stürzen and abstürzen
What is the difference?

I'm translating an episode of the German dubbed "UFO Files" to English and comparing to the original English, trying to learn vocabulary.

The German version:
Am vierten Oktober, 1967, stürzte bei Shag Harbor in Neuschottland ein Objekt in den Ozean. So etwas wie Shag Harbor ist ungewöhnlich das lässt sich nicht leicht erklären wenn überhaupt. Es war etwas am Himmel, hat aufgeleuchtet und ist dann abgestürzt.

The original American episode has "plunging into the water" for the first "stürzte". Then a Canadian speaks of his experience seeing the object, and he says, "something was there in the sky, and it was lit up, and the lights were on, and it came down"... came down = abgestürzt.

Do they both mean "to fall down" or when talking about flying things, "to crash".

Question

stürzen and abstürzen
What is the difference?

I'm translating an episode of the German dubbed "UFO Files" to English and comparing to the original English, trying to learn vocabulary.

The German version:
Am vierten Oktober, 1967,  stürzte bei Shag Harbor in Neuschottland ein Objekt in den Ozean. So etwas wie Shag Harbor ist ungewöhnlich das lässt sich nicht leicht erklären wenn überhaupt. Es war etwas am Himmel, hat aufgeleuchtet und ist dann abgestürzt.

The original American episode has "plunging into the water" for the first "stürzte".  Then a Canadian speaks of his experience seeing the object, and he says, "something was there in the sky, and it was lit up, and the lights were on, and it came down"... came down = abgestürzt.

Do they both mean "to fall down" or when talking about flying things, "to crash".

Answer

As Otto mentioned normally "stürzen" is often used to describe the "Sturz" of a person or the fall of a government or person in power while "abstürzen" always referres to a flying object crashing down. Yet this doesn't seem to explain your example, which are correct german sentences, right? In fact there is a grammatical reason to it.

The difference in your example is as follows:

If you use the verb "stürzen" it wouldn't make much sense to a german speaker because "stürzen" allone is as already mentioned mostly used for a person falling while walking, or the metaphorical fall of something.

If you said something like: "Das Flugzeug stürzte." It might be a grammatically sound german language, but everyone would wonder "Ok das Flugzeug stürzte, aber ist es auch abgestürzt, oder hat es sich auch abgefangen?"
The trick is, that the Prefix "ab-" indicates that the object fell from far up until the very crash of it. To just say "Das Flugzeug stürzte." Doesn't mean at all that the plane really did crash. 
Because it's possible that "Das Flugzeug stürzte 6km in die Tiefe, aber konnte sich dann abfangen, wodurch ein Unglück verhindert werden konnte."
If an object is "abgestürzt" it does mean that the object or bird did indeed crash to the ground or into the water. 
So the ab- indicates that the act of falling went on to the final consequence of crashing. 
Why don't we also use "abstürzen" for persons falling down on the street even though that person also has fallen to the ground? It's because with "abstürzen" we think of falling from quite some height. If we want to express the final sense of a person falling to the ground we would say "Er/sie ist hingefallen.". The hin- Prefix just as the ab- indicates that the action is completed.

Answer

hallo choppy,
"stürzen" kann bedeuten "fallen", to fall, fall down, particularly for a person walking, as well as a politician or a government is overthrown
see the list at
http://www.dict.cc/?s=st%FCrzen

"abstürzen" is often in connection to flying objects as in crashing down
see list at
http://www.dict.cc/?s=abst%C3%BCrzen