"She will execute the mission."
Translation:Ella ejecutará la misión.
I agree they are interchangeable. My question was about verb use. I'm not a grammarian, but it seems that will is a form of to be (ser) and is going to is a form of to go (var). It sometimes is confusing to know what is right. I used hara, and it was wrong according to Duobot.
The sentence is somewhat odd, but:
1.- "var" does not exist in Spanish. The inf of "va" is "ir" therefore the best translation is Ella realizará la misión. Else you are saying "she is going to execute the mission"
2.- To execute would be more "ejecutar" in Spanish. However , in the meaning of "to carry out" in Spanish, "llevar a cabo" cab be used. I believe the use of execute is down to make thing easier.
3.- Will is, when use on future, an auxiliary verb. There is no need of auxiliary verbs in Spanish in this case. For guidance see http://www.verbolog.com/0amar.htm