A concept shows Nasa's giant rocket, the Space Launch System soaring off a launchpad at the Kennedy Space Centre in Florida NASA

Nasa has said it is not ready to send its astronauts on a manned mission on board the untested Space Launch System (SLS) rocket to the moon. The announcement means the agency will not be fulfilling a request of the administration of US President Donald Trump to send humans to the moon by 2018 or 2019.

The White House had in February asked the premier space agency to speed up the mission in a bid to send humans to the moon. Since the moon is being looked at as a launchpad for a future manned Mars mission, Trump wanted the entire mission to be speded up. He also later expressed his desire for having humans on Mars by 2020.

Nasa has, however, refused to fulfil such requests, surprisingly not due to technical difficulties but financial. The overall additional cost of a crew, the time and risks outweigh the benefits, said Nasa.

"Putting astronauts on the first flight will add $600m (£465m) to $900m to the $24bn price tag, said Robert M Lightfoot Jr, acting Nasa administrator.

Besides, astronauts will require significant training in a short span of time and a fully operational life-support system for the Orion crew. Even without these additions, the launch of the unmanned SLS has been postponed to 2019 from 2018. Adding a crew would have delayed the mission further.

The SLS has never been tested in space and Nasa feels sending an unmanned crew will have its advantages when it comes to testing the craft. SLS will launch without a crew as originally planned in 2019 and in August 2021, the first crew of astronauts are expected to board the space shuttle.

Nasa has not been able to send people beyond low-Earth orbit since the final moon missions conducted over 40 years ago. Unlike the earlier ones, the mission to moon this time is not limited to the moon but, is a crucial stepping stone towards a successful manned mission to Mars.