Unlike the typical metallic color of plutonium, a sphere of plutonium-238 emits enough thermal energy during natural decay to glow red. Plutonium-238 has a half-life of around 88 years, which means that after continuous irradiation for 88 years, half of its mass will have transformed into other elements, while the other half will continue to emit radiation. This heat generated during plutonium-238 decay has practical applications in space propulsion or energy production systems. The emitted radiation is not harmful to human health because the alpha particles emitted cannot penetrate human skin. However, ingesting or inhaling plutonium-238 can be deadly.