At the edge of the Sun’s domain

The Solar System does not end with the planets. It extends into a vast, invisible bubble carved out by the Sun. A new mission is now beginning to map this hidden frontier, writes Stuart Clark .

Dr Stuart Clark

For decades, the Voyager spacecraft had been travelling towards a boundary that no one had ever seen. Beyond the eight classical planets of the Solar System, there lay a frontier defined not by celestial objects but by physics itself. Known as the heliopause, it was the outer limit of the Sun’s direct influence, the boundary between the Solar System and interstellar space.

Crossing it with Voyager would mark humanity’s first step into the wider Galaxy. But when the moment finally came, it was not immediately obvious that it had happened at all.

“There was a lot of discussion about ‘have we really crossed the heliopause?’” recalls Linda Spilker of NASA’s Jet Propulsion Laboratory, who was working on the mission at the time. It was 2012 and the data they were getting did not present a clean boundary or a sudden transition.

Instead, the spacecraft was recording a series of ongoing changes that looked emblematic but which had to be carefully analysed.

Part of the difficulty lay in the nature of the boundary itself. Unlike the surface of a planet or a defined orbit, the heliopause is neither fixed nor simple. It expands and contracts based on the activity levels of the Sun. So the frontier Voyager was approaching was not a line, but a shifting, ‘breathing’ interface. No wonder it was confusing to interpret. “Maybe there were a couple of crossings,” Spilker told Astronomy Now , referring to the movement that could have been overtaking the spacecraft and then shrinking back a number of times.

The team was looking for a sudden and dramatic increase in high-energy particles. This is because the heliopause marks the outer boundary of the heliosphere, which acts as a shield, diverting a proportion of these particles, known as cosmic rays, away from the Solar System – much like the Earth’s magnetic field deflects most of the particles emanating from the Sun. So a sudden upturn in the number of cosmic rays hitting Voyager would be a good indicator that they were now in interstellar space.

Part of the difficulty lay in the nature of the boundary itself. Unlike the surface of a planet or a defined orbit, the heliopause is neither fixed nor simple.

Sure enough, as Voyager pushed outward, it encountered what Spilker describes as “a huge jump in the energetic cosmic rays.” Finally convinced, after more than a year’s deliberations, NASA announced on 12 September 2013 that Voyager 1 had crossed the heliopause on 25 August 2012. Six years later, sister spacecraft Voyager 2 also crossed the heliopause. These were remarkable achievements for spacecraft that had been launched in 1977 to study the giant planets of the outer Solar System. Voyager 1 had encountered Jupiter and Saturn before heading into the unknown, while Voyager 2 had completed the Grand Tour, visiting Jupiter, Saturn, Uranus and Neptune.

“It just seemed to make sense to keep Voyager going,” Spilker says. So, with its cameras switched off and its operations simplified, the spacecraft became something new: interstellar probes in all but name.

Together they confirmed what had long been suspected: that the continuous outflow of charged particles from the Sun, known as the solar wind, which carries the Sun’s magnetic field into space, inflates a bubble (known as the heliosphere) in the surrounding interstellar medium, carving out a region in which the Sun’s magnetic field and plasma dominate. Earth, along with every planet, resides entirely within it.

The structure of the heliosphere

The heliosphere is not a simple boundary, but a layered structure formed by the interaction between the solar wind and the interstellar medium.

1: SOLAR WIND

The story begins at the Sun, where charged particles stream outward at speeds of hundreds of kilometres per second. This continuous flow carries the Sun’s magnetic field far into space, filling the Solar System.

2: TERMINATION SHOCK

At a distance of roughly 80–100 astronomical units (AU), the solar wind begins to feel the pressure of the interstellar medium. It slows abruptly from supersonic to subsonic speeds at a boundary known as the termination shock.

3: HELIOSHEATH

Beyond the termination shock lies the heliosheath, a turbulent region where the slowed solar wind is compressed, heated and deflected. This is the heliosphere’s ‘buffer zone’.

4: HELIOPAUSE

The outer boundary of the heliosphere is the heliopause, where the pressure of the solar wind is balanced by that of the interstellar medium. This is the frontier crossed by Voyager 1 in 2012 and Voyager 2 in 2018. It is not a sharp edge, but a dynamic and irregular boundary that can move over time.

5: BEYOND: INTERSTELLAR SPACE

Outside the heliopause lies the local interstellar medium, a sparse mixture of gas, dust and magnetic fields that fills the space between the stars. Here, the Sun’s influence gives way to that of the wider Galaxy.

What Voyager could not do, however, was show us what this vast structure actually looks like. Each spacecraft sampled only a narrow path through the heliosphere, providing exquisite detail along its trajectory but no sense of the global picture. “The Voyagers have measured it, but only in two places, very briefly,” says Tim Horbury of Imperial College London.

As such, reconstructing the full shape and behaviour of the heliosphere from such limited glimpses is rather like trying to map an ocean from a couple of dinghies. NASA’s Interstellar Mapping and Acceleration Probe (IMAP) aims to change that.

To see the invisible

Lofted into orbit in 2025, IMAP has been designed specifically to observe the heliopause as a whole. Rather than taking decades to travel to the boundary, IMAP will study it from afar, using a technique that turns the outer Solar System into a source of particles that can be detected near Earth.

IMAP sits at the first Earth–Sun Lagrange point (L1), around 1.5 million kilometres from Earth or about one percent of the distance from our planet to the Sun. In this gravitational sweet spot, it will keep pace with our planet, orbiting the Sun once every year. It will not image the heliopause in the conventional sense of taking a photograph, but by using energetic neutral atoms (ENAs) as messengers from the boundary.

At the edge of the heliosphere, charged particles from the solar wind collide with neutral atoms from interstellar space and undergo a charge exchange, becoming neutral themselves. Freed from the influence of magnetic fields, a small fraction of these atoms will travel in straight lines back towards the inner Solar System. By detecting these particles and measuring their direction, energy and composition, IMAP will determine where they originated. This will allow it to build up a map of the heliosphere’s outer regions.

As the spacecraft spins, it accumulates data. Over time, these measurements will be combined into a global, three-dimensional picture, allowing scientists to infer the shape and structure of the heliopause and how it changes with variations in solar activity.

As the spacecraft spins, it accumulates data. Over time, these measurements will be combined into a global, three-dimensional picture.

IMAP’s Instruments

NASA’s IMAP observatory will use 10 science instruments to study and map the heliosphere.

IMAP-Lo

maps particles created where the solar wind and interstellar medium meet, known as low-energy Energetic Neutral Atoms (ENAs).

IMAP-Hi

maps medium-range ENAs located at the edge of the heliosphere.

MAG

measures the interplanetary magnetic field originating from the Sun.

SWAPI

measures charged particles from the solar wind and particles from beyond the solar system.

HIT

studies high-energy charged particles originating from the solar wind and deep space.

GLOWS

studies the ultraviolet glow produced by the solar wind to better understand how it evolves over time.

IMAP-Ultra

maps high-energy ENAs from the edge of the heliosphere.

SWE

measures electrons found in the solar wind.

CoDICE

measures the mass and electric charge of charged particles originating from both interstellar space and the solar wind.

IDEX

measures the composition of interstellar and interplanetary dust particles.

Horbury is the Science Lead for IMAP’s magnetometer (MAG), which will constantly be recording the Sun’s magnetic field strength around the spacecraft. “As IMAP orbits the Sun, it will build a full cartographic map of these particles coming from the edges of the Solar System. From that, we can build up maps of the interaction and try and understand how the solar wind interacts with interstellar space,” he says.

A ribbon in the dark

The first attempt to see the heliosphere as a whole came with the Interstellar Boundary Explorer (IBEX), launched in 2008. Like IMAP, IBEX detected energetic neutral atoms returning from the heliosphere’s boundary. Its observations produced the first global maps of this invisible structure and immediately revealed something unexpected.

Instead of a fairly smooth variation in intensity across the sky, they found a narrow, arc-like band of much brighter emission stretching across the sky. It cut across the expected pattern. Quickly dubbed the IBEX ribbon, it immediately told scientists they were missing something important about how the heliosphere interacts with interstellar space.

What makes the ribbon particularly intriguing is that it appears to be closely aligned with the direction of the local interstellar magnetic field. In other words, it seems to mark where the heliosphere is interacting most strongly with the magnetic environment of the whole Galaxy.

The leading explanation is that the ribbon is produced just outside the heliopause, where charged particles from the solar wind are reflected or redistributed by the interstellar magnetic field before undergoing charge exchange and returning as ENAs. In this picture, the ribbon is not a physical structure like a shock wave or boundary, but a region of enhanced particle production tied to magnetic geometry.

However, the exact mechanism is still debated. Some models suggest the ribbon arises from particles that have been trapped at the boundary, while others place its origin farther out in the interstellar medium itself.

What makes the ribbon so important is that the IBEX observations now stretch for more than 17 years, even though the mission was only designed to last for 2 years. In that time, the ribbon can be seen shifting in intensity and structure in response to changes in the solar wind. This means that the behaviour of the ribbon encodes information about both sides of the heliopause: the outward flow of the solar wind and the properties of interstellar space beyond it. That makes it a uniquely powerful diagnostic of the boundary region, but also a complicated one to interpret. This is precisely where IMAP comes in.

Discovery to understanding

With a suite of ten instruments, greater sensitivity and a wider energy range than its predecessor, IMAP will measure energetic neutral atoms in far greater detail. It will track how structures such as the IBEX ribbon evolve, probe the processes that accelerate particles at the heliosphere’s boundary, and determine the composition of material flowing into and out of the Solar System.

Where Voyager provided the first in situ measurements, and IBEX revealed its global structure, IMAP will allow scientists to watch the ribbon changing in real time. It will turn a snapshot into a movie, showing how the Sun’s influence waxes and wanes and how the ribbon interacts with the wider Galaxy.

The implications reach beyond heliophysics. By shaping the flow of cosmic rays into the Solar System, the heliosphere plays a role in the radiation environment experienced by Earth and the other planets. As Spilker notes, the boundary Voyager crossed acts as a shield, reducing the flux of energetic particles that penetrate the Solar System. Understanding how that shield works and how it might vary is part of understanding the broader conditions that make planetary environments habitable.

The influence of cosmic rays on planetary environments is subtle but far-reaching.

A shield for life

The influence of cosmic rays on planetary environments is subtle but far-reaching. When high-energy particles penetrate an atmosphere, they alter its chemistry. On Earth, this is further moderated by our own magnetic field. Even so, cosmic rays contribute to the production of isotopes such as carbon-14, which we use to trace the history of life, civilisation and climate via the process known as ‘carbon dating’.

On worlds with thinner atmospheres, such as Mars, the effects are much more direct. Cosmic rays can break apart molecules, modify surface chemistry and, over long timescales, contribute to the loss or alteration of atmospheres. This has clear implications for habitability because it changes the radiation conditions under which life must arise and persist, perhaps favouring subsurface environments.

Modern human exploration of the Solar System is also affected directly. Beyond the protection of Earth’s magnetic field, astronauts are exposed to a complex radiation environment dominated by galactic cosmic rays (GCRs) and solar energetic particles (SEPs), which are produced during solar events such as flares and carry far more energy than the regular solar wind particles, though typically less than GCRs. These two sources present different kinds of risk.

Galactic cosmic rays are extremely energetic and can penetrate spacecraft hulls and even significant shielding. Although their flux is relatively low, they deliver a steady background of radiation and, because of their high energies, can damage biological tissue at the cellular level, increasing long-term risks such as cancer and degenerative diseases. They are therefore considered one of the principal limiting factors for long-duration missions to Mars.

Solar energetic particles, by contrast, are generally less energetic than GCRs but can occur in intense bursts, dramatically increasing radiation levels over short periods. Unlike GCRs, SEPs can often be forecast with some warning, allowing crews to take shelter.

In short, understanding how the heliosphere modulates the flux of galactic cosmic rays is therefore more than just a subject of astrophysical interest; it is also a practical concern for the future of human space exploration.

At the Sun's frontier

For now, the exploration of the heliosphere is entering a new phase. The Voyagers continue into interstellar space, still returning data from a region they were never designed to study. At the same time, IBEX continues its vigil and IMAP is preparing to deliver the first detailed, global maps of the boundary that the Voyagers crossed more than a decade ago.

The work is no longer concerned with reaching the boundary. Now it is about understanding it. In so doing, we will be investigating how our Solar System truly fits into the wider Galaxy beyond.

Dr Stuart Clark is the editor of Astronomy Now.