The rest is debt the Gov owes to itself, in the form of various trust funds. It pays interest to itself by debiting one account and crediting another.
The most pressing part of this is the extent to which real cash will be needed to redeem obligations to the Social Security and Medicare Trust Funds at such time as those funds need cash to pay for benefits. That date is currently pegged at 2016. It means that general revenue (or more borrowing from the public) will be used to pay interest to the funds.
The basic macro principle is that as long as debt to the public does not grow more rapidly than GDP, ever-growing debt is sustainable forever. Imagine your income growing at the same rate as your debt, barring unfortunate events like death. In this case, you're no worse off as time goes on. And the US Gov lives forever, if it doesn't take everybody down with it.
cheers, mbs
> http://www.publicdebt.treas.gov/opd/opdpdodt.htm
> I am very intrigued about the 6 trillion dollar debt that the US has. How
> long can a country function as a superpower with such a massive
> debt. It is more than 60% of the GDP. When will the "piper be paid" so to
speak.
> Is this sustainable in the long term? Help me understand this?