mase. The impact of comet Shoemaker-Levy 9 with Jupiter in 1994 resulted in maser emissions in the 22 GHz region from the water molecule. Despite the apparent rarity of these events, observation of the intense maser emission has been suggested as a detection scheme for extrasolar planets. Ultraviolet light from the Sun breaks down some water molecules to form hydroxides that can mase. In 1997, 1667-MHz maser emission characteristic of hydroxide was observed from comet Hale-Bopp. Planetary atmospheres It is predicted that masers exist in the atmospheres of gas giant planets. Such masers would be highly variable due to planetary rotation (10-hour period for Jovian planets). Cyclotron masers have been detected at the north pole of Jupiter. Planetary systems In 2009, S. V. Pogrebenko et al. reported the detection of water masers in the plumes of water associated with the Saturnian moons Hyperion, Titan, Enceladus, and Atlas. Stellar atmospheres The conditions in the atmospheres of late-type stars support the pumping of different maser species at different distances from the star. Due to instabilities within the nuclear burning sections of the star, the star experiences periods of increased energy release. These pulses produce a shockwave that forces the atmosphere outward. Hydroxyl masers occur at a distance of about 1,000 to 10,000 astronomical units (AU), water masers at a distance of about 100 to 400 AU, and silicon monoxide masers at a distance of about 5 to 10 AU. Both radiative and collisional pumping resulting from the shockwave have been suggested as the pumping mechanism for the silicon monoxide masers. These masers diminish for larger radii as the gaseous silicon monoxide condenses into dust, depleting the available maser molecules. For the water masers, the inner and outer radii limits roughly correspond to the density limits for maser operation. At the inner boundary, the collisions between molecules are enough to remove a population inversion. At the outer boundary, the density and optical depth is low enough that the gain of the maser is diminished. Additionally, the hydroxyl masers are supported chemical pumping. At the distances where these masers are found water molecules are disassociated by UV radiation. Star-forming regions Young stellar objects and (ultra)compact H II regions embedded in molecular clouds and giant molecular clouds, support the bulk of astrophysical masers. Various pumping schemes – both radiative and collisional and combinations thereof – result in the maser emission of multiple transitions of many