Yet again, NASA is delaying the launch of its flagship astrophysics mission, the James Webb Space Telescope (JWST), to 30 March 2021—the third schedule slip in less than a year. An independent review of the project concluded that there was excessive optimism in the launch schedule and suggested the delay, one of 32 recommendations to improve the project’s chances of success. NASA also revealed that the development cost of the telescope would rise from $8 billion to $8.8 billion, requiring it to be reauthorized by Congress, which set an $8 billion cap in 2011. (The total cost of JWST, including operations, is expected to be $9.66 billion.)

NASA was responding to the report of an Independent Review Board (IRB), headed by agency veteran Thomas Young. The board’s report highlighted a series of human errors and embedded problems in the hardware as the main drivers of the delay. Young told a press briefing today that because of the mission’s “awesome scientific potential” it was worth pursuing it to completion. Thomas Zurbuchen, head of NASA’s science mission directorate in Washington, D.C., says, “We have to get this right on the ground before we go into space.”

Young says JWST’s hardware includes seven noteworthy firsts for which NASA had no heritage. But most of the problems that led to his recommended delay were avoidable, he says. Human errors in the testing and integration of the spacecraft and its sunshield were responsible for many of them, including the use of an incorrect solvent in fuel valves and faulty fasteners which keep the sunshield folded up during its trip into space. During acoustic testing some of those fasteners broke loose and it took months to find and remove the pieces from the spacecraft. These problems had “simple fixes,” Young said, but caused a delay of up to 1.5 years.

That delay will cost the project nearly $1 billion. JWST has a long history of schedule slips and cost hikes . Around the turn of the century, while still in development, the projected cost was $1.6 billion with launch expected in 2011. By 2005, the cost had grown to $4.5 billion and the launch pushed back to 2013. When NASA admitted in 2011 that JWST wouldn’t take off until 2018 at a cost of $8 billion, Congress rebelled. It threatened the mission with cancellation and astronomers lobbied to save it. Lawmakers eventually allowed it to go ahead, but on condition that it remain on schedule and stay below an $8 billion cost cap.

With a new management structure, the project remained on track for several years. But last year, with rigorous testing of the flight hardware taking place , cracks began to show. Vibration testing of the telescope at the Goddard Space Flight Center in Greenbelt, Maryland, threw up some unexpected results that delayed the launch to the second quarter of 2019. Since then, the telescope and instrument package have endured a 100-day test inside a vacuum chamber at Johnson Space Center in Houston, Texas, before being shipped to main contractor Northrop Grumman’s facility at Redondo Beach, California, where the rest of the spacecraft was in final assembly.

In March, the Government Accountability Office (GAO) reported that combining the two remaining parts of the spacecraft and testing the whole assembly would likely bust the remaining wiggle room in the project’s schedule and funding . A few weeks later, the project’s Standing Review Board concurred, listing a string of problems the JWST team needed to resolve. In testing, the cables that tension the craft’s tennis court–size sunshield became unexpectedly slack during deployment and risked tangling. Tests also produced some tears in the thin fabric of the sunshield. Those are now repaired and some changes had to be made to stem leaks in the propulsion system. NASA pushed the launch back again to May 2020 . But NASA will now need more time to get the telescope off the ground.