Exclusive: Report Finds NASA Telescope $1 Billion Over Budget

As with other NASA science missions, the telescope's cost estimate has steadily risen over the years. Mikulski, who chairs the Senate panel that oversees the space agency budget, ordered an independent review this summer. Mikulski is a strong proponent of JWST, which is managed by NASA's Goddard Space Flight Center in her home state.

Although the Casani report is yet to be made public, word of the expected delay in JWST's launch is all but official. In a presentation made before the Astronomy and Astrophysics Advisory Committee of the National Science Foundation in October, Jon Morse, NASA's head of astrophysics, indicated that the launch schedule for JWST was likely to extend into the 2015-2017 timeframe.

Some U.S. astronomers fear that JWST's troubles could force NASA to shelve WFIRST and instead opt for a partnership with the European Space Agency (ESA) on a mission called Euclid that would focus on dark energy. Euclid is one of three projects competing for ESA funding; the agency is expected to pick a winner by February 2011.

The authors of the Astro2020 report expressly recommended that even though it was prudent to find international partners for WFIRST, the United States needed to maintain a leadership role in any such collaboration. In addition, joining Euclid as less than a 50% partner could mean sacrificing the exoplanet-hunting mission that is a key element of WFIRST but not included on Euclid.

However, that option would be better for the United States than the prospect of not participating in a dark energy mission at all. That's why NASA officials are pursuing both possibilities in parallel, proceeding with WFIRST—with the United States in the driver's seat—and joining Euclid as a partner. How these two plans evolve going forward will hinge on how NASA handles JWST.

A senior NASA official, who did not wish to be named, says the Casani report is not the final word on how much JWST will end up costing or when it will be ready for launch. The report, he says, will be only one of a number of inputs that NASA will consider in drafting a budgetary plan for completing the project.