NASA’s Orion spacecraft is about to face its final test, and it’s a big one

NASA’s Artemis I mission is nearly complete, and so far Orion’s daring flight past the Moon has gone about as well as the space agency could have hoped. However, to get a passing grade, the mission must still pass its final test.

That final exam will come Sunday, when the spacecraft begins re-entering Earth’s atmosphere at 12:20 p.m. ET (17:20 UTC). During the next 20 minutes, before Orion splashes into the Pacific Ocean off Mexico’s Baja Peninsula, it will have to slow down from a speed of Mach 32 to essentially zero before plunging into the water.

This is no small thing. Orion has a mass of 9 metric tons, about the same as two or three large elephants. Its base, covered with a thermal shield designed to slowly carbonize during passage through the Earth’s atmosphere, must withstand temperatures close to 3,000 degrees Celsius.

There are two main elements to this reentry that NASA wants to test: the performance of that heat shield and its parachute system. For mission planners, the heat shield is the biggest concern.

“Reentry is our priority goal for a reason,” said Mike Sarafin, who leads the Artemis I mission management team. “There is no arc or aerothermal facility on Earth capable of replicating hypersonic reentry with a heat shield the size of Orion. And this is a completely new heat shield design. It’s safety-critical equipment. It’s designed to protect the spacecraft and the astronauts on board. So the heat shield has to work. We can reduce some of that risk on the ground, but not in terms of getting back to Mach 32.”

A new design

NASA tested a normal version of the Orion spacecraft in December 2014, launching it to an altitude of nearly 6,000 km. From this orbit, Orion re-entered the Earth’s atmosphere at a speed of 9 km/s. For Artemis I, Orion will return at a speed of 11 km/s. That may not seem like such a large increase, but for reentry velocity, the increase in convective and radiative elements is exponential as the velocity increases, said Jim Geffre, Orion’s vehicle integration manager.

“So the velocity effect is tremendous, and so the increase in heat load from a low Earth orbit entry at lunar velocity is much larger,” he told Ars.

The Orion vehicle flown during the EFT-1 mission featured the same basic ablative material, an epoxy known as AVCOAT that was also used by the Apollo capsules during their lunar returns half a century ago. Like the Apollo capsule, this AVCOAT material was injected into honeycomb cells at the base of the spacecraft.

For the Artemis I flight and future missions, however, NASA has switched to an AVCOAT “molded” block design for the Orion base. This was done, in part, to make the production of these heat shields faster and more efficient. Unlike the honeycomb design, these molded block heat shields can be built parallel to the base of the spacecraft, rather than needing to be placed afterwards.

There are 186 different shaped blocks on the bottom of Orion, a real puzzle to cover the bottom of the 5 meter wide spacecraft. Sunday’s re-entry will test the design of NASA’s method of filling the seams and gaps between these molded blocks.

Parachutes and jumps

Another key element of Orion’s re-entry involves the deployment of its parachutes about 1,600 meters above the Earth’s surface. These channels are intended to slow Orion down to a speed of 30 km/h as it falls into the ocean.

However, unlike Orion’s heat shield, NASA officials believe they have adequately characterized the parachutes’ risk through an extensive test campaign. Geffre said that to date, NASA has conducted 47 drop tests of Orion’s parachute system.

NASA announced Thursday that it plans to land Orion further south in the Pacific than previously planned. This is due to poorer weather conditions further north, off the coast of California. As a result, Orion will splash near Guadalupe Island, which is about 240 km west of Mexico’s Baja Peninsula.

As part of its descent, Orion will follow a skip entry technique rather than a direct descent followed by the Apollo missions. This will allow Orion to land closer to shore and subject the astronauts to lower gravitational forces (about 4 Gs) than those experienced during Apollo reentry.

NASA will provide live coverage of Orion’s return on Sunday starting at 11:00 a.m. ET (4:00 p.m. UTC), with a flyby scheduled for 12:40 p.m. ET.