Retrorockets are a widely used tool for spacecraft, and a key staple in spaceflight. Retrorockets are rocket engines, typically smaller than main engines, that allow a craft to decelerate, usually during landing, although they are extensively used in space as well. Many satellites and spacecraft have retrorockets simply for the purposes of deorbiting, for example, if a mission has come back to Earth and is in orbit, it is critical for the craft to have retrorockets so that it may get to the surface as quickly as possible. Retrorockets are also used for spacecraft that are landing on planetary bodies which do not a lot of atmosphere. Two main examples of this would be the Apollo spacecraft, which landed on the Moon using retrorockets, and the Curiosity rover, which used parachutes and then retrorockets to land on Mars. All in all, retrorockets are a highly versatile piece of technology that will certainly be used for centuries to come.