"The space shuttles is the subject."
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)
Darry grows old and lives a nice life and marries Sasha but has huge pains
It is a sunny day
Tom is excited
He is adventurous
The water is blue
And feels warm
Answer: by taking normal nouns
and turning them to pronouns
Explanation:
example Baby It's A boy