In order to achieve orbit a space vehicle must acquire a forward velocity relative to the Earth's surface such that its rate of motion around the Earth is just enough to arrive at a balance between shooting off into space and falling back down to Earth due to the gravitational attraction of the Earth. The Moon has such an orbital velocity around the Earth that allows it to maintain its orbital distance from the Earth without fuel by relying only on its established momentum. Closer to the Earth, orbital velocity must be increased to compensate for the greater attraction of gravity that accompanies being closer to the source of gravity, the Earth.
Once an orbit has been established a small reduction in orbital velocity allows the downward pull of gravity to overtake the forward momentum necessary to maintain orbit and the vehicle will begin to descend. As the vehicle descends it encounters the thinner layers of the atmosphere and this acts as a break further slowing the vehicle and allowing gravity to have an even greater effect thereby lowering the vehicle into denser layers of the atmosphere, further slowing the vehicle, etc. etc . . .
While at first glance the atmosphere might seem a detriment to �safe� reentry, it is the gradual breaking effect of the air in the atmosphere that makes it possible to return to the Earth�s surface using very little fuel, however, the angle of descent and heat-shielding are critically important to the reentry process.