If everything goes well the JWST will be launched in June 2013. The new telescope is almost three times larger than Hubble, it will sit farther from Earth and will be able to see more.
Hubble, launched in 1990, has sent back pictures of our solar system, distant stars and planets, and remote fledgling galaxies formed not long after the Big Bang.NASA also said it will send a manned servicing mission to Hubble in 2008.
But scientists say the JWST will enable them to look deeper into space and even further back at the origins of the universe.
"Clearly we need a much bigger telescope to go back much further in time to see the very birth of the universe," said Edward Weiler, director of Nasa's Goddard Space Flight Centre.
Martin Mohan of Northrop Grumman, the contractor building the telescope, said that the team was making excellent progress.
"There's engineering to do, but invention is done, more than six years ahead of launch," he said.