I really really wouldn't. The whole point of an Air over a MacBook Pro is size and weight. Don't spoil that by adding an external drive. Either get a big enough SSD to hold what you need while travelling, use a cloud service as suggested by Shawbridge, or if you must have separable storage get one or more big usb sticks.

And if you're worried about data loss, SSDs can take a lot more abuse than traditional magnetic discs with floating heads. You would have to work quite hard to trash the SSD in an Air and make it unreadable.

Bob