Why Putting Data Centers in Space Is Not as Simple as It Sounds: Three Ways Energy Moves

Chuck Nice jokes that he is conserving his energy because the word is used so casually in everyday speech. Neil deGrasse Tyson responds that energy has a very specific meaning in physics. When Chuck asks whether it is the E in E=mc², Tyson confirms it is, referring to Einstein.

Tyson says he wants to talk about how energy moves from one place to another. Chuck first takes "move" as a dance reference before the conversation turns to physics.

Tyson argues that carrying a gallon of gasoline from one place to another moves potential energy, the energy stored in the gas molecules, rather than energy in the sense he is describing. He says that is not the kind of movement they are discussing. He then states that there are three, and only three, ways energy can move, and names the first as conduction, which he describes as the slowest.

To illustrate conduction, Tyson describes standing in front of a fireplace with a poker. If the poker's tip stays in the fire long enough, the hand holding the other end will eventually burn, though not immediately. The tip gets hot and its molecules vibrate, and the vibration is a measure of the heat energy present.

That vibration passes to the neighboring molecule, which begins to vibrate in turn, and the motion works its way up the poker. Tyson says the handle eventually starts feeling warm, and that by then a person would usually take action, such as grabbing a pot holder. He notes that this process might take 20 minutes, and calls it slow.

Tyson describes convection as the second way energy moves and says it is very efficient. He adds that convection requires gravity. Chuck initially responds that heat would need something to move it, and says that when people think of convection they usually think of a convection oven.

Tyson says the term "convection oven" is badly named and that nothing convects inside one. Chuck reacts with disbelief. Tyson clarifies that a regular oven is the one that actually convects, and that the so-called non-convection oven is in fact the convection oven. He adds that fans are placed in convection ovens to move the air.

Tyson describes convection in terms of a fluid, which in physics and chemistry is a medium that takes the shape of its container, such as water. A heat source at the bottom heats the medium, which then expands and becomes less dense than the surrounding medium. Because it is less dense, it rises.

As the heated fluid rises, it leaves a partial vacuum behind that draws in air that has not yet been heated. Tyson calls this very efficient, because the heated fluid is removed and more material is brought in to be heated. He stresses that it is not waiting for one molecule to touch another, but bringing hot fluid from one place to another.

In a regular oven the heat source is at the bottom, and Tyson notes that it is only at the top when broiling. The bottom air heats, rises, and is replaced by cooler air. Tyson says a regular oven is therefore a convection oven, and that manufacturers add fans to move the air however the cavity needs.

Tyson explains that when air moves quickly, a fresh hot molecule hits the food each time, so moving air transfers heat faster. Chuck points out that this is just a fan rather than convection, and Tyson responds that moving fluid always transfers heat faster.

Tyson connects moving air to the wind chill factor, which people also call the real feel temperature. He gives the example of a 32-degree day that can feel like about 23. In that case, cold air moves across the body, and because moving air transfers heat faster, heat flows away from the person's body into the air.

Tyson concludes that stationary air at the same temperature would not feel as cold, because moving air is whisking away the body's heat.

Tyson states that if one thing is hotter than another and they come into contact, heat moves from the hotter thing to the colder thing. Chuck jokes about his heating bill, and Tyson quips that he is heating the rest of the earth. Tyson gives the example of shaking hands: if the other person's hand is cold, your hand feels warm to them, and he says there is no exception to this.

Tyson extends the example to a couple getting into bed, where a warm-sounding compliment has only one reply, and concludes that both people cannot be warm at the same time.

Tyson introduces the third way, radiation, noting that the word has negative associations in modern times. Chuck mentions that he lives in an old house and gets his heat from radiators, and Tyson responds that radiators are not actually radiating. Tyson explains that radiation is simply the movement of photons from one place to another, happening at the speed of light.

He applies this to a broiler, which he says is not convecting. The top of the oven feels the radiant photons from the heating element, which is why food cooks on the side facing the element.

Tyson says radiant heat travels only in straight lines and consists of photons. He describes sitting in front of a fireplace in winter: when someone walks in front, it instantly feels cooler, even though the air temperature has not changed. Similarly, standing under a tree in summer feels cooler because you have stepped out of the direct sunlight.

Tyson concludes that what people experience as the temperature of their surroundings depends not only on air temperature but on how much radiant energy is coming toward them.

Tyson says heat can transfer by conduction, convection and radiation, and that a radiator is not really a radiator. He describes a radiator as having sections, like cells, filled with hot water or steam. Each section gets hot and emits infrared that hits the neighboring section, which heats the air between the sections. That air rises and more air comes in from below, which is convection.

Tyson suggests that adding a fan across the radiator would make it far more efficient, turning it into a fake convection oven. He concludes that the radiator is radiating, but convection is what heats the room.

Tyson says many people think space is cold, and he explains that temperature is a statistical average of the vibration of molecules in whatever is being measured. A thermometer in a room reads 72 degrees because of the air molecules around it, and near an oven the molecules vibrate faster, giving a higher reading.

In space there is no air. Tyson says the thermometer reading then has no meaning, and there is no medium for convection, so there is nothing to conduct heat away from a system. A vessel in space therefore needs a way to take in heat and move it somewhere else. Chuck asks whether this makes data centers in space nonsense, and Tyson says he is not saying that.

Tyson explains that in space the only way to get rid of heat is through radiation. Heat is created in one place, and a fluid conducts it to a broad, wide surface, where piping runs across the back of the panel. The panel then radiates the heat out.

Chuck compares this to radiant floor heat. Tyson agrees it is exactly like radiant floor heat, except that it is done deliberately, and not released because something is bad.

Tyson asks whether anyone has seen the space shuttle in orbit with its bay closed, and says the bay is always open because those are radiating surfaces. He explains that everything that makes heat must send it somewhere. A person at 98.6 degrees in a 72-degree room is making heat, which leaves the body through sweat evaporating or other means. On a spacecraft, the heat has to be whisked away and radiated, and Tyson says properly designed science-fiction spaceships have radiating panel sections for this purpose.

Tyson says not to think space is cold merely as a matter of principle, and says that this is not relevant to the question. He then describes the Apollo 13 mission, which went to the moon but did not land because something went wrong, and on the return the crew had to conserve energy.

As the cabin got colder, the crew realized that if they sat still their bodies would heat the air around them, and that air would not go anywhere. Tyson says they became their own insulated radiator, with conduction also at work as the air touched the skin. Because they were in zero gravity, the warm air pocket did not convect away, so they stayed warm.

Tyson says the advantage of physics is that understanding it is in fact understanding the universe itself, not just a set of facts in a manual. He says that if someone tells you physics is useless, they are lying to you, especially when stuck in space and needing to make your own pocket of warmth.

Tyson says energy in the center of the Earth escapes by convection. Because convection requires a fluid, he identifies the fluid as magma when it is below ground, and as lava when it spills out. Chuck asks whether this convection drives continental drift, and Tyson says yes: the continents move, subduct and circulate as the heat leaves the Earth through convection.

Tyson adds that volcanoes are an even more efficient way for the Earth to release heat. The conversation becomes lighthearted when Chuck says he feels bad for the people of Vesuvius, and Tyson corrects him that the people affected were in Pompeii, while the volcano that destroyed the area was Vesuvius.