This article originally appeared in the Jan. 15, 2018 issue of SpaceNews magazine.

Last January, at the 229th Meeting of the American Astronomical Society (AAS) in suburban Dallas, astronomers were growing increasingly excited about the progress the James Webb Space Telescope was making towards a launch then expected in late 2018. A town hall session about the mission spent only a little time on the assembly and testing of the spacecraft, focusing much more on planning for the initial rounds of observations it will perform.

“We’re in the phase of the program where there will be many new challenges, different kinds of challenges than we’ve had before,” Eric Smith, the JWST director at NASA Headquarters, said at that town hall. “The team that has been working together so well, so when problems arise, I’m really confident that the team will solve them.”

A year later, at the 231st Meeting of the AAS (it meets twice a year) outside Washington, the mood wasn’t nearly as celebratory. While project officials promoted the progress they had made in the last year, including recently completing a thermal vacuum test of the telescope and instruments at the Johnson Space Center, they couldn’t avoid the fact that JWST’s launch had slipped from October 2018 to sometime between March and June 2019.

At a December hearing by the House Science Committee’s space subcommittee, Thomas Zurbuchen, NASA associate administrator for science, said the delay was not because of a specific technical problem with the spacecraft but simply testing delays at prime contractor Northrop Grumman that exhausted the program’s margin.

“The sunshield and spacecraft bus experienced delays during their integration and testing at Northrop Grumman,” he said. “Following a schedule assessment of the remaining activities, the Webb launch date was changed from October 2018 to between March and June 2019.”

In an Jan. 9 interview during the AAS meeting, Scott Willoughby, vice president and program manager for JWST at Northrop Grumman, said the company had recently successfully tested the deployment of the sunshield, made of five layers of Kapton material the size of a tennis court, that will keep the telescope cold in space. The overall test, including repackaging of the sunshield, took longer than anticipated.

“The actual harder part is not deploying it, in terms of time, but folding it back together,” he said. “Deploying took a couple weeks, but folding it takes nearly two months.”

That was one of the reasons, he said, NASA decided to push back JWST’s launch. Another problem was with thrusters on the spacecraft bus. “We had to resolve an issue with the thrusters in terms of how the valves close,” he said. Northrop decided to refurbish all of the valves in question, and is in preparing to reinstall the thrusters on the bus and test them again.

Those issues, he said, led to a mutual decision by NASA and Northrop to delay the launch. “It was really the team coming together and saying we’ve now done most— not all — of the first-of-a-kind type of operations,” he said. There was also a decision, he said, to back off trying to do some activities in parallel.

“When we looked at all of that, we said that, for the work we have in front of us, we need more time,” he said.

Much more of the work in the coming months will be similar to what’s been done before, such as additional deployments of the sunshield after various tests to the spacecraft. That will help save some time, but won’t drastically accelerate the schedule. “There will be a little bit of a learning curve, but it won’t be two months down to two weeks” for refolding the sunshield, he said.

The sunshield and bus will soon begin a series of acoustic, vibration and thermal vacuum tests. During that time, the telescope and instrument section of JWST, known as the Optical Telescope Element and Integrated Science, or OTIS, will arrive at Northrop’s facility in Southern California from Houston. Willoughby said OTIS will be installed on the spacecraft bus in August or September, after another series of deployment tests.

The observatory, at long last a single spacecraft, will undergo yet another set of environmental and deployment tests at Northrop before it’s loaded onto a ship for transport to French Guiana. That will likely take place in early 2019, he said, or about three months before the spacecraft’s launch on an Ariane 5.

NASA has not yet provided a more specific launch window for JWST than March to June of 2019. Zurbuchen said in his congressional testimony in December that NASA expected to provide an updated launch date in January or February, after an independent review of the mission’s plans. “At this moment in time, with the information that I have, I believe it’s achievable,” he said of the March-to-June window.

Willoughby emphasized the overall progress Northrop has made on JWST, despite the schedule slip. “A year ago, it was about deploying the sunshield. That was the biggest newness,” he said. “That was really big.”

Lost in that test were other milestones. He said another major milestone last year was commanding the spacecraft bus for the first time from the mission operations center at the Space Telescope Science Institute in Baltimore. “We’re going to have to command it from a million miles away, so we should be able to do it from 2,500. But still, it’s a big deal to do flight commanding, and that was a fantastic success.”

“By the end of 2018, we’ll have an observatory,” Willoughby said, referring to the completion of JWST’s assembly.

Astronomers who have been waiting for years, through many previous delays and cost overruns, can probably handle a delay of another six months or so for a telescope that still promises to revolutionize their field.