500 degrees?
Th 210 decays quickly. In a single mass of a half gram, the heat cannot dissipate as fast as it is generated until it reaches that temperature.
Interstingly, some iron-type meteorites have enough uranium and thorium in them so that they remain a few degrees above room temperature.
The effect is used in Radioisotopic thermoelectric generators for space missions, like the 60 Watt Voyager "Batteries".(Still running fine after all these years.)