There is a wonderfully counterintuitive fact about the outer reaches of our solar system, something worth thinking about on a Saturday. The two Voyager spacecraft, launched in 1977 and built with technology that now belongs in museums, eventually reached a region where scientists measured temperatures of tens of thousands of degrees Kelvin. Voyager 2 encountered plasma temperatures estimated at roughly 30,000 to 50,000 K around the boundary between the Sun's domain and interstellar space. Yet the spacecraft did not melt, burn or even become particularly warm. It simply kept going.
At first sight this seems impossible. Tungsten melts at around 3,700 K, iron at around 1,800 K and aluminium at less than 1,000 K. A spacecraft travelling through material at 50,000 K should apparently be transformed into a rapidly expanding cloud of vapour. Yet Voyager, constructed largely from perfectly ordinary materials and carrying electronics designed in the 1970s, passed through unharmed. There is no exotic force field protecting it and no forgotten piece of Star Trek technology aboard.
The solution is that temperature and heat are not the same thing, although everyday experience encourages us to think that they are. When the weather forecast says it will be 40 degrees tomorrow, we know it will feel hot. Put your hand into a 200-degree oven and you know you will soon be burned. Consequently, when someone tells us that Voyager entered a region at tens of thousands of degrees, our intuitive response is to imagine an unimaginably fierce oven.
But an oven contains an enormous number of molecules compared with interstellar space. Those molecules continually strike whatever is inside it, transferring energy to the cooler object. Space is different because it is astonishingly empty. The particles that do exist can be travelling at tremendous speeds, but they are separated by enormous distances compared with molecules in the air around us.
Temperature, at the microscopic level, is closely connected with the kinetic energy of particles. If the particles in a gas are moving faster, their temperature is higher. Consequently, scientists can encounter an extremely thin plasma in which the relatively few particles present are travelling at enormous velocities and legitimately describe that plasma as having a temperature of tens of thousands of Kelvin.
The crucial question for Voyager, however, is not merely how energetic each individual particle is. It is how many energetic particles are actually striking the spacecraft and transferring their energy to it. Out near the heliopause (outer-solar system), the density is extraordinarily low. There simply aren't enough particles colliding with Voyager to deliver the tremendous quantity of energy that the number "50,000 K" leads our terrestrial intuition to expect.
Imagine somebody firing a single red-hot grain of sand at you, low velocity. The grain might have a temperature of 1,000 C degrees, but you would not burst into flames because the tiny grain contains very little total thermal energy; but it may cause an isolated burn. Now imagine instead being dropped into a swimming pool containing water at 100 C degrees. The temperature is vastly lower, but the enormous quantity of hot water can transfer enough energy into your body to kill you.
Another comparison is even simpler. A cup of boiling water is at 100 degrees Celsius, while a large swimming pool on a warm summer day might be at only 30 degrees. The cup has the higher temperature, but the swimming pool contains enormously more thermal energy. Temperature therefore cannot by itself tell us how much energy is available to heat something else.
This distinction becomes spectacular in space because space takes low density to an extreme that human beings never encounter naturally on Earth's surface. Our instincts concerning heat developed in an environment containing atmospheres, liquids and solids, all packed with atoms and molecules. We therefore expect hot surroundings to contain enough matter to heat us rapidly. That assumption works wonderfully well in kitchens, deserts and bushfires, but it becomes badly misleading in interstellar space.
There is another important complication. Physicists use the word "heat" more carefully than we generally do in ordinary conversation. Heat is not simply something stored inside an object like water inside a bottle. Strictly speaking, heat refers to energy being transferred because of a temperature difference. An object possesses internal energy, while heat describes energy passing from one system to another.
This means that asking "How hot is space?" can itself be misleading. Different components of the space environment can be assigned temperatures according to the energies of their particles, but a spacecraft's own temperature depends upon the balance of energy entering and leaving it. In the near-vacuum of space, ordinary conduction and convection become extremely weak, while radiation becomes enormously important.
That is why spacecraft designers worry so much about sunlight, reflective surfaces, insulation and radiators. Close to the Sun, a spacecraft can become dangerously hot because it absorbs intense electromagnetic radiation even though the surrounding space is almost empty. Far from the Sun, a spacecraft can become extremely cold despite travelling through a plasma whose particles technically possess a temperature of thousands or tens of thousands of Kelvin.
This produces one of those delightful situations in physics where two apparently contradictory statements can simultaneously be true. Voyager can be surrounded by plasma at perhaps 50,000 K while remaining far colder than the plasma. There is no violation of thermodynamics because the transfer of energy is extraordinarily slow. The surrounding particles may individually be highly energetic, but there are simply too few of them to deliver enough energy to roast the spacecraft.
A useful analogy would be standing in the middle of an enormous battlefield while one extremely powerful rifle is fired at you from thousands of kilometres away once every century. The individual bullet carries dangerous energy, but the average amount of energy arriving at your position is practically nothing. Increase the number of bullets to billions every second and the situation changes completely, even if the energy of each bullet remains exactly the same.
This is also why some of the hottest places in the universe would not necessarily "feel" hot in the everyday sense if we could somehow visit them while protected from other hazards. The solar corona reaches temperatures of more than a million Kelvin, vastly hotter than the Sun's visible surface, yet it is extraordinarily tenuous. The temperature describes the energies of the particles rather than telling us that the corona behaves like an oven containing million-degree air.
The Voyager story therefore exposes a useful limitation in everyday language. We casually use "temperature," "heat" and "thermal energy" as though they were interchangeable. Usually we get away with it because the environments of ordinary human life have densities within familiar ranges. A hot saucepan contains plenty of matter, a hot bath contains plenty of matter and hot air contains enough molecules to transfer noticeable amounts of energy.
Voyager entered a completely different regime. The particles were moving with energies corresponding to enormous temperatures, but there were astonishingly few particles available. What sounds like a wall of 50,000-degree fire is actually something much stranger: an almost unimaginably empty region containing a sparse population of extremely energetic particles.
There is something philosophically appealing about the example because it shows how easily intuition can mistake a measurement for the thing being measured. We hear "50,000 degrees" and unconsciously import everything we know about furnaces, flames and molten metals. The number is correct, but the mental picture is wrong.
Voyager did not survive because the laws of heat somehow cease operating at the edge of the solar system. It survived because those laws operate perfectly well there. The error lies in our assumption that temperature alone tells us how much heating will occur.
Nearly half a century after their launch, the Voyagers continue to provide lessons their designers could scarcely have anticipated. One of the simplest is also one of the best: something can have an extraordinarily high temperature without containing enough matter to make anything else very hot. Sometimes 50,000 degrees is not nearly as hot as it sounds.