Voyager 1 and Voyager 2 are the farthest human-made objects from Earth, both now traveling through interstellar space after launching in 1977. Despite being nearly five decades old, both spacecraft continue to return scientific data from beyond the Sun’s protective bubble, making measurements in a region of space no instrument had ever sampled before. Their stories are intertwined but distinct: Voyager 1 took a faster route and crossed into interstellar space first, while Voyager 2 visited two extra planets and carries a working plasma instrument that Voyager 1 lost decades ago.
Two Spacecraft, Two Different Paths
Both Voyagers launched in the summer of 1977, taking advantage of a rare alignment of the outer planets that occurs roughly once every 175 years. Voyager 2 actually launched first, on August 20, followed by Voyager 1 on September 5. Despite leaving later, Voyager 1 was placed on a faster, more direct trajectory and reached Jupiter and Saturn before its twin. After its Saturn flyby in 1980, Voyager 1’s path was bent sharply upward, away from the plane of the solar system, ending its planetary tour but setting it on the quickest route outward. Voyager 2, meanwhile, continued on to Uranus in 1986 and Neptune in 1989, becoming the only spacecraft ever to visit either planet. That additional tour came at a cost in speed: Voyager 2 trails its sibling by billions of kilometers and crossed into interstellar space more than six years later.
The two spacecraft are structurally identical, each weighing about 825 kilograms and built around a central bus carrying a suite of instruments for imaging, magnetometry, plasma science, cosmic ray detection, and radio science. Both carry a Golden Record, a gold-plated copper disc containing sounds and images intended to represent life on Earth to any hypothetical finder. The records were something of a cultural afterthought to the engineering mission, but they have become the most famous feature of the spacecraft in public imagination.
What Voyager 2 Found at Uranus and Neptune
Voyager 2’s encounter with Uranus in January 1986 produced some of the mission’s most surprising science. The planet’s magnetic field turned out to be wildly tilted relative to its rotation axis. Initial analysis showed the dipole was offset from the planet’s center by about a third of Uranus’s radius, with the angle between the magnetic axis and the spin axis measured at roughly 60 degrees.1PubMed. Magnetic fields at uranus A more detailed spherical harmonic model placed the tilt at about 59 degrees and revealed an unusually large quadrupole moment, meaning the field was far from the simple bar-magnet shape seen at Earth, Jupiter, and Saturn.2Journal of Geophysical Research: Space Physics. The magnetic field of Uranus Before Voyager 2 arrived, most planetary scientists expected a field more or less aligned with the rotation axis, as at most other planets. The extreme tilt suggested that the field might be generated in a shallow conducting layer rather than deep in the core, a hypothesis that remains under active study.
Voyager 2 also discovered ten previously unknown moons at Uranus and found that the planet’s known rings were darker and more tenuous than Saturn’s. The encounter revealed that Miranda, the smallest of Uranus’s five major moons, had a surface so geologically jumbled that it looked like it had been shattered and reassembled.
At Neptune, three years later, Voyager 2 found a planet far more dynamic than expected. A storm system dubbed the Great Dark Spot, rivaling Jupiter’s Great Red Spot in relative size, churned in the southern hemisphere. Wind speeds exceeded 2,000 kilometers per hour, the fastest recorded in the solar system. Neptune’s largest moon, Triton, turned out to be one of the most remarkable worlds in the outer solar system. Voyager 2 imaged four active geysers on Triton’s south polar region, eruptions of nitrogen gas and dark particles shooting up to eight kilometers above the surface.3Birkbeck Institutional Research Online. Physics of dissociating clathrates in cyrovolcanic vents: application to Enceladus, Triton and Titan Triton’s retrograde orbit and composition strongly suggested it was a captured Kuiper Belt object rather than a moon that formed alongside Neptune, making it a kind of cousin to Pluto.
Crossing Into Interstellar Space
After the planetary encounters ended, both Voyagers began the long, quiet coast toward the edge of the Sun’s influence. The Sun blows a constant stream of charged particles outward, forming a vast bubble called the heliosphere. Where that wind can no longer push back against the pressure of interstellar gas, there is a boundary called the heliopause. Crossing it means leaving the Sun’s domain and entering the interstellar medium.
Voyager 1 reached this boundary first. On April 9, 2013, the spacecraft’s plasma wave instrument began detecting electron oscillations at about 2.6 kilohertz, a frequency that corresponds to an electron density of roughly 0.08 per cubic centimeter. That density was close to the value expected in the interstellar medium and much higher than what exists inside the heliosphere, providing strong evidence that Voyager 1 had crossed the heliopause.4PubMed. In situ observations of interstellar plasma with Voyager 1 Later analysis suggested the actual crossing occurred in August 2012, at roughly 121 astronomical units from the Sun. One astronomical unit is the distance from the Sun to Earth, so Voyager 1 was about 121 times farther out than we are.
Voyager 2 made its own crossing in November 2018, at about 119 astronomical units. The slight difference in distance is real and meaningful: the heliosphere is not a perfect sphere, and its shape depends on factors like the local interstellar magnetic field and solar activity. Pressure analysis using ion measurements from Voyager 2 and remotely sensed data from the Cassini spacecraft confirmed the crossing distance and yielded an estimated interstellar neutral hydrogen density of about 0.12 per cubic centimeter and a magnetic field strength of roughly 0.5 nanotesla upstream of the heliopause in Voyager 2’s direction. Those values were consistent with what had been measured at Voyager 1’s location years earlier.5Geophysical Research Letters. Plasma Pressures in the Heliosheath From Cassini Ena and Voyager 2 Measurements: Validation by the Voyager 2 Heliopause Crossing
Voyager 2 had a critical advantage at the heliopause: its plasma science instrument still worked. Voyager 1’s equivalent had failed in 1980, forcing scientists to rely on indirect plasma wave detections. Voyager 2 could directly measure the temperature, density, and speed of the plasma on both sides of the boundary, giving a much richer picture of what the crossing looks like.
What They Are Finding in the Interstellar Medium
The interstellar medium is not the empty void many people imagine. It is filled with an extremely thin plasma, threaded by magnetic fields, and peppered with cosmic rays from sources across the galaxy. Before the Voyagers, everything scientists knew about this environment came from remote observations and theoretical models. The spacecraft are now providing ground truth.
One puzzle that emerged almost immediately was the behavior of the magnetic field. Simulations had predicted that the interstellar magnetic field should change direction significantly as the spacecraft crossed the heliopause. Instead, the field directions measured by Voyager 1 on both sides of the boundary turned out to be surprisingly similar.6The Astrophysical Journal Letters. WHY ARE THE MAGNETIC FIELD DIRECTIONS MEASURED BY VOYAGER 1 ON BOTH SIDES OF THE HELIOPAUSE SO SIMILAR? Follow-up modeling showed that the solar magnetic field strongly distorts the draping of the interstellar field around the heliopause. As the interstellar flow stagnates in front of the boundary, the field twists into a configuration where it acquires a strong east-west component, reducing the expected rotation across the boundary to only about 10 to 20 degrees.7The Astrophysical Journal Letters. ON THE ROTATION OF THE MAGNETIC FIELD ACROSS THE HELIOPAUSE In other words, the two magnetic environments end up looking more alike than anyone anticipated, not because they share a source, but because the interaction between them smooths out the transition.
Voyager 1 has also been detecting a faint, nearly continuous hum of plasma oscillations at the local electron plasma frequency. These oscillations persist even between the louder bursts triggered by solar events. The evidence strongly suggests they are driven by suprathermal electrons exciting a type of emission similar to what plasma instruments with long antennas commonly pick up in other space environments.8The Astrophysical Journal. Origin of the Weak Plasma Emission Line Detected by Voyager 1 in the Interstellar Medium: Evidence for Suprathermal Electrons This continuous signal gives scientists a way to track the density of the interstellar plasma between the dramatic events, which had been difficult before.
Both spacecraft also serve as outposts for monitoring galactic cosmic rays, high-energy particles that originate outside the solar system. Inside the heliosphere, the Sun’s magnetic field deflects many of these particles, so their intensity increases the farther you go from the Sun. Observations from Voyager 1 and 2, combined with data from New Horizons (which is still inside the heliosphere, near 60 astronomical units), create a network of monitors stretching across the full depth of the Sun’s influence and beyond. This network has revealed that solar disturbances can still modulate cosmic ray intensities even in the interstellar medium, months after the disturbance left the Sun.9The Astrophysical Journal. Influence of Solar Disturbances on Galactic Cosmic Rays in the Solar Wind, Heliosheath, and Local Interstellar Medium: Advanced Composition Explorer, New Horizons, and Voyager Observations
Running Out of Power
Both Voyagers are powered by radioisotope thermoelectric generators, which convert the heat from decaying plutonium-238 into electricity. There are no solar panels; at their distances, sunlight is too faint to be useful. Plutonium-238 has a half-life of about 87.7 years, which means the raw heat output drops slowly and predictably. But the thermoelectric couples that convert heat to electricity degrade on their own, so the actual power loss is somewhat faster than the isotope decay alone would produce. A telemetry analysis spanning the full mission found that a best-fit model combining both effects gives an effective decay constant of about 0.0148 per year.10Measurement. Long-term electronics reliability in deep space: Lessons from 47 years of voyager mission telemetry analysis
In practical terms, each spacecraft now generates somewhere around 250 watts, down from about 470 watts at launch. NASA has been progressively shutting down instruments and heaters to keep the most scientifically valuable sensors running. Voyager 2 still has five working instruments; Voyager 1 has four. Engineers have gotten creative about power management, including turning off heaters for instruments that turned out to operate at colder temperatures than originally specified. The current expectation is that at least some science instruments can be kept alive into the early-to-mid 2030s, though the exact date depends on which systems degrade fastest.
Communication is another challenge that gets harder every year, though not for power reasons. The radio signals from the Voyagers travel at the speed of light but spread out over distance. Voyager 1’s signals currently take more than 22 hours to reach Earth. NASA’s Deep Space Network, a trio of large antenna complexes spread around the globe, can still pick up the faint signals, but the data rate has been reduced to about 160 bits per second for Voyager 1, roughly a thousand times slower than a basic dial-up modem from the 1990s.
Common Misconceptions About the Voyagers
A persistent misunderstanding is that the Voyagers have “left the solar system.” They have left the heliosphere, but the solar system extends much farther than the reach of the solar wind. The Oort Cloud, a vast shell of icy bodies loosely bound to the Sun by gravity, is thought to extend out to perhaps 100,000 astronomical units. Voyager 1, at roughly 165 astronomical units as of mid-2025, has barely begun to cross this region. It will take tens of thousands of years to pass through the Oort Cloud entirely, if the cloud exists in the form current models predict.
Another common confusion is the idea that the two spacecraft are heading in roughly the same direction. They are not. Voyager 1 is heading generally toward the constellation Ophiuchus, climbing away from the ecliptic plane. Voyager 2 is heading below the ecliptic, roughly in the direction of the constellation Sagittarius. They are diverging from each other as well as from the Sun, sampling different corridors of the local interstellar medium. This separation is scientifically valuable because it allows measurements at two independent points, revealing how the interstellar environment varies from place to place.
People also sometimes assume the spacecraft are speeding up as they coast through space. In fact, both Voyagers are very slowly decelerating due to the Sun’s gravity, though at their distances the effect is negligible. Voyager 1 moves at about 17 kilometers per second relative to the Sun, and Voyager 2 at about 15 kilometers per second. Neither will ever return to the inner solar system.
Where the Voyagers Are Headed
With their planetary missions over and their fuel nearly exhausted, both spacecraft are now ballistic objects drifting through the galaxy. Their trajectories have been computed far into the future using data from the Gaia star-mapping satellite. The closest encounters with other stars for all four outbound spacecraft (both Voyagers and both Pioneers) fall between 0.2 and 0.5 parsecs within the next million years.11Research Notes of the American Astronomical Society. Future stellar flybys of the Voyager and Pioneer spacecraft One parsec is about 3.26 light-years, so even these “close” approaches would be separated by roughly a light-year or more. Voyager 2’s closest known approach comes sooner, in about 40,000 years, but at a larger separation of about 0.5 parsecs.12arXiv Central / Research Notes of the American Astronomical Society. Future stellar flybys of the Voyager and Pioneer spacecraft – Section: Conclusions
Because the spacecraft are not leaving the Milky Way, they will continue to orbit the galactic center along with everything else in the stellar neighborhood. Over extremely long timescales, they will inevitably pass closer to some stars than any currently predicted flyby. The estimated timescale for a spacecraft to actually collide with a star is on the order of 10^20 years, a hundred billion billion years, far longer than the current age of the universe. For all practical purposes, the Voyagers will drift as inert artifacts essentially forever, long after their plutonium has decayed, their electronics have failed, and even the Golden Records have been eroded by cosmic dust impacts. Whether anything will ever find them is a question no measurement can answer.
Why No Successor Has Launched
Given the wealth of science the Voyagers have returned from the boundary of the heliosphere and beyond, it might seem obvious that a follow-up interstellar probe should have launched by now. Several concepts have been studied, including missions variously called Interstellar Probe, Innovative Interstellar Explorer, and more recently a dedicated study by the Johns Hopkins Applied Physics Laboratory. The basic challenge is speed: even with modern propulsion, reaching the interstellar medium takes decades. The Voyagers benefited from gravity assists at Jupiter and Saturn, which gave them the speed to reach the heliopause within a human career span. A new probe would need to go faster and farther to answer the questions the Voyagers have raised but cannot resolve with their 1970s-era instruments.
Solar Oberth maneuvers, where a spacecraft dives close to the Sun and fires its engine at perihelion to gain maximum velocity, are one proposed technique. Nuclear electric propulsion is another. But the funding and institutional will to commit to a mission whose primary data return lies 30 to 50 years in the future has been difficult to muster. For now, the Voyagers remain the only functioning instruments in interstellar space, and every month of continued operation is scientifically irreplaceable. When they finally fall silent, there will likely be a gap of many decades before any human-made object again measures the interstellar medium directly.