NASA was created to get Americans into space, and then to the Moon. It succeeded brilliantly in those two missions. Then it became the Shuttle operator and science investigator, launching planetary probes and Hubble space telescope. Now that the shuttle is going away, what is NASA supposed to be doing?
How about building and launching a bigger and better space telescope? Hubble is 20 years old. Sooner or later it's going to die. And with the Shuttle gone, we won't be able to fix it. With 20 years of technical progress we should be able to build a bigger and better one. Right now NASA is getting $19 billion a year of taxpayer money. I'd kinda like that money to get us something worthwhile rather than just paying a bunch of salaries.
A new Space Telescope is easily fundable. A manned mission to Mars is not. Back of the envelope calculations show a Mars trip would cost $1 trillion and up. That's more than the US defense budget and is politically impossible. A lot of voters think we need a space program, but few of them are willing to pay for that size of space program. More do able, would be setting up a manned Moon base, a worthy goal, but when you consider the success of Hubble, I think a bigger and better Hubble follow on would be just as successful, giving us a view of the entire universe, where as a Moon base would only be able to do Lunar geology and seismology. Seeing back to the Big Bang is more exciting than categorizing Lunar rocks. Cheaper too.