Ariane 5 launch of JWST
ESA and Arianespace said they are finalizing corrective actions to address a payload fairing separation issue sene on two Ariane 5 launches in 2020, setting up for a launch of JWST no earlier than November. Credit: ESA - D. Ducros

Updated 3:30 p.m. Eastern Dec. 18 with formal confirmation of the new launch date.

REYKJAVÍK, Iceland — With a communications glitch resolved, NASA and the European Space Agency have rescheduled the launch of the James Webb Space Telescope on an Ariane 5 for Dec. 24.

While a formal announcement from NASA and ESA was still pending as of late Dec. 17, various sources, including the Space Telescope Science Institute, said that they had rescheduled the launch for Dec. 24 at 7:20 a.m. Eastern after the communications problem between the spacecraft and ground equipment delayed the launch. The agencies finally publicly confirmed the new launch date early Dec. 18 after encapsulation of JWST within the Ariane 5 payload fairing was complete.

#Webb is confirmed for the target launch date of 24 Dec, 12:20 GMT / 13:20 CET.
Encapsulation of the observatory inside @ariane5 is successfully completed. Webb’s launch final readiness review will be held on 21 Dec and, if successful, roll-out on 22 Dec. #WebbFliesAriane #JWST

— ESA Webb Telescope (@ESA_Webb) December 18, 2021

NASA announced Dec. 14 that the launch, previously scheduled for Dec. 22, had slipped to no earlier than Dec. 24. The agency said that a “communication issue between the observatory and the launch vehicle system” caused the postponement but didn’t disclose any additional details.

At a Dec. 16 ESA briefing about the launch, scheduled before this latest delay, NASA and ESA officials discussed the issue. “It’s an interface issue in the electrical network connecting the observatory and the ground support equipment. Or, to be more precise, it’s a cable located in the launch table which is experiencing intermittent losses of data,” said Daniel Neuenschwander, ESA director of space transportation.

The cable issue, said Thomas Zurbuchen, NASA associate administrator for science, delayed a final “aliveness test” of spacecraft systems that runs for several hours. After the test is completed, the spacecraft is encapsulated within the payload fairing. “From that moment on, if you will, it’s all rocket stuff that’s on the critical path.”

At the time he said he believed the problem, which he described as “finicky,” had been found. “Those of us in the launch business are aware of these occurring from time to time. It’s just, when it’s Webb, there are no small problems.” Zurbuchen tweeted several hours later that the problem was resolved and the aliveness test underway.

Getting the test complete, though, does allow for the rest of the launch preparations to proceed much like a typical Ariane 5 launch. “From this moment onwards, it is more of a standard approach,” said Neuenschwander. “Of course, as Webb is a very special payload, we have further enhanced some aspects,” like additional oversight.

“Nothing is routine about Webb, but it’s a lot more routine after that,” Zurbuchen said of completing the aliveness test.

The cable issue was not the only communications problem. At the time of the announcement of the problem, NASA provided no detailed information about what had gone wrong. Early Dec. 17, ESA Director General Josef Aschbacher tweeted that the launch was rescheduled for Dec. 24, but deleted the tweet a short time later without explanation. NASA Administrator Bill Nelson told The Associated Press that the launch was set for Dec. 24, and the Space Telescope Science Institute informed both staff and media of that new launch date. But neither NASA nor ESA released a formal update as of the close of business Dec. 17.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...