The spaceship, which has been on a three-week journey around the Moon, is targeting a splashdown in the Pacific Ocean off California.
The parachute-assisted drop into the sea should occur at about 09:40 local time (17:40 GMT).
The incoming velocity will be extremely high. The capsule will be moving at 40,000km/h (25,000mph) – or 32 times the speed of sound – when it touches the top of our planet’s atmosphere.
Friction and pressure on Orion’s forward-facing surface will generate temperatures likely to reach close to 3,000C (5,000F).
The shielding that covers this part of the spacecraft has to handle this challenge if Orion is ever to be trusted to carry astronauts in the future.
The USS Portland will be waiting to pick up Orion.