Nasa believes humans will set foot on Mars within 30 years.
Head of Nasa Daniel Goldin also thinks that astronauts may be sent to explore some asteroids within the same time period.
Mr Goldin, currently waiting to hear whether he will be reappointed by incoming president George W Bush, wouldn't rule out another moon landing - as long as there is a good reason for one.
But, according to ExoSci.com, he said the true commercialisation of space is unlikely to happen until cheap reusable craft are perfected.
He said private companies would not be taking the lead: "If you would have asked me five years ago, I would have said 'you bet'. But that boom we all expected ha