Why NASA Is Trying to Dodge the Moon

The biggest, most powerful space telescope in history is currently sitting on top of a rocket in French Guiana, on the northeastern coast of South America, awaiting its blazing departure from this planet. The James Webb Space Telescope is designed to point its 18 gold-coated mirrors into the darkness and reveal hidden wonders in the universe. But its last few months on Earth have been a little stressful.

The Webb telescope arrived at its launch site in October unscathed after a days-long journey at sea. Yay! But then a hardware malfunction during launch prep jolted and shook the entire observatory, sparking fears that something inside might have been damaged. Yikes! Technicians checked out Webb and eventually deemed it fine, so they proceeded with fueling the observatory and hoisting it on top of its rocket. Great news! But now they’ve discovered a communications issue between the observatory and the rocket, which have to talk to each other in order to reach space. Oh no! It’s as if the entire astronomy community has piled into a car, and their driver, a $10 billion space telescope, keeps alternating between pressing the gas and hitting the brakes, determined to lurch all the way to their final destination.

The communications problem, which technicians were still troubleshooting as of this morning, has pushed Webb’s blastoff back a couple of days, to December 24. If new problems arise, the launch could be delayed again, to Christmas Day or sometime later in December. At this point, a reasonable observer might wonder whether the people in charge should just postpone the launch until January. Why not take a break, let everyone working on the mission enjoy the holidays, and then try again in the new year?

Well, if the schedule slips to January, program managers could run into a new kind of obstacle, one that no amount of troubleshooting can avoid: the moon. Our very own satellite, lovely and gray and minding its own business, could thwart the multibillion-dollar mission if the Webb telescope were to launch at the wrong time.

[Read: The biggest ‘oh no’ moment in the solar system]

The Webb telescope is headed to a spot about a million miles from Earth, four times farther than the moon. To get there, it must follow a specific trajectory, nudging itself along the way with the help of its propulsion system. And during this journey, depending on where the moon is in its own orbit around Earth, our celestial companion can get in Webb’s way, explains Karen Richon, a flight-dynamics engineer at NASA’s Goddard Space Flight Center who has provided analyses on Webb’s launch trajectory for a decade. If the moon comes too close to Webb’s path, its gravity will tug on the observatory. “It either pulls us back, because it wants to try to capture us into orbit, or it gives some acceleration,” Richon told me.

Either effect could be bad news for the telescope. A tug backward would require Webb to expend more fuel than planned just to stay on track, which could, in the long run, shorten the observatory’s operational lifespan. A boost could be helpful, and even save Webb some fuel, but it could also send the observatory toward the wrong orbit altogether. The Webb telescope’s trajectory is so sensitive, Richon said, that, in addition to the moon, engineers even have to take into account the gravitational forces of the other planets in the solar system. If Webb struggles to reach its intended orbit, it risks becoming a very shiny, very expensive piece of space junk.

Richon and other engineers are prepared for some tiny deviations from their preferred trajectory. They’re planning to closely monitor where exactly the rocket deposits Webb in space, about a half hour after liftoff, and use the observatory’s thrusters to make adjustments as needed. But adding the moon to the mix would be a mess, which is why mission managers want to avoid it altogether. The moon becomes inconvenient once a month, and for December, the risk has already passed. But if Webb hasn’t launched by New Year’s Day, it’ll have just about a week to do so before the moon makes its move, closing the launch window sometime between January 9 and 13.

[Read: Who would kidnap a space telescope?]

Richon says she is hopeful the telescope can find a good launch window before that deadline. But Webb can’t just lift off any old time in the next few weeks. The observatory can launch only during a certain time of day—morning in French Guiana. “We have at least 30 minutes every day through January 6,” Richon told me. Arianespace, the company that has provided the Ariane 5 rocket at the launch site in French Guiana, has vetoed the two following days, Richon said. And, after that, well, there’s our very beautiful, very rude moon. Richon has run the trajectory simulations out until early February, just in case.

The Webb telescope still has several important checkpoints to clear before it’s ready to fly. Once technicians figure out the latest glitch, they will enclose the observatory, its gleaming mirrors all folded up for the ride, inside the nose cone of the rocket. No rocket has carried a payload quite like Webb before, so engineers had to redesign this part to suit the observatory, Daniel de Chambure, a project manager at the European Space Agency who oversees Ariane 5 launch operations, told me. “We had to develop a specific procedure to be able to do this encapsulation in the safest way,” said de Chambure, who has been in French Guiana preparing for the launch since early November. After that, the rocket and its precious cargo will have a dress rehearsal, final reviews, a careful transport to the launchpad—and technicians must check on the hardware at every step.

There may be more lurching, more stops and starts, to come. NASA, along with its partners in this international effort—Arianespace, the European Space Agency, the Canadian Space Agency—are assembling many of these parts for the first time, in a way they couldn’t really practice until now. After about a quarter century of development, the Webb team is closing in on the finish line, but that’s precisely why mission managers seem willing to stop at any moment if they discover any new surprises. Unlike the Hubble telescope, Webb wasn’t designed to be repaired in orbit. When it’s been 25 years, what’s another few days?

It’s another few days of keeping technicians and engineers and other officials in a tiny seaside town with very few hotels. It’s extra spending for an already over-budget project. And it’s running the risk that while Webb waits, whether for technicians to fix something or for the moon to get the heck out of the way, something else could go wrong. “We definitely want to get there before the moon starts affecting our trajectory too much,” Richon said. But “space doesn’t care about the holidays.”

This article was originally published in The Atlantic. Sign up for their newsletter.