Hypergolic propellants are fuel-oxidizer pairs that ignite spontaneously upon mixing, eliminating the need for an ignition system. The most common combination is monomethylhydrazine (MMH) with nitrogen tetroxide (NTO). Their extreme reliability and ability to restart indefinitely make them the standard choice for spacecraft reaction control systems, orbital maneuvering engines, and upper stages requiring multiple burns.
The primary drawback of hypergolics is their extreme toxicity, which drives up ground handling costs and environmental risk. As a result, the industry is actively developing "green" alternatives such as AF-M315E (now called ASCENT) and LMP-103S that offer similar ignition properties with dramatically lower toxicity. Many next-generation spacecraft are also replacing hypergolic systems with storable bipropellants or electric propulsion.