If the United Sates treated it as a bigger version of Apollo and poured Apollo+ levels of funding into it, no problem. But getting humans to mars in 20 years time when no money is being provided for it, and at a time when the U.S. government going further into debt and the U.S. has a large number of baby boomers on the verge of retirement makes it hard to see it being done within 20 years time. And the money NASA does get could well be wasted on sending humans to the moon with nothing left over for mars. But as far as tech is concerned, we could do it with what we have today, but it would be expensive.