Helium is a noble gas. Periodic table symbol He. It can be ionized and used as fuel for an ion drive, as well as a coolant for high temperature gas core nuclear reactors and as a cryogenic coolant. Helium does not form bonded molecule and the natural form is He. Helium has two stable isotopes, He3, rare, and He4, common. He3 is regarded as a possible fuel for aneutronic nuclear fusion. He3 and He4 may be rare on Mars.