The scale of the delay and cost overrun blighting Nasa's James Webb Space Telescope has been laid bare by a panel called in to review the project.

The successor to Hubble will probably cost at least $6.5bn to launch and operate, and is unlikely to get into orbit before September 2015.

Even that assessment is optimistic, say the panel members.

The head of the US space agency has accepted that "cost performance and coordination have been lacking".

Charles Bolden has ordered a reorganising of the project and has changed the management at its top.

Estimates for JWST's total cost - to build, launch and operate - have steadily increased over the years from $3.5bn to $5bn. Along with the cost growth, the schedule has also eroded. The most recent projected launch of 2014 has looked under pressure for some time.

The independent panel chaired by John Casani of Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California, was asked to examine the root causes of JWST's problems.

It found the original budget for the project to be insufficient and poorly phased, and blamed the management for failing to pick up and deal with the issue.

"This is a very large complex project and to estimate something with any real degree of precision that's never been done before is a tough job," John Casani told reporters. "But the bottom line is that there was never enough money in the budget to execute the work that was required."

The panel did however commend the technical success of the project. Mr Casani said the technology on JWST was in "very good shape".

The telescope was always regarded as major undertaking. Its primary mirror is 6.5m (21ft) across - close to three times wider than Hubble's.

The huge reflector will sit behind an even more expansive sun shield, the area of a tennis court.

This structure will protect the observatory from radiation from the Sun and the Earth.

Whereas Hubble sees the Universe mostly in visible light, JWST will observe the cosmos at longer wavelengths, in the infrared. It will see deeper into space and further back in time, to the very first population of stars.

When it is finally built, it will be launched on Europe's Ariane 5 rocket and sent to an observing position 1.5 million km from Earth. It is expected to have a 10-year lifespan.

Its distance from Earth means the telescope cannot be serviced by astronauts, as was the case with Hubble.

Casani's panel still has concerns. It says a total life-cycle cost of $6.5bn and a launch in September 2015 may still be possible, but only if the budget for the project is increased by about $200m in each of the financial years 2011 and 2012.

"If those amounts are not available then of course the $6.5bn number is going to move out and the launch date might move out too," Mr Casani said.

The James Webb Space Telescope is named after the former administrator of Nasa who led the agency in the run up to the Apollo landings.

The independent review panel was established at the request of Senator Barbara Mikulski, chairwoman of the Senate Appropriations Subcommittee on Commerce, Science and Related Agencies.

Source : bbc.co.uk