Hydrazine may also be mixed with monomethyl hydrazine (MMH). It is fuelled by very toxic unsymmetrical dimethyl hydrazine and nitrogen tetroxide. Draco are hypergolic liquid-propellant rocket engines that utilize monomethyl hydrazine fuel and nitrogen tetroxide oxidizer. Guest molecules include dimethyl sulfoxide (DMSO), hydrazine, and urea. Propulsion for midcourse maneuvers was supplied by a monopropellant (anhydrous hydrazine) 225 N retro-rocket. This was reportedly changed to a hypergolic nitrogen-tetroxide/hydrazine propulsion system. The spacecraft uses a fuel tank filled with of hydrazine monopropellant. It was originally prepared by the reaction of aqueous hydrazine with nitrous acid. UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. At launch, the spacecraft carried of liquid hydrazine monopropellant in a diameter spherical tank. There is also a spherical tank that contains the hydrazine monopropellant fuel. Some other strategies like stable amide, thiazolidine, oxime and hydrazine linkage have also been developed. Each spacecraft contains 12 thrusters connected to four hydrazine fuel tanks. Aniline was later replaced by hydrazine. Three fuel tanks mounted inside the drum held of hydrazine pressurized by helium.