Nitrogen tetroxide and monomethylhydrazine are "hypergolic" propellants, meaning they ignite spontaneously when they come in contact with each other, a useful property for maneuvering systems.
CNN: Four compounds from shuttle can harm on contact
2.
Another form of the chemical, monomethylhydrazine, fueled the shuttle's maneuvering systems while it was in space, said Randy Azera, a former NASA engineer and consultant to CNN.
CNN: Four compounds from shuttle can harm on contact