Heat is transferred by basically 3 methods:
Conduction - where it touches
Convection - via air (or a fluid - think solar wind)
Radiation - (not radioactivity) Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid). It is the transfer of energy by means of photons in electromagnetic waves.
We are receiving convected heat, solar wind of particles, and photons.
Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing 17.59 MeV as kinetic energy of the products while a corresponding amount of mass disappears, in agreement with kinetic E = Δmc2, where Δm is the decrease in the total rest mass of particles.
So we can also be hit with any number of particle isotopes like Hydrogen, Helium, etc. including heavy metals, from the sun. All of this can be in the solar wind.
Without the protection of Earth's atmosphere and magnetosphere, astronauts are exposed to high levels of radiation. A year in low Earth orbit results in a dose of radiation 10 times that of the annual dose on earth. The photons can range anywhere in the spectrum (see pic), and we can also be hit by ionizing radiation (that's the really bad stuff).