What is the simple subject in the following sentence? The space shuttles have made travel to the International space station convenient and possible. A travel
Simple formula: find the verb of the sentence then ask the question "who or what performs the verb" and the answer to that question is the SUBJECT of that sentence!
In your sentence; ask the question; "Who or what have made travel to the International space station convenient and possible? Answer: The space shuttles (that's your subject)