Based on your response to my comment, I'll take a crack at my own answer.
The first question I'd as is if these 'rumors' are, in fact, true. If you were given an assignment that a senior developer felt too difficult, then management has unrealistic expectations, or your manager has run into a brick wall above him, and is looking for his sacrificial lamb. Conversely, if a senior developer looked at this, said "I've never done this before, maybe the new kid can take a crack at it", management is interested in your abilities, and giving you a chance to demonstrate your value and earn a more permanent or senior spot on the team. I suspect the second option is closer to the truth.
Now, what can you do??!?!?!?
You can do the assignment. This looks a lot like someone up the food chain is giving you the opportunity to prove your worth to the company and move you up the food chain. Communicate to your manager that you are worried about your ability to complete this on time. Their reaction will tell you a lot, I suspect they will tell you that this is a high priority to give to you and they're hoping you pull through.
Based on the type of question you're asking, this sounds more like someone wants you to bring new information into the company, in the form of a new API, design paradigm, new technology or vendor or whatever. In that case, you're in really good shape. Learn everything you can, create some simple examples, and give an awesome presentation. Plan on having more senior support when it comes time to integrate this project with others the company is currently involved in. If they really expect a junior-level dev to generate enterprise-quality code in a new environment in a month and then have it ready to integrate with other products, they are smoking some very weird-smelling oregano. If that's truly the case, do your best and be ready to explain why you were unable to meet the objectives. In some cases, that can be just as valuable as it showcases shortcomings of the original plan, some technical oversight, or just an under-estimation of how much work this project requires.
Now I'm going to assume that you want to downvote this answer because you asked "How do I turn this down?" and I answered "I'm afraid of this new project, what do I do?", so I'll answer your original question.
If you're going to turn down an assignment from your boss, have a reason, a good reason. "This is hard" or "This is not what I do" are not even reasons, let alone good ones. Right now, my current company is transitioning ISPs. My boss asked me to do the actual cut-over, and I told him I didn't think it was a good idea I should do that. My argument was that we had a vendor who would be able to do the cut-over better than myself because they have more experience. I volunteered to oversee the project and advise the vendor where needed. We wound up with more of my valuable time to devote to other, non-outsource-able projects, and a better setup because the vendor knew exactly what to do.
Unfortunately, at this junior point in your career, your time is worth relatively little to the company, even if you are paid 'a lot' (define that as you choose). Your best bet is to tell your manager that you are unqualified for this. You'll gain some maturity points, but likely be passed over the next time something interesting comes along. You'll be 'that guy' who didn't want to try that one time, so why would you be willing to try this time?