Voyager 1 Crossed the Heliopause, Not the Solar System's Edge

This article was prepared from the source material provided, originally written by Brian Koberlein and published on One Universe at a Time.

Voyager 1 has crossed the heliopause, the boundary where the solar wind gives way to interstellar wind, according to a recent paper that measured a 40-fold increase in electron density around the probe. The finding confirms the spacecraft entered interstellar space, but it does not mean Voyager 1 has left the solar system.

The Sun emits a stream of charged particles, mostly electrons and ionized hydrogen, that flows outward as the solar wind. Together with the Sun's magnetic field, this wind creates the heliosphere, a diffuse bubble of charged particles that blocks interstellar particles and some cosmic rays from reaching us. Where the solar wind slows enough for interstellar wind to push back, the heliopause forms.

Announcements that Voyager 1 had left the solar system have appeared off and on for years because the heliopause is not a hard line; it shifts with solar activity. The probe's age and condition also complicate measurement. Voyager 1 is 36 years old, most of its detectors no longer work, and its transmission power is only about 23 watts, roughly that of a refrigerator light bulb. Two antennae still detect magnetic fields. By observing magnetic field oscillations, scientists can estimate the density of ionized particles around the probe, since oscillation frequency depends on plasma density.

What the 40-Fold Density Increase Shows

The recent paper reported that electron density around Voyager 1 rose by a factor of 40, exactly what was expected as the probe moved from the tenuous outer heliosphere into interstellar space. If the edge of the heliosphere is defined as the edge of the solar system, then Voyager 1 has achieved that milestone.

But that definition is not what most people mean by the solar system's edge. Voyager 1 remains under the Sun's gravitational influence, and many objects orbit the Sun much farther out. Sedna, a 1,000-kilometer-wide trans-Neptunian object, travels more than 900 AU from the Sun. Voyager 1 is about 125 AU away. If the outer edge of the Oort cloud marks the solar system's limit, the probe is just getting started.

As the source puts it: Elvis has left the building, but he hasn't yet left town.