fly through the galaxy. Inside this bubble, we are safe. Leaving the solar system means leaving this protective shield behind.
This is a tricky question because there is no single “finish line” for the solar system. Scientists define the edge in three different ways. The first edge is the Kuiper Belt, which is the end of the region where planets and other large objects orbit in a flat disk. If you are just talking about the planets, the solar system ends after Neptune and the Kuiper Belt.
The second, and more important, boundary is called the heliopause. This is the true edge of the Sun’s bubble. It is the exact point where the Sun’s solar wind becomes too weak to push back against the interstellar medium. The solar wind stops, and you are officially in interstellar space. This boundary is incredibly far away, more than 120 times farther from the Sun than Earth is. This is the border that our Voyager spacecraft have crossed.
The third and final edge is defined by the Sun’s gravity. The Sun has a very strong gravitational pull that can hold onto objects even in deep space. The farthest region where the Sun’s gravity is still in charge is a giant, spherical shell of icy objects called the Oort Cloud. This cloud surrounds our solar system in every direction, like a massive, thick bubble of comets. This is the true gravitational boundary of the solar system. Anything beyond this point is no longer tied to our Sun and is truly lost to interstellar space.
We know what is past the heliopause thanks to two amazing robots, Voyager 1 and Voyager 2. These spacecraft were launched in 1977, first to visit Jupiter, Saturn, Uranus, and Neptune. After they finished their main mission, they just kept going. In 2012, Voyager 1 became the first human-made object to ever cross the heliopause and enter interstellar space. Voyager 2 followed it, crossing in a different location in 2018. As of 2025, they are still traveling, billions of miles away from home.
When they crossed the border, they sent back amazing data. It was like popping a bubble. Suddenly, the spacecraft detected a huge drop in the particles from our Sun’s solar wind. At the same time, they measured a big jump in the level of galactic cosmic rays. This was the proof. They were “outside.” They found that the magnetic field in interstellar space is different and stronger than ours, and it does not come from our Sun. These probes are our first tiny steps into the galactic ocean, and they are teaching us what the environment is really like out there. They are so far away that a radio signal, traveling at the speed of light, now takes almost a full day to get from Voyager 1 back to Earth.
When we leave the heliosphere, we enter interstellar space. It is often imagined as a perfect, empty vacuum, but that is not true. It is filled with the interstellar medium. While this medium is much, much thinner than any vacuum we can create on Earth, it is not empty. It is mostly made of hydrogen and helium gas, with tiny bits of dust and other elements that were created inside ancient, exploded stars. We are currently passing through what scientists call the “Local Interstellar Cloud.”
The biggest difference, however, is the radiation. Interstellar space is flooded with galactic cosmic rays. These are high-energy particles, like protons, that have been shot out of distant supernovas (exploding stars) and black holes. They fly through the galaxy at nearly the speed of light. Our heliosphere bubble deflects most of these, but out in interstellar space, there is no protection. Any spacecraft, and especially any human, traveling there would be exposed to this constant, high-energy radiation, which can damage DNA and destroy electronics. It is one of the single biggest dangers of traveling between the stars.
Even after a spacecraft has passed the heliopause and is flying through interstellar space, it is still technically not free of the solar system. It is still moving inside the Sun’s giant gravitational field. The very last part of our solar system is the Oort Cloud. This is not a belt, like the Kuiper Belt. It is a gigantic, spherical shell that surrounds the entire solar system from all sides. Scientists believe it is made of trillions of icy comets, each in a long, slow orbit around the Sun.
The scale of the Oort Cloud is almost impossible to understand. Its inner edge is thought to begin around 2,000 to 5,000 AU from the Sun. Its outer edge might stretch to 100,000 AU or more. At this distance, the Oort Cloud reaches almost halfway to the nearest star, Proxima Centauri. Our Voyager 1 spacecraft, which is traveling at over 38,000 miles per hour, will take about 300 years just to reach the inner edge of the Oort Cloud. It will then take an estimated 30,000 years to fly through it. The Oort Cloud is the true, final frontier of our solar system. Only after leaving it would a spacecraft be completely free from our Sun’s pull.
The distances in interstellar space are the biggest challenge of all. Our solar system is like a tiny island in a vast, empty ocean. The next island, or the next star system, is incredibly far away. The closest star to us is Proxima Centauri, which is about 4.24 light-years away. A light-year is the distance light travels in one year, which is about 5.88 trillion miles. So, our nearest neighbor is over 25 trillion miles away.
How long would it take to get there? Let us look at our fastest spacecraft. The Voyager 1 probe is fast, but it would take it about 75,000 years to reach Proxima Centauri. Even the New Horizons probe, which flew to Pluto at incredible speeds, would still take over 54,000 years to make the journey. With our current technology, sending humans to another star is simply not possible. The journey would take many, many generations, and the ship would have to be a self-sustaining world. This is without even mentioning the other dangers, like radiation and the risk of hitting even a small dust particle at such high speeds, which would destroy the ship.
If we ever build a ship capable of traveling to the stars, the human crew would face dangers unlike anything we have ever known. The first and greatest danger is radiation. Without the Earth’s magnetic field or the Sun’s heliosphere, the crew would be bombarded by galactic cosmic rays. We would need a ship with very thick shielding, perhaps made of water or other special materials, to protect the crew from getting cancer and radiation sickness.
The second danger is the effect of long-term zero gravity. We already know from astronauts on the International Space Station that living in space for just six months causes serious problems. Astronauts lose bone density, their muscles waste away, and their eyesight can be permanently damaged. A journey to another star could take hundreds or thousands of years. This would require the ship to have artificial gravity, perhaps by spinning, to keep the crew healthy.
Finally, there is the mental challenge. Humans are not built to live in a small, metal box for their entire lives, seeing nothing but darkness outside. The isolation, the confinement, and the knowledge that they can never return to Earth would be an incredible psychological burden. Any interstellar mission would have to be a “generation ship,” where the great-great-grandchildren of the original crew would be the ones to finally arrive at their destination.
Leaving the solar system is a journey of layers. It begins with passing the orbits of the planets and crossing the icy Kuiper Belt. The next great step is pushing through the heliopause, the invisible wall where our Sun’s wind stops, and entering the true, radiation-filled space between the stars. Finally, after a journey of thousands of years, a traveler would pass through the Oort Cloud, the final, ghostly shell of comets held by the Sun’s gravity.
Today, only the Voyager 1 and 2 probes have made it to interstellar space, becoming our silent ambassadors to the galaxy. While a human journey to the stars remains in the realm of science fiction for now, it shows us just how big our universe is, and how special and protected our small “bubble” of a solar system truly is. What new, unimaginable things will we discover as we continue to listen to our tiny robotic messengers in the dark?
The Kuiper Belt is a flat, donut-shaped disk of icy bodies that orbits beyond Neptune, on the same plane as the planets. The Oort Cloud is a massive, perfectly spherical shell of comets that surrounds the entire solar system at a much, much greater distance.
Yes. As of 2025, two spacecraft, Voyager 1 and Voyager 2, have crossed the heliopause and entered interstellar space. They are the only human-made objects to have done so and are still sending back data.
The Voyager probes are too far from the Sun to use solar panels. They are powered by Radioisotope Thermoelectric Generators (RTGs), which are basically nuclear batteries. They create electricity from the heat of decaying plutonium, but this power source is slowly running out.
When Voyager 1 crossed the heliopause, its instruments measured two key things. First, the particles from our Sun (the solar wind) dropped to almost zero. Second, the levels of high-energy galactic cosmic rays from deep space suddenly and dramatically increased.
The nearest star to our Sun is Proxima Centauri. It is about 4.24 light-years away, which is more than 25 trillion miles. With our current fastest technology, it would take a spacecraft over 50,000 years to get there.
Interstellar space is the space that exists between the stars within a galaxy. It is not a perfect vacuum. It is filled with a very thin mixture of gas (mostly hydrogen), dust, and high-energy particles called cosmic rays.
The main danger is radiation. Outside the Sun’s protective heliosphere, humans would be exposed to a constant, high dose of galactic cosmic rays, which can damage DNA and cause cancer. Other dangers include the health effects of zero gravity and the immense psychological stress of total isolation.
The fastest object made by humans is the Parker Solar Probe, which is designed to study the Sun. It has reached speeds of over 430,000 miles per hour by using the Sun’s gravity, but it is not traveling away from the solar system.
The probes themselves will drift through space for billions of years. However, their power sources are running out. NASA expects to lose contact with both Voyager probes sometime around 2025 to 2030, after which they will go silent forever but continue their journey.
The hydrogen wall is a region at the outer edge of our heliosphere where hydrogen gas from the interstellar medium slows down and piles up as it meets our Sun’s solar wind. This “wall” was first detected by the Voyager probes.