A lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. Lander means the soft landing after that probe stays active while impact probe (as a rule, preceding the lander) just achieves the surface by hard landing with crush. For bodies with atmospheres, the landing occurs after atmospheric re-entry (or less precisely for other planets, atmospheric reentry) and the lander is first a re-entry vehicle. In these cases landers may employ parachutes to slow down and to maintain a low terminal velocity. Sometimes small landing rockets are fired just before impact to reduce the impact velocity. Landing may be accomplished by controlled descent and setdown on landing gear, with the possible addition of a post-landing attachment mechanism for celestial bodies with low gravity. Some missions (since, as example, Luna 9 and Mars Pathfinder) used inflatable airbags to cushion the lander's impact rather than a more traditional landing gear. When a high velocity impact is planned not for just achieving the surface but for study of consequences of impact, the spacecraft is called an impactor. Several terrestrial bodies have been subject of lander and/or impactor exploration: among them Earth's Moon, the planets Venus and Mars, the Saturn moon Titan, the asteroids and comets. Of the inner Solar System planets, Mercury is the only one that is yet to be visited by a lander.

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