The most recent slippage of the James Webb Space Telescope, which now will launch no earlier than March, 2021, has raised some questions about how it will get into space. This is because NASA's chosen rocket for the mission, the proven Ariane 5 launcher, is likely to fly for only a few more years before it is phased out in favor of a newer, less expensive booster.

Back in 2015, when NASA formally reached an agreement with Arianespace to launch on the Ariane 5 rocket, the projected launch date was 2018. NASA partnered with the European Space Agency and its affiliated rocket company for the launch to keep costs down. Essentially, Europe provided a rocket in exchange for some of the observing time. The telescope's massive heat shield was then designed to fold 12 times to fit within the Ariane 5's payload fairing.

Last year, when the telescope's launch date was delayed into 2019, this was still no problem. But the telescope's launch has since been delayed twice more: first into 2020 and then into 2021. The Ariane 5 can still launch during these years. Further delays, however, may prove problematic.

According to Alain Charmeau, who as the head of Ariane Group oversees a family of launch vehicles including the Ariane 5, European states have created a transition plan to the Ariane 6. A separate launch pad is being constructed at the European spaceport in French Guiana for the Ariane 6, and this will allow the Ariane 5 to continue flying for a few more years—but not indefinitely.

Could accommodate 2023

"One can back up the other one," he told Ars. "We will have the Ariane 5 for at least until the end of 2022, but it’s not clear cut. If we need to have another launch in 2023, we can extend it; it is just a matter of maintaining the team and maintaining the infrastructure. But our plan today is to start Ariane 6 in 2020 and stop Ariane 5 at the end of 2022."

Charmeau said he was not aware of any discussions with NASA on launching the Webb telescope on Ariane 6, and the American space agency confirmed this as well. "NASA has not initiated any discussions with ESA or the Ariane Group about flying on an Ariane 6," NASA's Felicia Chou told Ars. "We still plan to launch Webb on the Ariane 5."

However, the Webb telescope could probably make a relatively simple transition from one vehicle to the other, as they have similarly sized payload fairings.

Two competing factors in any decision to move from one rocket to the other are likely to be the success of Ariane 6 and costs. For now, NASA is comfortable with the long and largely successful track record of the Ariane 5 rocket, which has flown about 100 times. The Ariane 6 needs to establish something of a track record before it flies the most expensive instrument NASA has ever built. However, it will be costly for Ariane Group to keep its Ariane 5 supply lines and facilities open for just a single launch if Webb continues to be delayed into 2023 or beyond.