Of course it's possible and we will pay it. It's in the Constitution that we have to. It would take a constitutional amendment to keep us from doing so. We actually pay off our debts all the time. Currently we just get new loans to pay back the old ones.
I'm sure though that you're wondering if we could pay it all off. It isn't necessary but it's possible. There are 2 questions that I think are more important. The first question is at what point does the interest on the debt become too much of a burden to taxpayers, because right now the interest is all we are effectively paying anyway. So far this fiscal year we have spent 219 billion dollars on interest service.
LINK The answer to this question is basically "Deal with it". Like I said we have to pay it. If we don't want to pay a larger portion of our GDP in the future to interest then we either balance the budget or stimulate growth now. (<between those two is where the political argument lies)
The second question is when do bondholders lose faith in our ability to pay them back faster than inflation eats up their investment or they believe the likelihood that we will ignore/change the Constitution in order to avoid paying them is high enough that they quit buying them at a reasonable interest rate. The answer to this question in Greece(iirc)was about 120% debt to GDP ratio but Greece doesn't have such a constitutional guarantee, Brussels is less fond of Athens than Washington is of Washington, the US doesn't have the kind of social spending that Greece has, and Greece doesn't have the 18 Trillion dollar economy that gives us an economy of scale advantage. The answer to the second question is we don't know at what level the bond market will get spooked but we don't want to find out we have hit our limit in the middle of the next financial crisis, war, or whatever.
I think most people agree that our debt to GDP ratio is too high and needs to come down so that we have a little more breathing room. How we get there? As far as I can tell vitriol seems to be the path most people believe in.