First came the Hubble Space Telescope. Now, NASA is finalizing development of the James Webb Space Telescope for launch in 2019. And finally, the space agency is beginning to design and develop its next great space telescope, the Wide Field Infrared Survey Telescope, or WFIRST.

This instrument will have a primary mirror of 2.4 meters, the same size as the Hubble's, and be designed to hunt for dark energy and spy on exoplanets. Although similar in size to Hubble, the WFIRST telescope's infrared instrument would have a field of view that is 100 times greater than the Hubble, allowing it to observe much more of the sky in less time. It was also supposed to carry a special coronagraph, which could block the light of stars and allow astronomers to observe exoplanets directly.

But a new report—released without fanfare on the Wednesday before the Thanksgiving holiday—calls into question the viability of the project. "The risks to the primary mission of WFIRST are significant and therefore the mission is not executable without adjustments and/or additional resources," the report states. It estimated the cost of the project at $3.9 billion to $4.2 billion, significantly above the project's $3.6 billion budget.

Produced by an independent and external team to review the technical aspects of the program, its management, and costs, the report is critical of a series of key decisions made by NASA. The addition of a coronagraph and other design choices have made for a telescope that is "more complex than probably anticipated" and have substantially increased risks and costs, according to the report.

It also offered a scathing review of the relationship between NASA headquarters and the telescope's program managers at Goddard Space Flight Center. "The NASA HQ-to-Program governance structure is dysfunctional and should be corrected for clarity in roles, accountability, and authority," the report states.

Even before the review's public release, NASA has been attempting to fix some of these issues. In an October memo, NASA science chief Thomas Zurbuchen told the director of Goddard Space Flight Center, Chris Scolese, to modify the telescope's design to reduce its cost and complexity. Such revisions would include treating the coronagraph as a "technology demonstration instrument" and using additional commercial components to bring the cost to $3.2 billion.

"WFIRST remains NASA's highest priority for a large astrophysics mission following the James Webb Space Telescope," Zurbuchen wrote. "Making these adjustments to WFIRST in response to the findings in the (new) report will ensure its success while preserving a balanced Astrophysics program."