Why some people call the edge of the solar system ‘the wall of fire’

The heliopause, the region where the influence of the solar wind stops and interstellar space begins, has been called a “Wall of Fire” surrounding the solar system. The name is hyperbolic and technically inaccurate, but it does refer to a remarkable discovery that was one of the major achievements of the Voyager missions.

The Voyager spacecraft have been through so much that it is amazing that one of them is still functioning and that there is still hope for the recovery of the other. A ‘Wall of Fire’, however, sounds like a tougher test than all they’ve survived combined, more of a rhetorical testament to dedication than something a space probe would have to navigate.

Nevertheless, that is the expression some people have used to describe the heliopause, and at first glance they have an inkling of it. After all, the brave probes measured temperatures of 30,000-50,000 Kelvin (54,000-90,000 degrees Fahrenheit) during their passage through the heliopause, making terrestrial fires cool by comparison.

Of course, there is no literal fire in the sense of fuel burning by reacting with oxygen. Like the sun, the heliopause consists of hot plasma. Yet crossing the heliopause was nothing like meeting the sun; not even the solar corona.

The reason the two craft didn’t vaporize, let alone malfunction, is that the density of the material outside the boundaries of the solar wind is unfathomably low.

To understand how something so far from the sun could be so hot, and also why that didn’t affect the first vehicle that went into it, it’s important to understand something about the physics of heat.

Temperature is a measure of the speed at which atoms and molecules vibrate. Energy is needed to create faster vibrations. Once the speed of the vibrations has increased, regardless of the source of that energy, they are more likely to collide with something nearby and transfer some of that energy to whatever they are contacting. So if you put your hand in hot gas, the fast-moving molecules will collide with it, so very quickly your hand will be very hot too. (Should we say, “Don’t try this at home”?)

The fewer molecules there are, the less energy it takes to make them move very quickly, but also the less likely it is that an intruding solid object will bump into them. Without such meetings, energy cannot be transferred and the newcomer will remain cool.

That’s the situation the Voyager probes find themselves in, as will be the case for future spacecraft leaving the solar system. The heliopause may be denser than the space on either side, which partially justifies its description as a “wall,” but it’s still more of a vacuum than the innards of the thing you use to clean your house. Even if the few molecules present move phenomenally fast – and therefore at a very high temperature – they will not be able to heat up anything as substantial as the Voyagers, each weighing 722 kilograms.

This still leaves open the question of how those scarce atoms and molecules got so hot in the first place.

The heliopause was expected to be hot, but previous estimates were about half that of Voyagers, again demonstrating the pair’s high value.

The solar wind in the heliosphere is hot and has had little opportunity to dissipate its energy, but the interstellar medium outside the heliopause is cold, so we might expect the boundary to be somewhere in between. Instead, the heliopause is much hotter than either.

The temperatures measured by the Voyagers are attributed to compression of the plasma as the solar wind encounters the interstellar medium, or to magnetic reconnection. Reconnection occurs in electrically conducting plasmas when rearrangement of the structure of the field causes magnetic energy to be converted into fast-moving waves, thermal energy, and particle acceleration.

Magnetic reconnection has been observed where the magnetic fields around Earth and other planets come into contact with the solar wind. Despite its name, it can refer to the disconnection of connected magnetic fields, but also to the reunification of disconnected fields.

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