Imagine a robotic probe that has left Neptune behind and keeps moving away from the Sun. The known major planets are behind it. Can we say it has left the Solar System? And if its instruments already measure interstellar gas, why might a cloud of Sun-associated bodies still lie ahead?
Passing the planetary orbits, leaving the heliosphere and moving beyond the Solar System’s distant bodies are different events. Beyond the heliopause, a probe enters the interstellar environment, but solar gravity still acts and the proposed Oort Cloud extends much farther. Sunlight becomes weaker, communication takes increasingly long, and another star remains an enormous distance away. NASA: heliosphere, Oort Cloud.
The planets end before the solar wind does
Neptune orbits roughly 30 astronomical units from the Sun. One astronomical unit, or au, is about 150 million kilometres, the characteristic Earth–Sun distance. Beyond Neptune lies the Kuiper Belt with its small icy bodies. The solar wind travels far beyond the major planets’ orbits.
The solar wind is a stream of charged particles flowing from the Sun. Gas containing such charged particles is called plasma. Along with the magnetic field, this outflow forms a large region within the surrounding interstellar environment: the heliosphere. A probe that has only passed Neptune’s orbit therefore remains inside this plasma environment. NASA: how the heliosphere forms.
“Bubble” helps us picture a region around the Sun. But it is a changing environment of flows and magnetic fields; its shape and size depend on interaction with the external gas and on solar activity. A round shell in a drawing is a simplification, not an established surface with the same radius in every direction.
At the heliopause, the environment changes
In the heliosphere’s outer regions, the solar wind slows and changes under the influence of the interstellar environment. The heliopause separates the solar-wind region from the external plasma. When a probe crosses it, instruments register different particle and plasma properties.
Along the Voyagers’ paths, this change occurred at distances of around 120 au from the Sun. That number gives the scale of two particular crossings, not a universal heliopause radius. The interstellar environment also contains tenuous gas, dust and magnetic fields. NASA: Voyager 2’s crossing.
The heliosphere partly reduces the flux of galactic cosmic rays, high-energy particles arriving from outside. Beyond it, the Voyagers measured an increase in that flux. This is a real change in spacecraft conditions, but it does not justify assigning one “protection” factor or one radiation dose to all particles, directions and journeys.
To test the meaning of “boundary,” choose different criteria in the diagram. Could crossing one boundary happen long before crossing another?
Which boundary do we mean by the edge?
Predict whether passing the planets means leaving the solar wind. Then choose a criterion and compare the explanation. The keyboard arrows change the selection.
Voyager identified the transition through measurements
Voyager 1 crossed the heliopause on 25 August 2012. The conclusion was announced in September 2013, after researchers compared changes in particle flux with plasma-oscillation data. Those oscillation frequencies can reveal electron density. The evidence was therefore the behaviour of matter and fields, rather than a photograph of a visible line in space. NASA: the 2013 announcement.
Voyager 2 crossed the heliopause on 5 November 2018. Its plasma instrument directly recorded a sharp decrease in solar-wind flow at the time. Other instruments showed consistent changes in particles and the magnetic field. The probes crossed at different places and times, so their data complement one another without providing a complete map of the entire boundary. NASA: Voyager 2 measurements.
Instrument status changes too. According to NASA’s table checked on 29 October 2026, Voyager 1 operates a magnetometer and a plasma-wave instrument; Voyager 2 operates those two plus a cosmic-ray instrument. The table itself is marked as updated on 17 April 2026. Voyager 2’s plasma instrument, important to the 2018 observations, was switched off in September 2024 to save power. Both probes’ cameras have long been off. A historical list of measurements does not mean every instrument involved is still operating. NASA: instrument status.
Interstellar plasma can surround solar comets
The heliopause describes an environment. A comet’s membership of the Solar System, however, depends on its motion and gravitational bond to the Sun. These criteria need not share a boundary.
The Oort Cloud is a proposed distant reservoir of icy bodies. Its existence and structure are inferred primarily from long-period comet orbits and models, rather than a complete image of a shell. This is a different kind of knowledge from a direct Voyager instrument measurement.
Estimates place its inner region at roughly 2,000–5,000 au and its outer edge at 10,000–100,000 au. The broad intervals express uncertainty, not two precisely marked surfaces. At these distances, bodies can remain bound to the Sun while surrounded by interstellar material. Stars and the Galaxy’s gravitational tides also affect their motion. NASA: Oort Cloud origins, distances and orbits.
Solar gravity weakens with distance, but it does not switch off at the heliopause. Even an approximate outer edge of the cloud is not a place where gravity suddenly disappears. We can therefore correctly say “Voyager entered interstellar space” about the plasma environment without claiming that the probe has passed every distant region of the Solar System.
Sunlight weakens and messages take longer
For a probe, travelling outward means an ever-smaller flux of solar energy. At 100 au, a given area receives approximately 10,000 times less direct sunlight than at 1 au. This changes gradually with distance; the Sun does not go out at the heliopause. JPL: solar energy and distance.
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Why flux falls with distance squared
With constant luminosity, the same solar power passes through imaginary spheres of different radii. A sphere’s area is proportional to its radius squared, so less energy reaches each unit of area. Let D be the numerical distance in au and F_(1) the solar flux at 1 au. Without absorption:
(F)/(F_(1)) = (1)/((D)^(2)).
At D = 10, the flux is 1/100 of the Earth-orbit reference; at D = 100, it is 1/10 000. Doubling distance reduces flux fourfold. This estimates the Sun’s contribution, not illumination from every star or the spacecraft’s temperature. NASA: area and brightness.
The Voyagers obtain electricity from radioisotope thermoelectric generators, which convert heat from radioactive decay into electrical energy. They power systems and instruments; they are not engines continuously pushing the probes forward. Available power declines over the years, so the team switches off some equipment. In August 2026, NASA reported that power-management changes on Voyager 2 had allowed its three science instruments to keep operating longer. NASA: Voyager 2 power savings.
Communication delay also grows. Light crosses 120 au in about 16.6 hours, and 10,000 au in about 58 days. Here we divide distance from the Sun by light speed; actual radio-link distance is measured from Earth and varies as bodies move. A reply to a command also needs the return journey and processing time. This explains the scale of delay, not the current communication time for a particular Voyager.
Interstellar space does not yet mean arrival at a star
The Sun’s nearest stellar neighbour, Proxima Centauri, lies about 4.24 light-years away. A light-year is the distance light travels in a year, not a spacecraft’s journey time. Voyager 1 is heading in a different direction; crossing the heliopause did not make it a ship about to arrive at Proxima. NASA: light distance and Voyager 1’s direction.
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How long would a journey at 20 km/s take?
Take a hypothetical constant speed v = 20 km/s. For a length d ≈ 4.01 × (10)^(13) km, corresponding to about 4.24 light-years, time is t = d/v:
t ≈ (4.01 × (10)^(13))/(20) s.
That is around 2 × (10)^(12) seconds, or 64,000 years. We have compared distance with speed, not calculated a mission. The example omits acceleration, braking, gravity and the star’s motion; its speed is not presented as Voyager’s measured speed. It shows why entering the interstellar environment and reaching another star are very different scales of travel.
Check your understanding: a probe already measures interstellar plasma but has not reached the Oort Cloud. Is that a contradiction?
No. The heliopause separates plasma environments, whereas the Oort Cloud concerns distant bodies and their orbits. A probe can cross the first boundary and remain closer to the Sun than most of that distant reservoir for a very long time.
On an outward journey, everything does not change at once: the major planets fall behind first, then the solar-wind region, while distant solar bodies extend much farther. To understand news about “the edge of the Solar System,” first ask the edge of what was measured. Return to the map of the Solar System to compare these regions. Next in the course, we will explore the physical laws used to describe such journeys.