How NASA Achieved the Historic Artemis II Splashdown

Every crewed mission to the moon starts with fire and ends with water. Artemis II was no different. The spacecraft and crew began their journey on April 1, under the power of six rocket engines pouring flames that generated 8.8 million lb. of thrust. The mission ended Friday night, April 10, at 8:07 p.m. EDT, off the coast of San Diego. The Orion capsule hissed into the ocean, gently settling down atop the waves at just 17 miles per hour, under the control of three 116-ft. diameter parachutes. Less than two hours later the four astronauts—commander Reid Wiseman, pilot Victor Glover, and mission specialists Christina Koch and Jeremy Hansen—will be safely on the deck of the USS John P. Murtha, successfully concluding the first crewed mission to the moon in 54 years. 

The astronauts’ safe return tonight was by no means a sure thing. They were flying a spacecraft that had never carried crew before and entering the atmosphere on a trajectory that had never been attempted before, protected by a heat shield that had caused engineers worry ever since the flight of Artemis I in 2022.

The trip home began in earnest on April 7, after Artemis II had completed its swing around the lunar far side and traveled about 40,000 of the 250,000 homeward miles that separate the Earth and the moon. At that point in space, the ship left the moon’s so-called sphere of influence, entering the region where the Earth’s distant but more powerful gravity takes over from the moon’s closer but weaker pull. When the astronauts reached that spot they were traveling approximately 2,100 miles per hour—but that speed would change dramatically.

During the remainder of the trip the crew was not so much flying toward the Earth as falling toward it, following a gravitational plunge that saw them moving faster and faster as they drew closer and closer until they were moving at a blistering 24,000 mph—or 6,500 mph faster than spacecraft in orbit around the Earth move. That’s a lot of energy with which to crash into Earth’s thick atmosphere, and the crew spent all of today preparing for that high-stakes rendezvous.

Read more: The Significance of The Artemis II Moon Photos

The astronauts’ workday began at 11:35 a.m. EDT, when NASA piped up the song “Lonesome Drifter,” by Charly Crockett—the latest in a playlist of wake-up calls that, on earlier days, included "Working Class Heroes (Work)" by CeeLo Green, "Good Morning" by Mandisa and TobyMac, and “Under Pressure” by Queen and David Bowie. The crew spent the next few hours stowing 10 days’ worth of gear and tools, securing all loose objects to ensure that nothing would fly free during the violent shaking and the increase in gravity that would mark the capsule’s entry into the atmosphere. At 2:53 p.m., they fired their thrusters for what is called the Return Trajectory Correction 3 Burn, which put them on the proper path to enter the atmosphere over Hawaii and hit the waters off San Diego.

At just after 5:00 p.m., the crew wriggled into their bright orange pressure and survival suits, and prepared for the next major milestone, at 7:33 p.m., when they separated their conical Orion command module from the cylindrical service module, which contained their main engine and their four solar panels. Then, at 7:53 p.m., when the ship was just under 76 miles above the surface of the Earth, the crew oriented their capsule rump forward, so that the heat shield at the base of the module would be the first part of the ship to encounter the atmosphere. This is when the worrying began.

What is the Artemis II heat shield made of?

As with the Apollo spacecraft, Orion’s heat shield is made principally of a material known as Avcoat, a combination of epoxy resin and silica fibers, designed to absorb the fires of reentry and slowly burn off, or ablate, carrying the reentry heat away from the capsule and crew. That’s a big job, given that the heat shield must withstand temperatures of 5,000°F, half as hot as the surface of the sun, and significantly hotter than the 3,000°F a spacecraft returning from Earth orbit endures. Unlike Apollo’s heat shields, however, Orion’s had serious R&D problems.

The Apollo spacecraft measured 12 ft., 10 in., at the base; Orion is larger, at 16.5 ft. During construction, the base of the Apollo was covered in a honeycomb-like mold with 360,000 individual cells, all of which were filled with Avcoat. The larger Orion is instead covered with 200 Avcoat tiles. That, as it turned out, did not work so well.

In November of 2022, the Artemis I spacecraft took off for an uncrewed flight around the moon, as a first test of the Orion capsule and the giant Space Launch System moon rocket. The mission flew flawlessly—until it didn’t. When the crew capsule was recovered from the ocean, technicians were alarmed to find that the heat shield was covered with more than 100 cracks and large divots where the Avcoat had failed and flaked away. Any one of those locations could have allowed the heat of reentry to burn through the aluminum alloy walls of the spacecraft, killing any luckless crew that may have been inside, in much the kind of reentry breakup that doomed the shuttle Columbia and its seven-person crew in 2003. It was little more than dumb luck that none of the flaws in the Artemis I heat shield led to such a burn-through event.

To correct the problem on Artemis II, NASA was faced with two choices: reinvent the heat shield entirely—which would have delayed the mission by two years or more—or modify the shield they had and change the reentry route to reduce the heat load. They chose the second option. A 2024 report by NASA’s Office of the Inspector General found that the heat shield failure was caused at least in part by the build up of hot gasses beneath the Avcoat during reentry. For Artemis II, the agency thus used a different, more porous Avcoat formulation that would let the gasses escape. 

What was the Artemis II re-entry trajectory?

Additionally, NASA eased Artemis II’s coast back to Earth. Artemis I followed a reentry route pioneered by the Apollo missions called skip-entry. Rather than plunging directly into the atmosphere the way slower-moving Earth-orbiting spacecraft do, the Apollo ships roller-coastered into the atmosphere, then up and out again, and then back in, bleeding off heat and gravitational forces along the way. That proved too much for Artemis I. So for Artemis II, NASA split the difference, not quite following the as-the-crow-flies route of Earth-orbiting craft, but taking a somewhat shallower path than Artemis I did. In the run-up to today’s reentry, NASA brass knew they were playing with fire—literally—but remained cautiously optimistic about their plan.

“[There are] thirteen things that have to go right,” said Artemis II lead flight director Jeff Radigan at a Thursday, April 9 press conference. “I have a whole checklist in my head that we’re going through—all the things that have to happen.”

“It’s impossible to say that you don’t have irrational fears left,” said Amit Kshatriya, NASA associate administrator. “I would tell you I don’t have any rational fears about what’s going to happen. We’ve done the work we have to, and we have full confidence in the recovery team, the flight control team, and the analysis of the work we did.”

That confidence was rewarded tonight, with a safe and whole crew returning to the planet that sent them aloft. In an early interview shortly after the astronauts were selected for the mission in 2023, TIME asked mission pilot Glover about the larger historical significance of visiting the moon after more than half a century. “Let’s get to splashdown successfully,” he said. “Then maybe we can revisit the question.”

Splashdown has been achieved. That history can now be written.