In order to land on a planet
safely, one must slow down from orbital speed i.e. tens of thousands of miles
per hour, to a full stop on the surface. Re-entry friction with Planet Earth's dense
atmosphere decreases the spacecraft‘s speed sufficient for parachutes to deliver
a gentle descent. Object like the moon, which has no atmosphere at all, rockets
are used all the way down to guarantee an easy landing. On the other hand Mars
has very thin atmosphere and it cannot offer sufficient resistance to slow a
spacecraft to a safe landing speed with just only a parachute and heat shield.
