Why are the launch windows for NASA’s Artemis 2 moon mission so short?

NASA is gearing up to launch Artemis 2, the first crewed mission to the moon in more than half a century.

There are five potential Artemis 2 launch dates next month (March 6-9 and March 11) and six in April (April 1, April 3-6 and April 30), according to the space agency. The liftoff window lasts 120 minutes on all of these dates except March 11, when it’s 115 minutes long.

That works out to just 11 opportunities over a 61-day stretch — and some of them may be nixed by bad weather or the need to replenish commodities like rocket fuel. Why does Artemis 2 have so few chances to fly?

It’s all about orbital mechanics and the specific requirements of the mission and its hardware.

Artemis 2 will lift off from Kennedy Space Center in Florida, sending Canadian astronaut Jeremy Hansen and NASA’s Reid Wiseman, Victor Glover and Christina Koch on a roughly 10-day trip around the moon aboard an Orion crew capsule.

But the astronauts won’t head directly to lunar realms: Artemis 2’s Space Launch System rocket will first deliver Orion to high Earth orbit, where the crew and ground teams will conduct a variety of checkouts to make sure the capsule is ready for its epic journey.

Therefore, “the launch day and time must allow SLS to be able to deliver Orion” to that orbit, NASA officials wrote in an Artemis 2 explainer last month.

“Orion also must be in the proper alignment with the Earth and moon at the time of the trans-lunar injection burn,” they added. As its name suggests, that maneuver will put Orion on course for the moon, which it will fly by on a “free return” trajectory. The capsule will use lunar gravity to slingshot itself back toward Earth, without the need for another significant engine burn.

Orion’s power requirements further constrain the mission’s launch window.

“The trajectory for a given day must ensure Orion is not in darkness for more than 90 minutes at a time so that the solar array wings can receive and convert sunlight to electricity, and the spacecraft can maintain an optimal temperature range,” NASA officials wrote. “Mission planners eliminate potential launch dates that would send Orion into extended eclipses during the flight.”

Finally, viable launch windows also have to take into account Orion’s return to Earth, which will feature a specific entry profile.

NASA is not officially targeting any of the March or April launch dates; that won’t happen until Artemis 2 passes its wet dress rehearsal, a two-day-long practice run of the key operations that will occur in the leadup to liftoff.

The Artemis 2 team has already conducted one wet dress, which ended early on Feb. 2 due to a leak of liquid hydrogen propellant. That outcome took February out of consideration for the Artemis 2 launch. A second try is expected soon, perhaps as soon as this weekend.

And don’t worry if it takes a while to get the hydrogen leak under control. The same happened on Artemis 1, a delayed but ultimately successful uncrewed mission to lunar orbit that flew in late 2022. There are potential Artemis 2 launch dates beyond April; NASA just hasn’t published them yet.

“We’ve got opportunities in every month,” Lori Glaze, acting associate administrator for NASA’s Exploration Systems Development Mission Directorate, said during a press conference on Feb. 3. “So, you know, if necessary, we’ll go beyond that.”

Whenever Artemis 2 flies, the mission will be historic: No astronaut has been beyond low Earth orbit since December 1972, when the four Apollo 17 astronauts returned from the moon.