Helium is named from the Greek helios, the sun and its existence was first obtained by Janssen during the solar eclipse of 1868 when a new line was detected in the solar spectrum. After hydrogen, helium is the most abundant element found throughout the universe. Helium is widely used as an inert gas shield for arc welding, as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, as a cooling medium for nuclear reactors and as a gas for supersonic wind tunnels. A mixture of 80% helium and 20% oxygen is used as an artificial atmosphere for divers and others working under pressure. Helium is extensively used for filling balloons as it is much safer than hydrogen. One of the largest uses for helium is pressurizing liquid fuel rockets, such as those on the space shuttle.