This will entirely depend on how much you like to study Astronomy and how good you are at independent learning.
If you enjoy the subject you are more likely to study the content for longer periods of time, more likely retain information and more likely to find yourself opening a textbook or trying out practise questions. If you study it in Year 10 for 1 year, will you take the exam in Year 10 as well? In which case, you will already have 1 GCSE under your belt by the time you come to take the majority of your GCSE's in Year 11. If you don't do as well as you expected, would you be wanting to re-take this exam? In which case, you will be retaking it amidst the rest of your GCSE's.
Only you (potentially with help from parents and teachers) can make this decision.
The questions to ask yourself are; will this help me in a potential career or getting into University? Will I enjoy it? Am I willing to spend the extra time learning this subject? Will I be able to study for this exam without it negatively impacting my other subjects?
If the answers to the above questions are yes, then definitely; go for it!
If not, then I would suggest having a look at the exam board specification to see what content will be covered, and the specimen exam papers, to see what sorts of questions will come up.
If you can manage your time effectively, then you should have no problems with Astronomy negatively impacting on your other subjects. You just need to make sure you don't slack off, basically.