Move asteroids first, then Mars: NASA

How do you prepare for a mission to Mars? By redirecting asteroids of course.

Head of NASA, Charles Bolden, told students at Sydney's Australian National Maritime Museum on Monday that the organisation wants to put astronauts on the red planet by the 2030s.

He said they would redirect the orbit of asteroids to around the moon as training for the Mars mission.

An "Asteroid Redirection Mission" would involve meeting an asteroid and slightly varying its flight path over a period of a year or so.

But getting to Mars is a lot more difficult.

"We actually don't know how to do that", Mr Bolden said.

Though an astronaut could survive the eight-month trip to the fourth planet from the sun, scientists aren't sure how they would react to surface radiation.

And how should astronauts interact with the locals?

"If there is some kind of life there, my suggestion is to ask it something really simple like "do you speak English?" or whatever language that the person speaks," Mr Bolden said.

"Because you really want to establish some rapport with whatever that life form is before you ... [not] make the immediate decision it is evil."

NASA has been the victim of government funding cuts since the Global Financial Crisis hit the US in 2008.

It has now begun using private companies to put its astronauts into space so it can focus its resources on exploring the "oceans of our universe".

NASA works closely with the Russian Federal Space Agency, which in the wake of current tensions over Crimea, has both agencies trying to keep below the waterline.

"We both realise how incredibly valuable the International Space Station is to the world community," Mr Bolden said.

"It's like a mini United Nations."