WASHINGTON — NASA’s James Webb Space Telescope is facing a schedule conflict for its Ariane 5 launch with a European planetary science mission that could, in one scenario, delay the telescope’s launch by several months.

Current plans call for the launch of JWST on an Ariane 5 from the spaceport at Kourou, French Guiana, in October 2018. The European Space Agency is providing the launch of JWST as its contribution for the mission, in exchange for a share of observing time on the telescope.

However, ESA is also planning an October 2018 launch of BepiColombo, its first mission to Mercury, in cooperation with the Japanese space agency JAXA. That mission will also use an Ariane 5 launching from Kourou.

At a meeting of the NASA Advisory Council’s science committee July 24, Alan Boss, an astronomer at the Carnegie Institution and a member of the Astrophysics Advisory Committee, warned that BepiColombo could take precedence over JWST for that October 2018 launch slot.

“BepiColombo has rights to launch before James Webb does,” he said in a summary of a meeting of that advisory committee earlier in the month.

Boss didn’t elaborate on the reasons for that precedence, but BepiColombo, unlike JWST, has a narrow launch window in order to reach Mercury. ESA officials said earlier in July that the mission’s current launch window opens Oct. 5 and runs through Nov. 28. JWST does not have similar launch window restrictions.

While the Ariane 5 is capable of flying at a relatively high cadence — three Ariane 5 rockets launched in May and June of this year — the extensive payload processing requirements of both BepiColombo and JWST appear to rule out launching both missions around the same time.

“It’s unclear if BepiColombo will be out of the way” before JWST arrives at Kourou for launch preparations, Boss said. He believed JWST needed three to six months of “full access” to facilities at Kourou to prepare for launch. “You really want to have BepiColombo long gone before you move in and start taking over.”

If BepiColombo sticks to its current schedule, that could mean delaying JWST by several months. “There’s some concern that that October 2018 launch may actually slip into the spring of 2019,” he said.

That schedule conflict is due in part to delays in the development of BepiColombo. The mission’s launch has slipped several times in the last decade. In 2007, when ESA approved moving the mission into its development phase, it was expected to launch on a Soyuz rocket in 2013.

In 2011, ESA announced the mission would instead launch on a more powerful Ariane 5 in July 2014. The launch slipped in 2012 to August 2015, then later to July 2016, January 2017 and April 2018. Last November, ESA announced that the launch was now scheduled for October 2018 because of a problem with a power processing unit on the spacecraft.

Boss noted BepiColombo’s delays in his presentation, suggesting that the mission could face additional delays. ESA officials, though, said at an event in early July that the spacecraft was on scheduled to ship to French Guiana in early 2018 to being final launch preparations.

“We are looking forward to completing the final tests this year, and shipping to Kourou on schedule,” Ulrich Reininghaus, project manager for BepiColombo at ESA, said in a July 6 statement about the completion of the latest series of tests of the spacecraft. That statement added that the launch schedule for the mission would be confirmed later this year.

JWST has also suffered significant delays in its development, although it has maintained an October 2018 launch date since a re-plan of the mission several years ago. The telescope is currently in an Apollo-era thermal vacuum chamber at NASA’s Johnson Space Center for testing, and will be shipped later this year to a Northrop Grumman facility in Southern California to be integrated to its spacecraft bus and sunshield.

JWST currently has about three and a half months of schedule reserve, an amount that has been gradually decreasing to account for development issues. That current level of schedule reserve is nearly a month below what the project’s plan called for having at this stage in development, but still above recommended levels for projects set by NASA’s Goddard Space Flight Center.

Additional problems, however, could lead to delays in JWST regardless of any launch site conflicts. “There’s some concern that they might be running out of funded schedule reserve,” Boss said, particularly as the project goes into critical final assembly and testing activities. “There’s some concern, but the JWST folks are confident they will overcome the remaining hurdles and get it done on time.”