Recent progress on NASA’s James Webb Space Telescope (JWST) is encouraging but it also underscores the precarious state of affairs with the agency’s biggest and most expensive science mission, already well behind schedule and billions of dollars over budget. 

During a recent briefing in Germany, Eric Smith, NASA’s acting JWST project manager, said the program officials had resolved a number of issues flagged last year by the U.S. Government Accountability Office. These included balky micro shutters, cryo-cooler valves and light detectors. He also said weight growth on the giant observatory had been addressed by a combination of trimming mass from parts of the structure and the fact that the Ariane 5 rocket slated to launch JWST in 2018 is more powerful than when it was selected for the mission.

The telescope’s four main instruments have all been delivered; the last one, the European Space Agency-supplied Near InfraRed Spectrograph, arrived Sept. 20 at NASA’s Goddard Space Flight Center, which is managing the project. That instrument was one year late following the 2012 discovery of cracks in its support structure that required some rework.

Late-arriving instruments — the Lockheed Martin-built Near Infrared Camera was also a year late — forced NASA to begin thermal vacuum testing of the telescope’s core science module before their installation. Nonetheless, the testing started late and is expected to take longer than originally planned, forcing NASA to ship its four Magnetosphere Multiscale heliophysics probes, which were scheduled to follow JWST in Goddard’s thermal vacuum chamber, to the Naval Research Laboratory to keep that program on schedule.

So far NASA and its JWST prime contractor, Northrop Grumman Aerospace, seem to be keeping it all together. But the effort has the look of a high-wire act where one small slip up could lead to disaster — another cost-growth spurt that NASA’s congressional overseers have vowed not to tolerate. 

The situation is especially treacherous with sequestration — NASA is doing its best to protect JWST — and the ever-present possibility of a government shutdown, which would wreak havoc on the program. NASA will need nothing short of the best in its engineering and program management talents, and perhaps a little bit of luck, to keep its flagship observatory from going off the rails again.