Following the Curiosity landing I noticed that the possible landing site (the so-called 'landing footprint') was demarcated by an ellipse. Here is a picture of it:
Now obviously such a footprint cannot be circular because atmospheric entry is at an angle. But is it as simple as that? Is the landing ellipse simply the cross section of the cone of possible trajectories a given spacecraft may assume after contact with the atmosphere with the (flattened) surface of the planet? Obviously that's an ellipse as a conic cross section (unless, I guess, things go horribly wrong and there are possible trajectories in which the spacecraft misses the planet and shoots into space.) Or is there more going on here?
(In writing this question, I realised that it's highly unlikely that 'more is going on' than what I've suggested above, but since I've taken the trouble to write it down I will post it nonetheless.)