Satellites orbiting the earth have a finite life and sometimes debris of satellites fall to the earth. This is because:
| 1. | the solar cells and batteries in satellites run out. |
| 2. | the laws of gravitation predict a trajectory spiralling inwards. |
| 3. | of viscous forces causing the speed of the satellite and hence height to gradually decrease. |
| 4. | of collisions with other satellites. |
