Attainable, a quick thesaurus search of that word lists; within reach, possible, achievable, realistic, reasonable and manageable within its description. I'm commenting on this article by breaking my opinion up between them, to try to answer this question.
Is peace on Earth within reach? Presently, no it's not. Look around, can you just about reach out and touch peace? I see it being well out of our grasp and between the lack of peace worldwide and right here at home, I don't see us getting any closer to it. No (presently) I do not see peace as being realistically attainable.
Is peace on Earth possible? I believe it is. Realistically, anything is possible. (Where there's a will there's a way.) It's up to us. It's always been up to us. Since the beginning of existence, we have always had that luxury/burden of choice. Peace is possible but only if we will it.
Is peace on Earth achievable? Now, there you have me. I don't know if it's achievable or not because there are infinite free wills floating around. Would it be enough if only a majority of them wanted peace? Would the majority rule or would the vote for peace have to be a unanimous one? Every individual will (soul) coming together in agreement; is that achievable?
Is peace on Earth reasonable? Definitely yes, why (on Earth) wouldn't it be? I think it is very reasonable that one should not want to have to battle or suffer continuously on a day-to-day basis. I think what is unreasonable is the fact, that one should be expected to.
Lastly, is peace on Earth manageable? Yes and no, it IS being managed but since its beauty lies in its vastness, that's not a good thing. When you try to manage something, you try to control it. When you try to control it, it's because you want to be in charge of it. No one is in charge of peace (there's no need to control serenity). Maybe that's why we have so little of it today. After all, we all know how people like to be in charge of things.
In conclusion, I'd like to sum up my comment on this article by saying that (though still out of reach) I believe we can realistically attain peace on Earth if we all choose to implement our will to do so. I don't know if it's possible to achieve such a thing but I do think (considering the options) it's a reasonable goal. As for the management, I say we fire the management, pull together and reach our own agreement for peace on Earth.