Why do space vehicles heat up when they enter the atmosphere?
Due to drag by fluid (air) friction that is directly proportional to air density, face area and square of velocity of the space vehicle
Like meteors entering the atmosphere with high speed, space vehicles also suffer heating due to air friction. The vehicle's resistance to fluid friction generates heat that might lead to disintegration and burning.
Vehicle-streamlining and coating with heat-withstanding materials, and stroking for retardation to reduce speed, safe landing is possible.
The formula for drag force for higher entry- velocities is
For related formulas: