The heliopause, the region where the solar wind’s influence 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 point to a remarkable discovery that was one of the major achievements for the Voyager missions.
The Voyager spacecraft have been through so much, it is astonishing that one of them is still operating and that hopes remain for restoring the other. However, a “Wall of Fire” sounds like a tougher test than everything they have survived put together, more a rhetorical proof of dedication than something a space probe should have to navigate.
Nevertheless, that’s the phrase some people have used to describe the heliopause, and a first sight they have a case. After all, the plucky probes measured temperatures of 30,000-50,000 kelvin (54,000-90,000 degrees Fahrenheit) on their passage through the heliopause, which makes Earthly fires cool by comparison.
Of course, there is no literal fire in the sense of fuel undergoing combustion by reacting with oxygen. Like the Sun, the heliopause is composed of hot plasma. Still, passing through the heliopause was nothing like encountering the Sun; not even the solar corona.
The reason the two craft didn’t vaporize, let alone malfunction, is that the density of material outside the bounds of the solar wind is unfathomably low.
To understand how something so far from the Sun can be so hot, and also why that hasn’t affected the first craft to enter it, it’s important to understand a little about the physics of heat.
Temperature is a measure of the speed with which atoms and molecules vibrate. It takes energy to create faster vibrations. Once the speed of vibrations has increased, irrespective of the source of that energy, they are more likely to bump into anything nearby, and to transfer some of that energy to what they hit. Consequently, if you stick your hand into hot gas, the fast-moving molecules will collide with it so that very soon your hand will be very hot as well. (Do we need to say, “Don’t try this at home”?)
The fewer molecules there are, the less energy it takes to get them moving very fast, but also the less chance an intruding solid object will run into them. Without such meetings, energy can’t be transferred, and the new arrival will stay cool.
That’s the situation the Voyager probes are in, as will be the case for future spacecraft leaving the Solar System. The heliopause may be denser than the space on either side, partially justifying the 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 are moving phenomenally fast – and therefore at a very high temperature – they won’t be able to heat up something as substantial as the Voyagers, which weigh 722 kilograms (1,600 pounds) each.
This still leaves the question of how those sparse atoms and molecules got so hot in the first place.
The heliopause had been expected to be hot, but previous estimates were about half as high as those the Voyagers measured, demonstrating again the pair’s great value.
The solar wind within the heliosphere is hot, having had little opportunity to shed 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 the Voyagers measured have been attributed to either compression of the plasma when the solar wind encounters the interstellar medium or magnetic reconnection. Reconnection occurs in electrically conducting plasmas when rearrangement of the structure of the field causes magnetic energy to be converted to fast-moving waves, thermal energy, and particle acceleration.
Magnetic reconnection has been witnessed where the magnetic fields around the Earth and other planets encounter the solar wind. Despite its name it can refer to connected magnetic fields disconnecting, as well as disconnected fields reuniting.