Washington, Feb 25 (DPA) A group of senators has called for a clear vision for the future of the US space programme, characterising President Barack Obama's plans for space agency NASA as 'without a mission'.

Earlier this month, Obama's 2011 budget proposal included the scrapping of existing plans for next-generation spacecraft to return to the moon. The move unleashed a stream of criticism from politicians who supported the plans, but NASA officials defend the move as allowing NASA to focus on longer term goals.

Under the budget proposal, NASA would end the so-called Constellation programme of rockets and spacecraft that was in development to replace the space shuttle. Instead, the US would encourage commercial operators to fly astronauts into low-Earth orbit.

Members of the Senate's Science and Space Subcommittee told NASA chief Charles Bolden at a hearing Wednesday that the new plans were too vague and left too much to chance, without a clear path for where the US space agency should go and how it would achieve those goals.

'You don't accomplish great things without a clearly defined mission, and this budget has no clearly defined mission,' Republican Senator David Vitter said.

Bolden made clear that the ultimate goal is for manned missions to Mars; however he said that could not happen within the next decade because there are too many unknowns. He said NASA will now focus on developing new technology and utilising the International Space Station to learn what is necessary for humans to undertake the sort of extended mission necessary for a trip to Mars.

But the firm commitment to a Mars mission came as news to Senator Bill Nelson, himself a former astronaut and chairman of the space subcommittee. He said Obama needs to present a clear plan for manned spaceflight, much like John F. Kennedy did in the 1960s by announcing the Apollo programme to land a man on the moon.

'The president either himself or through the administration or through his administrator needs to clearly state what the goal is,' Nelson said, stressing the needs for a 'bold statement, vision for the ultimate destination of our space programme, and that is Mars'.

NASA is on track to retire its existing fleet of space shuttles by the end of the year, leaving US astronauts reliant on Russia for rides into space until a commercial space travel industry can be developed.

'This senator fears the US is going to be on the sidelines while other countries continue to make incremental progress toward destinations like the moon,' said Nelson, who represents Florida, where the Kennedy Space Centre employs thousands.