The study of chemical reactions that occur naturally in space. Molecules are able to form in space at very low temperatures (e.g. 20 K) and at pressures far lower than are achievable in a laboratory on Earth. Many chemical species that are unstable on Earth exist in space, and are detectable by their spectral lines at radio, infrared, optical, and ultraviolet wavelengths. There are two broad types of formation process: chemistry in gas clouds (including photodissociation) and chemistry on the surfaces of dust grains. Many astrochemical reactions involve only gases. In some cases atoms or molecules in the gas may become ionized by the passage of cosmic rays, and the subsequent reactions between ions and molecules are more rapid as a result. In the other main process, dust particles act as catalysts, providing a surface on which atoms, ions, and molecules can stick and then react. The dust also acts as a shield and prevents starlight from breaking up the molecules again. Heating of the dust by newly formed stars may release complex molecules from the grains back into the gas cloud and drive a new series of chemical reactions. See also Interstellar Molecule.