I'm talking about within the Sol or some other inhabited system. The Nostromo's detonation sure looked violent and scary, but Ripley couldn't have been very far away from it at the time either. I imagine, under normal circumstances, they'd probably scuttle it a few thousand klicks away from anything or anyone just to be safe, but that's nothing in astronomical terms.
I don't really care either way if the Narcissus has an FTL drive. I haven't done the math, though, so I don't know how far it could have possibly traveled in 57 years without one.