General Antonyms:
* Impossibility: This is a direct opposite, implying something is impossible to achieve or happen.
* Disappointment: This suggests a negative outcome, the opposite of a hopeful prospect.
* Threat: This implies danger or harm, the opposite of a positive or promising prospect.
Context-Specific Antonyms:
* Setback: If "prospect" refers to a hopeful development, a setback would be the opposite.
* Decline: If "prospect" refers to a rising trend, a decline would be the opposite.
* Failure: If "prospect" refers to a potential for success, failure would be the opposite.
It's crucial to consider the specific context to choose the most appropriate antonym. For example, if you are talking about the prospect of finding a new job, the antonym could be "unemployment" or "joblessness."
Let me know the context of your usage, and I can give you a more precise antonym!