From the Council on Foreign Relations Greenberg Center for Geoeconomic Studies:
Near zero T-bill yields throughout 2009 is keeping U.S. debt service low even though the amount of outstanding debt continues to rise. A forecast increase in U.S. interest rates, along with growth in the amount of debt, will lift interest expenses sharply over the next ten years. In fact, as this chart shows, interest payments are projected to surpass defense spending by 2017. According to the Bureau of Economic Analysis, which collects data back to 1929, interest payments have never surpassed defense spending.