Astronomer Royal Martin Rees believes that planets resembling Earth may be discovered in the next two to three years - although it may take a further 20 years before we are able to capture an image of them.

Rees, Professor of and Astrophysics at Cambridge University, also said the need for manned space exploration is rapidly diminishing in the face of new technologies and advances in robotics.

He made the remarks during an interview as part of the University's Cambridge Ideas series of videos, podcasts and slideshows.

During the discussion, recorded to mark more than forty years since man walked on the moon, Rees also says that those walking on the surface of planets like Mars in the future are likely to be adventurers of the sort who conquered Everest, rather than astronauts on state-backed missions of the type undertake.

Rees also reveals that his biggest wish is to have answers to the questions about how life began in the first place.

He said: "Kepler (NASA ) will tell us whether other stars are orbited by planets like the earth and that's going to be important. If there is other life elsewhere in the universe, the most obvious place to look would be a planet like the Earth. I'm sure that in two or three years we'll know from the Kepler observations that there are many other planets like the earth orbiting other stars. But I think it may be 20 years before we get an image of a planet.

"As to whether they will have life on them, I would not take any bets at all. Biology is a much harder subject than astronomy and we don't know how life began on earth. I would hope in 20 years we might understand that. Once we understand how life got started here, we'll have a better idea for how likely it was to start in other environments and where the best places in the cosmos to look are."

Talking about the Apollo missions and moon landings, Rees also says that such a golden age of expensive manned missions, historically backed because of nationalist agendas, are most likely a thing of the past.

"The moon landings were an important impetus to technology but you have to ask the question what is the case for sending people back into space? I think that the practical case gets weaker and weaker with every advance in robotics and miniaturisation. It's hard to see any particular reason or purpose or going to the moon or indeed sending people into space at all.

"I hope that some people living today will walk on Mars, but I think they will do this not for any practical purpose but with the same motive as those who climb Everest or the pioneer explorers.

"I think the future for manned space exploration will be a cut-price, high-risk programme, perhaps even partly privately funded which would be an adventure, more than anything practical."

On his hopes for the future of science, Rees is optimistic about the pace and direction of future discoveries.

He added: "We are now trying to answer scientific questions that couldn't have been posed twenty years ago and 20, 40, 60 years from now I'm sure the questions we as scientists will address are questions we can't even conceive of today.

"The scientific question I'd most like to have the answer to is whether there is life out in space and how began. This is a question that would have fascinated Darwin and Galileo, 400 years after he made his telescopes and looked at craters on the ."