For six years one of Voyager 1’s instruments had counted the same steady drizzle of low-energy particles streaming off the Sun. On 25 August 2012, over a matter of days, that drizzle fell away and did not return. The spacecraft was about 122 astronomical units out, roughly 18 billion kilometres, and it had just logged the sharpest environmental change of its flight.

That much is not in dispute.

What the moment meant took another year to settle, and the surprise for the mission’s scientists was not the sharpness. It was what stayed put.

What the instruments recorded

Three papers published together in the journal Science in 2013 documented the crossing, from Stamatios Krimigis, Leonard Burlaga, and Edward Stone with their respective co-authors. Energetic particles gave the clearest signal. Krimigis and colleagues reported that low-energy ions produced inside the heliosphere dropped to near nothing, while galactic cosmic rays from beyond it climbed sharply. That changeover ran across a distinct edge Voyager 1 seemed to cross more than once over about thirty days, as the boundary flexed back and forth past the spacecraft.

By the particle count, Voyager 1 was now sampling interstellar space, the first human-made object to do so, as NASA sets out in its account of the interstellar mission.

The signature that did not arrive

Here the neat version breaks down. Textbook physics of the heliopause, the outer skin of the Sun’s bubble, held that any spacecraft crossing it should feel the magnetic field swing to a new heading. Inside, that field is wound into a spiral by the turning Sun. Outside, it was meant to fall into line with the separate field of interstellar space, pointing elsewhere. A clean rotation was supposed to be the unmistakable proof of a crossing.

No rotation arrived. Burlaga and colleagues found the field strengthening, as predicted, but the heading barely moved, by no more than two degrees, the figure the team reported through NASA and JPL in mid-2013. The one signature everyone had trusted was the one that misbehaved, which is why the instruments seemed for a while to be telling different stories, and why the news came with caution rather than fanfare.

A second surprise is easy to miss. Voyager 1 had passed the termination shock, the inner boundary of this region, at 94 astronomical units in 2004. Several models of the solar wind meeting the interstellar medium put the layer between the two boundaries at somewhere between 50 and 70 astronomical units thick.

Voyager crossed it in roughly 28.

In this direction, at least, the Sun’s bubble was thinner than much of the modelling had assumed.

Why “sharp boundary” needs care

Resist the image of a wall. The particle drop was genuinely abrupt, but the approach to it looked nothing like a clean surface. For months beforehand, Voyager 1 threaded a transition zone the team nicknamed the magnetic highway, where particles leaked inward from interstellar space and outward from the heliosphere along connected field lines. What the craft reached was layered and porous, not a single hard face.

One instrument recorded a sharp change while the magnetic geometry refused to behave as the standard model demanded. The solar system did not end at a line. It frayed into a structure nobody had cleanly mapped.

How the crossing was finally confirmed

Confirmation came from an unrelated event. A coronal mass ejection that erupted from the Sun in March 2012 reached Voyager 1 thirteen months later, in April 2013, and set the surrounding plasma ringing. From those oscillations the plasma wave instrument, run by Donald Gurnett and colleagues, could work out the electron density around the spacecraft. It read more than forty times denser than the plasma inside the bubble, a value that fitted the interstellar medium and nothing closer to home. Working through the earlier data, the team fixed the crossing at 25 August 2012, and NASA confirmed it on 12 September 2013.

What Voyager 2 later added

A cleaner check arrived in November 2018. Voyager 2 crossed the heliopause in a different part of the sky, at about 119 astronomical units, and it carried a working plasma instrument where its twin’s had failed in 1980. That let it gauge the boundary density directly rather than by inference. Voyager 2 also met a sharp plasma edge, and it too found the inner region thinner than the older models allowed. Two craft, crossing years apart on different headings, returned the same awkward result. That is what turned a quirk from one probe into something the field has had to sit with.

What remains unsettled

More than a decade on, the thin inner region still has no agreed explanation, and nobody has settled why the magnetic field keeps roughly its old heading across the boundary. Mission scientists have said publicly they are still waiting to see whether that field ever rotates the way theory once demanded. Both craft are low on power now, their instruments switched off one by one to stretch the years that remain. Whether either lasts long enough to catch that rotation, if it ever comes, is the open question.