Here's a look at the "carefully timed sequence" of the final stages of Orion's descent, according to NASA. All times Eastern.

7:33 p.m.: Orion's crew module will separate from the European Service Module, exposing the heat shield that will protect the spacecraft as it travels through the Earth's atmosphere.

7:37 p.m.: Orion will perform an 18-second raise burn that fine-tunes the reentry angle of the spacecraft to minimize the time the heat shield will experience high temperatures.

7:53 p.m.: Orion will make first contact with the upper atmosphere, beginning a planned 6-minute communications blackout. The crew is expected to experience up to 3.9 Gs as the spacecraft travels nearly 35 times the speed of sound.

7:59 p.m.: NASA will regain communications with the crew.

8:03 p.m.: At around 22,000 feet in altitude, drogue parachutes will deploy to help slow the capsule ahead of splashdown.

8:04 p.m.: Three main parachutes will deploy at around 6,000 feet, reducing Orion's speed to less than 136 mph.

8:07 p.m.: Orion will splash down in the Pacific Ocean, off the coast of San Diego, having slowed to 20 mph.

The U.S. Geological Survey warned residents of Southern California that they should expect to hear a sonic boom from the Artemis II reentry in a few hours.

The sonic boom is expected between approximately 5-5:15 p.m. PT, it said.

It will take Orion about 14 minutes to travel the 400,000 feet from space to splashdown off the coast of San Diego.

Everyone inside the spacecraft will be laser-focused on monitoring the onboard systems during their descent through the atmosphere, according to retired NASA astronaut Barry "Butch" Wilmore.

The astronauts train for "all types of failures," so their mindset is just to concentrate on the jobs at hand, he said.

"You can't let apprehension involve you in those time frames," Wilmore told ABC News. "You have to focus on your task, whatever that might be, and you have to perform because if you don't, the consequences are pretty dire."

-ABC News' Briana Alvarado

The Orion crew module will be falling to Earth at over 24,000 mph when it encounters the atmosphere. As the atmosphere thickens, friction will slow the spacecraft. It will also create a lot of heat -- up to 5,000 degrees Fahrenheit, which is more than twice as hot as lava from a volcano -- turning Orion into a fireball.

Orion's heat shield, located on the underside of the spacecraft, will protect the spacecraft and its crew from the intense temperatures of reentry. Onboard maneuvering jets ensure Orion maintains the orientation required to keep the heat shield pointed toward Earth during reentry.

"Importantly, there's this coating on the outside that's meant to melt upon being heated rather than absorbing that energy," ABC News contributor and astrophysicist Hakeem Oluseyi said. "So, it's much in the same way that a car is meant to crumple in an accident rather than being a stiff object. You want to dissipate that energy. You need the energy to go somewhere that is not inside the capsule."

The four Artemis II astronauts are suiting up, officially stepping into their Orion Crew Survival System (OCCS) spacesuits as they prepare for their long-awaited launch.

These specialized suits are worn during the most critical phases of flight: liftoff and reentry.

The astronauts are also undergoing leak checks to ensure that their suits are airtight. They conduct a total of three leak checks in the suit-up room and then three more checks once in the Orion crew capsule, according to NASA.

-ABC News' Briana Alvarado