Throughout human history, our fascination with the cosmos has driven us to gaze at the stars and send probes into space. However, only two spacecraft equipped with functioning instruments have successfully left the solar system’s domain.

In 1977, the twin Voyager spacecraft embarked on a monumental journey to the outer planets. Both spacecraft visited Jupiter and Saturn, and Voyager 2 also made stops at Uranus and Neptune. Since then, they have been moving further into space, with some instruments still operational despite aging technology and decreasing power. Voyager 1 encountered the termination shock, marking its long journey to interstellar space, on December 16, 2004, and Voyager 2 followed in 2007. These spacecraft have since been our sole direct sources of information about the space beyond the sun’s influence, known as the heliosphere.

“Our understanding of the heliosphere is still very limited,” explains Merav Opher, a space physicist at Boston University. “It is much more complex and dynamic than we initially thought.”

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Generally, we might think of the sun as just a distant, luminous sphere, partly because our atmosphere shields us from its more dangerous effects. Yet, the sun is actually a tumultuous body of plasma and magnetism, projecting its influence billions of miles away through solar wind. This stream of charged plasma is accompanied by the sun’s magnetic field, affecting the space between planets. The heliosphere expands and contracts with the sun’s 11-year activity cycles.

“These 11-year cycles create dramatic fluctuations throughout the heliosphere,” notes Jamie Rankin, a space physicist at Princeton University and deputy project scientist for the Voyager mission. She points out that astronomers are influenced by this chaotic backdrop, which could impact their data and interpretations. “All our measurements have been influenced by the sun’s layers until the Voyagers crossed the heliopause,” says Rankin.

On their path to interstellar space, the Voyager probes had to navigate through several boundaries. First, they encountered the termination shock, located billions of miles from the sun, where solar wind suddenly slows down. Next was the heliopause, where the outward push of solar wind balances with the inward pressure of the interstellar medium. Between these boundaries lies the heliosheath, where solar winds continue to decelerate and sometimes reverse. Voyager 1, the faster probe, took nearly eight years to traverse these regions due to the immense distances involved.

Beyond the heliopause lies interstellar space, which Voyager 1 entered in 2012 and Voyager 2 in 2018. This region is quieter than the heliosphere but still active. “It reflects the environment from which our solar system originated,” explains Rankin. It contains galactic cosmic rays and interstellar dust from ancient stars, among other components.

The interstellar medium varies throughout the galaxy, with denser and thinner regions across the spiral arms of the Milky Way. The sun and its heliosphere move through this medium, influencing the heliosphere’s shape. However, the exact form of the heliosphere remains unknown. It might resemble a comet, with a compact nose and a long tail, or it might be more like a croissant with two trailing lobes due to the interplay of the sun’s magnetic field and the interstellar medium. “It’s like trying to measure our goldfish bowl from the inside without reaching the edges,” says Sarah Spitzer, a space physicist at the Weizmann Institute of Science in Rehovot, Israel.

The Voyager probes, initially designed for planetary exploration, have inadvertently become our only means of closely studying the outer heliosphere. After their planetary missions ended in 1989, NASA continued their journey, turning off non-essential instruments. The probes have since provided a unique perspective on the distant reaches of our solar system.

However, the Voyagers alone can’t reveal everything about the heliosphere’s three-dimensional structure, akin to taking biopsies from just two points, according to David McComas, a space physicist at Princeton University and principal investigator of the IBEX mission. Launched in 2008, IBEX orbits Earth and analyzes energetic neutral atoms from the edge of the heliosphere to infer distant activities. Although IBEX discovered a significant ribbon of these atoms, the Voyager probes missed it, highlighting the limitations of localized observations.

IBEX has surpassed its expected lifespan, observing through a complete solar cycle. McComas is now preparing for the launch of the Interstellar Mapping and Acceleration Probe (IMAP) next year, describing it as “IBEX on steroids”—offering clearer resolutions and additional measurements.

Following the Voyagers, NASA’s New Horizons mission, which passed Pluto in 2015, is on a trajectory to exit the heliosphere in about a decade. Scientists hope it can provide a third set of observations from beyond the sun’s realm. Additionally, a proposed mission known as the Interstellar Probe could offer more detailed insights into the outer heliosphere, but it has not been prioritized in recent strategic surveys, potentially delaying further U.S. exploration of the interstellar medium. Meanwhile, Chinese scientists might advance in this area with their own interstellar mission.

While we continue to rely on data from the Voyagers, these distant probes only offer glimpses into the boundary of interstellar space, raising as many questions as they answer. Despite the limitations, the data they provide is invaluable. “Even though the mission will end before we completely understand interstellar space, extending the Voyagers’ journey is invaluable,” Opher states.