Learning from Chandrayaan-2’s shortcomings, India’s new lunar mission aims to make a “soft landing” on the moon’s south pole. A landing is considered “soft” if the space probe reaches the surface without damaging itself.
A lander usually uses reverse thrust as a counterweight to gravity, decreasing its speed to ensure a soft landing.
However, a hard landing is different from a crash. It is called a hard landing if a space probe is intact but suffers enough damage to put the rest of the mission in peril.
The critical process of soft-landing has been dubbed by many including ISRO officials as "17 minutes of terror", with the entire process being autonomous when the lander has to fire its engines at the right times and altitudes, use the right amount of fuel, and scan the lunar surface for any obstacles or hills or craters before finally touching down.
According to ISRO officials, for landing, at around 30 km altitude, the lander enters the powered braking phase. It begins to use its four thruster engines by "retro firing" them to reach the surface of the moon by gradually reducing the speed. This is to ensure the lander doesn't crash as the Moon's gravity will also be in play.
Also see: Chandrayaan 3: first Moon images by Vikram lander after separating from main module
Noting that on reaching an altitude of around 6.8 km, only two engines will be used, shutting down the other two, aimed at giving the reverse thrust to the lander as it descends further, they said, then, on reaching an altitude of about 150-100 metres, the lander using its sensors and cameras, would scan the surface to check whether there are any obstacles and then start descending to make a soft-landing.
(With PTI inputs)