Historic moon mission concludes with splashdown of Orion capsule

(CNN) — The Artemis I mission — a 25½-day uncrewed test flight around the moon meant to pave the way for future astronaut missions — is coming to an end as NASA’s Orion spacecraft is expected to make an ocean splashdown Sunday.

The spacecraft is finishing the final stretch of its journey, closing in on the thick inner layer of Earth’s atmosphere after traversing 239,000 miles (385,000 kilometers) between the moon and Earth. It’s set to splash down at 12:40 p.m. ET Sunday in the Pacific Ocean off Mexico’s Baja California. NASA will air live coverage of the event, beginning at 11 a.m. ET Sunday.

The Orion capsule had been slated to splash down near San Diego, but NASA officials said Thursday that rain, wind and large waves had moved into that area, and it no longer complied with the space agency’s weather criteria.

This final step will be among the most important and dangerous legs of the mission.

“We’re not out of the woods yet. The next big test is the heat shield,” NASA Administrator Bill Nelson told CNN in a phone interview Thursday, referring to the barrier designed to protect the Orion capsule from the excruciating physics of reentering the Earth’s atmosphere.

The spacecraft will be traveling about 32 times the speed of sound (24,850 miles per hour or nearly 40,000 kilometers per hour) as it hits the air — so fast that compression waves will cause the outside of the vehicle to heat to about 5,000 degrees Fahrenheit (2,760 degrees Celsius).

The extreme heat will also cause air molecules to ionize, creating a buildup of plasma that’s expected to cause a 5½-minute communications blackout, according to Artemis I flight director Judd Frieling.

INTERACTIVE: Trace the path Artemis I will take around the moon and back

As the capsule reaches around 200,000 feet (61,000 meters) above the Earth’s surface, it will perform a roll maneuver that will briefly send the capsule back upward — sort of like skipping a rock across the surface of a lake.

There are a couple of reasons for attempting the skip maneuver.

“Skip entry gives us a consistent landing site that supports astronaut safety because it allows teams on the ground to better and faster coordinate recovery efforts,” said Joe Bomba, Lockheed Martin’s Orion aerosciences aerothermal lead, in a statement. Lockheed is NASA’s primary contractor for the Orion spacecraft.

“By dividing the heat and force of reentry into two events, skip entry also offers benefits like lessening the g-forces astronauts are subject to,” according to Lockheed, referring to the crushing forces humans experience during spaceflight.

As it embarks on its final descent, the capsule will slow down drastically, shedding thousands of miles per hour in speed until its parachutes deploy. By the time it splashes down, Orion will be traveling 20 miles per hour (32 kilometers per hour).

While there are no astronauts on this test mission — just a few mannequins equipped to gather data and a Snoopy doll — Nelson, the NASA chief, has stressed the importance of demonstrating that the capsule can make a safe return.

The space agency’s plans are to parlay the Artemis moon missions into a program that will send astronauts to Mars, a journey that will have a much faster and more daring reentry process.

Upon return from this mission, Orion will have traveled roughly 1.3 million miles (2 million kilometers) on a path that swung out to a distant lunar orbit, carrying the capsule farther than any spacecraft designed to carry humans has ever traveled.

A secondary goal of this mission was for Orion’s service module, a cylindrical attachment at the bottom of the spacecraft, to deploy 10 small satellites. But at least four of those satellites failed after being jettisoned into orbit, including a miniature lunar lander developed in Japan and one of NASA’s own payloads that was intended to be one of the first tiny satellites to explore interplanetary space.

On its trip, the spacecraft captured stunning pictures of Earth and, during two close flybys, images of the lunar surface and a mesmerizing “Earth rise.”

Nelson said if he had to give the Artemis I mission a letter grade so far, it would be an A.

“Not an A-plus, simply because we expect things to go wrong. And the good news is that when they do go wrong, NASA knows how to fix them,” Nelson said. But “if I’m a schoolteacher, I would give it an A-plus.”

If the Artemis I mission is successful, NASA will dive into the data collected on this flight and look to choose a crew for the Artemis II mission, which could take off in 2024.

Artemis II will aim to send astronauts on a similar trajectory as Artemis I, flying around the moon but not landing on its surface.

The Artemis III mission, currently slated for a 2025 launch, is expected to put boots back on the moon, and NASA officials have said it will include the first woman and first person of color to achieve such a milestone.

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