In thermodynamics, heat is basically energy moving in or out of a system, but not in the form of work or matter transfer. Instead, it moves around through radiation, conduction, or convection.
It’s essentially all about motion. Specifically, the motion of atoms. When atoms are zooming around like crazy, we experience heat. When they’re sluggish, we feel cold. You don’t actually feel individual atoms moving, but when a whole bunch of them jiggle against your skin, your own atoms start jiggling faster too. If they go way too fast, they can even break the chemical bonds in your skin, aka, you burn yourself. So, in a way, your fingers are like tiny atomic speedometers, constantly sensing how fast atoms are wiggling.
Each “mode” of atomic movement holds about 1/2 kT of energy, where T is temperature and k is Boltzmann’s constant, the conversion factor between energy and temperature.
If you’ve got a gas made of single atoms, those atoms are free to move in three dimensions, so their total energy is 3/2 kT. Molecules like oxygen (O₂) get more. Being made of two atoms, they can vibrate relative to each other, which is a whole new way to store energy, and being shaped like tiny dumbbells rather than single points, they can also rotate, which is two more modes. Tally it up and an O₂ molecule carries around 6/2 kT.
How do you warm things up in space?
We’ve got three main ways to move heat around:
- Conduction: atoms bump into each other and transfer energy, like when you touch a hot metal pan. This mostly happens in solids or between objects in direct contact.
- Convection: Heat gets carried around by flowing liquids or gases: how a pot of water heats up on a stove or how warm air rises in a room. Technically, this is a mix of conduction and advection (aka heat transfer by bulk fluid motion).
- Radiation: Heat gets turned into electromagnetic waves, aka infrared radiation, which then travel through space (that warm feeling on your face when you’re near a fire is thermal radiation in action).
Space is a vacuum, so convection is out of the picture. That leaves conduction and radiation. You can either stick a hot object directly onto something to warm it up, or blast it with electromagnetic waves. To warm up a spacecraft you either turn it toward the Sun or use onboard heaters that radiate infrared energy. It’s also why the Moon’s surface can be scorching in direct sunlight and bitterly cold in shadow, and why spacesuits need built-in temperature control.