It's a tough call to make.
I would say it could either be a 15 or 20 depending on the circumstances, and it would be a piloting manouver, rather than a command. Perhaps you could make it a 20, but if the player was able to make a physical sciences (mathematics or physics) OR Space sciences (Planetery bodies) roll of TN 15, then he could lower that roll to a 15 - since it would require a certain amount of knowledge on the workings of such things.. allot of pilots wouldn't be all that familiar with landing on planetary surfaces (though they might have bought that as a speciality, in which case it's all part of their repetoire!)
Depends on when it's happening and weather or not you are in combat. In combat you want to simplify it for speed, but if it were a drama thing - rescuing a ship of innocents spiraling towards an atmosphere with no shields, then it would be better to have plenty of rolls and complications to up the tension!