Chasing the Edge of the Solar System

For most of its lifetime, Voyager 1 has been traveling through uncharted territory. Initially launched to study the outer planets, Voyager 1 has soldiered on past Jupiter and Saturn and on to the outer edges of the solar system. It’s currently the farthest human-made object from Earth, but when will it be the first spacecraft to travel between the stars? Well, we won’t know until we answer two more fundamental questions: Where does our solar system end and the rest of the space between the stars begin? And if you were at the “edge” of our solar system, how would you know you had left? Recent scientific discussions on the Voyager spacecraft missions have captivated many people. And as the scientific debate swirled around the internet in near-real time, it became clear that these questions are not easy to answer.

Voyager spacecraft
The identical Voyager 1 and Voyager 2 are currently probing the farthest reaches of the solar system.

As the Principal Investigator for NASA’s Interstellar Boundary Explorer, or IBEX, spacecraft, I lead a team that is also studying this last frontier of our solar system. Data from IBEX complements the Voyager spacecraft—both missions are working together to find the very farthest reaches of the solar system. Unlike the Voyager spacecraft, which are careening out into interstellar space, IBEX orbits the Earth, collecting particles that have traveled in from the solar system’s boundary region and beyond. From those particles, we can determine many things, including what the boundary is like and what, exactly, is happening out there.

More Than Planets

Most everyone knows our solar system is composed of small solid objects orbiting the Sun—planets, comets, and asteroids. But there’s more to it than that. Our Sun continuously emits a “wind” of material outward in all directions, typically at speeds of about a million miles per hour (1.6 million kilometers per hour). The solar wind is composed mostly of charged particles, such as electrons and protons. It also carries the Sun’s magnetic field. As the solar wind streams away from the Sun, it races out past all the planets, past Pluto, and toward the space between the stars more than 10 billion miles away. We tend to think of that space as empty, but it’s not. Rather, it contains cold hydrogen gas, dust, ionized gas, and traces of other material. Called the interstellar medium, it’s a very thin mix that comes from exploded stars and the stellar wind of other stars. When the magnetic fields of the solar wind hit the magnetic fields of the interstellar medium, they do not intermix. The expanding solar wind pushes against the interstellar medium, clearing out a cavity in interstellar space known as the heliosphere. The boundary of that bubble is where the solar wind’s strength exactly matches the pressure of the interstellar medium. We call it the heliopause, and it’s often considered to be the very outer edge of our solar system.

The Heliopause

A few things about the heliopause: It isn’t an impermeable wall. Instead, it’s more like the edge of a forest clearing—the boundary is well defined, but easily negotiated. It’s also shaped more like a drop of water than a uniform sphere. That’s because our entire heliosphere, which contains our Sun, the planets, and everything else in our solar system, is moving through the interstellar medium at about 50,000 miles per hour (80,000 kilometers per hour). That motion creates a wake in the interstellar medium, much like a boat moving through water. As the solar system travels through the interstellar medium the heliopause is closest at the “front,” or the foremost point in the direction in which our solar system is traveling. At that point, the heliopause is still over 10 billion miles, or 16 billion kilometers, from the Sun.

Heliosphere and heliopause
As solar wind pushes out against the interstellar medium, it creates a bubble known as the heliosphere; the boundary between the two is known as the heliopause. The termination shock is where the solar wind slows as it presses against more of the interstellar medium, which also raises the plasma's temperature. The bow wave is where the interstellar medium material piles up in front of our heliosphere, similar to water in front of a moving boat

At least, that’s our best guess. We don’t know exactly where the boundary is or what it’s like. That’s what the IBEX and Voyager missions are trying to find out. IBEX lets us peer into the boundaries of our solar system to get a better idea of what it’s like and what’s happening there. However, because IBEX orbits the Earth, we cannot use it to mark where the boundary is located. That’s where Voyager 1 and 2 come in. Currently, they are directly sampling the boundary region. Several of the instruments on Voyager 1 and 2 are no longer working, including the cameras used to snap the stunning fly-by photos of Jupiter, Saturn, Uranus, and Neptune, but others that detect charged particles and magnetic fields are still gathering data. Both Voyagers are traveling in roughly the same direction as our solar system through the interstellar medium. We expect Voyager 1, the quicker and farther out of the two, to reach the heliopause first. Currently, it’s just over 11 billion miles, or 18 billion kilometers, from the Sun. This is so distant that radio signals from Voyager 1, which are traveling at the speed of light, take 17 hours to reach Earth.

Three Criteria

Before we can declare that Voyager 1 has crossed the heliopause, we are waiting to observe three main changes:

  1. A decrease in highly energetic charged particles from inside our heliosphere,
  2. An increase in highly energetic charged particles from outside our heliosphere,
  3. And a change in the strength and direction of the magnetic field, matching that outside the heliosphere.

Voyager 1 observed the first two in late 2012, and IBEX has provided what are likely the best observations of the third. By using IBEX to look at particles that have traveled in from outside the heliosphere, we have an idea of the direction of the magnetic field beyond the solar system, and it’s very different from the Sun’s, which is carried out by the solar wind. So far Voyager 1 hasn’t observed this change direction of the magnetic field. That’s why we don’t think that Voyager 1 has crossed the heliopause—yet.

IBEX satellite
The IBEX satellite orbits the Earth, capturing particles that have traveled into the solar system from beyond the heliosphere.

Now, Voyager 1 has clearly passed into a new region of space, one that we have not detected before. Every new bit of data coming from the venerable spacecraft is teaching us more about this uncharted territory. All of this information is new, and we are learning more every day. So, do we know when Voyager 1 will cross the heliopause? We really have no idea. And that’s part of the fun. But learning about the edge of space is more than just an esoteric pursuit. Our heliosphere is a protective cocoon, a crucial layer of shielding against dangerous charged particles, known as galactic cosmic rays, that are harmful to living things. Understanding it will help us understand how the heliosphere has protected our solar system, enabling life to flourish on this planet we call home. And someday, that knowledge will help us prepare for our first voyage beyond the protective cocoon of the solar system, when we step across the threshold and venture into deep space.