A probe’s landing procedure is a complicated and dangerous one. Because scientists do not know everything about a target planet, they can never be sure what the conditions will be like when a probe lands. Mars, for example, suffers from enormous dust storms that could seriously damage a probe descending to the surface. The diagram below shows a procedure for a landing.
A Probe lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. By contrast with an impact probe, which makes a hard landing and is damaged or destroyed so ceases to function after reaching the surface, a lander makes a soft landing after which the probe remains functional.
For bodies with atmospheres, the landing occurs after atmospheric entry. 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 set down on landing gear, with the possible addition of a post-landing attachment mechanism for celestial bodies with low gravity. Some missions (for example, Luna9 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 or impactor exploration: among them Earth’s Moon, the planets Venus, Mars, and Mercury, the Saturn moon Titan, the asteroids and Comets.
Beginning with Luna 2 in 1959, the first few spacecraft to reach the lunar surface were impactors, not landers. They were part of the Soviet Luna program or the American Ranger program.
In the year 1966, the Soviet Luna 9 became the first spacecraft to achieve a lunar soft landing and to transmit photographic data to Earth. The American Surveyor program (since 1966) was designed to determine where Apollo could land safely. As a result, these robotic missions required soft landers to sample the lunar soil and determine the thickness of the dust layer, which was unknown before Surveyor.