Temperature of relic radiation and thermal equilibrium The temperature or relic radiation is 2.7 K. Does it mean that any flying body sufficiently far from any stars will reach this temperature?
As I understand it, you can warm up water with sufficient amount or radiation with 2.7K spectrum and you can freeze helium even in presence of a source of photons with 100000 K spectrum if the source is dim enough.
So what temperature will reach a body placed in interstellar medium?
 A: 
The temperature or relic radiation is 2.7 K. Does it mean that any flying body sufficiently far from any stars will reach this temperature?

Theoretically, if the body interacts only with the background radiation, yes.

As I understand it, you can warm up water with sufficient amount or radiation with 2.7K spectrum

The water would be warmed by the background radiation only up to the temperature of the radiation, 2.7 K.
A: A body sufficiently far away from any star would interact solely with a bath of 2.725 K photons (the cosmic microwave background, or CMB). This means that the body would indeed warm up (or cool down) to 2.725 K temperature. 
The photon flux for isotropic thermal radiation such as the CMB is fixed by the temperature of this radiation (according to Stefan-Boltzmann's law). This means that the only way to increase the photon flux is to increase its temperature. In other words: there is no way you can radiatively heat up an object beyond the photon temperature in which the object is immersed.
