The US space agency Nasa is all set to bring home its Orion capsule.
The spaceship, which has been on a three-week journey around the Moon, is targeting a splashdown in the Pacific Ocean off California.
Uncrewed for this test flight, the capsule is expected to carry astronauts on its next outing, assuming everything works as planned in the coming hours.
The parachute-assisted drop into the sea should occur at about 09:40 local time (17:40 GMT).
The exercise is part of Nasa’s Artemis programme, a quest to take people back to the lunar surface later this decade.
Fittingly, Sunday marks exactly 50 years since this feat was last achieved by the crew of Apollo 17.
Orion was lined up for the day’s event by its European propulsion module.
This part of the spacecraft executed a big engine manoeuvre last Monday that pushed the ship away from the Moon and in the direction of Earth.
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.
What happens next is critical to the success of the whole endeavour.
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.
“This is our priority one objective, for a reason,” said Artemis mission manager Mike Sarafin.
“The heatshield is a safety-critical piece of equipment. It is designed to protect the spacecraft and its passengers, the astronauts on board. So it needs to work.” (BBC)