[horsebox]

Since heat is the kinetic energy possessed by atoms how does temperature work in a vacuum? I hear space is cold but isn't what we perceive as cold the loss of energy from our atoms due to collisions with gas molecules that have less energy? Also what would happen if you had your hand inside a sealed vacuum? Would your hand burst open and all the gas in your blood stream get released into the vacuum?

[Borek]

Since heat is the kinetic energy possessed by atoms how does temperature work in a vacuum?

Space is filled with homogeneous electromagnetic radiation. Try a simple thought experiment. Put an atom into space. When it will be too fast, it will have tendency to emit radiation, when it will be too slow, it will have tendency to absorb radiation. Its equilibrium speed reflects temperature of the empty space (around 2.725 K in deep space, google cosmic microwave background). You may also think about it in terms of spectrum of this radiation - and compare it with black body radiation.

[Juan R.]

Since heat is the kinetic energy possessed by atoms [...]

That is not exact. If you work in the approximation of free particles (e.g. in an ideal gas) then heat is at all related to the kinetic energy, but beyond that approximation potential energy also plays a role. If U is internal energy

U = Sum_j p_j E_j

and E_j is energy of level j and p_j its probability, then heat is often defined as

dQ = Sum_j (dp_j) E_j

Notice that E_j is not restricted to be only a kinetic energy.