Terrain features have some restrictions. All movement restrictions are applied as the ship tries to leave the sector. If exit is randomized, you may spend one extra movement point to insure exit in the proper direction.
Nebulas: +1 to strategic movement cost, no detection, ship may exit the sector in an unexpected direction.
Asteroid Fields: +1 to strategic movement cost, -1 to the detection range of the ship.
Dust Cloud/Heat Zone/Radiation Zone: -1 to the detection range of the ship.
Ion Storm: +2 to strategic movement cost, no detection, ship may exit the sector in an unexpected direction, ship may take damage.
[Super]nova: Ummm... it would be better if you didn't try to enter this sector.