Imagine being cramped in the back seat of a car for several hours as the twists and turns of the highway intensify already uncomfortable circumstances.

Now imagine similar conditions on a 10-day trip, traveling a distance of 685,000 miles at over 20,000 miles per hour.

That's what lies ahead for the four astronauts on the Artemis II mission onboard the Orion crew module.

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-ABC News' Mason Leib

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

All of the Artemis II SLS rocket fuel tanks are fully loaded and ground crews are in the replenishment phase, which replaces the cryogenic propellent that naturally boils off.

They will soon deploy the close-out crew and prepare for the flight crew to board the Orion crew module.

The crew will then walk out of the Operations and Checkout Building and head to the spacecraft.

Weather remains at an 80% likelihood for a go for launch. Tonight's window is between 6:24 p.m. ET and 8:24 p.m. ET from Florida's Kennedy Space Center Launch Complex 39B.

-ABC News' Matthew Glasser

Despite all the preparation, safety systems and redundancies, the nature of human spaceflight is inherently risky, according to experts.

"I think it'd make everybody feel comfortable to say that all the risk was knocked down to zero," Danny Olivas, former NASA astronaut and engineer, told ABC News. "The truth is it is not and never will be."

NASA is using a variety of tools and systems to ensure crew safety from launch to splashdown, including customized lifesaving spacesuits, a launch abort system, radiation monitoring, an emergency egress system and other contingencies.

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-ABC News' Briana Alvarado

Weather is expected to be a green light at the Kennedy Space Center in Cape Canaveral, Florida, for the Artemis II mission to launch.

The two-hour window from 6:24 to 8:24 p.m. will feature temperatures in the mid to low 70s, forecasters say.

Winds will be between 8 and 16 mph, with a rare gust up to 20 mph, max.

There will be a mix of clouds and clear skies with no forecast chance for rain or lightning.

-ABC News’ Kenton Gewecke