Once they are on their way, coming home will not be easy or quick – the astronauts are now committed to their long journey around the Moon and back.
During this time, the crew will continue to evaluate the spacecraft systems, but the astronauts themselves will also be evaluated and monitored.
It is an opportunity to use the crew as guinea pigs – experiments on board will reveal how their bodies are affected this far out in space.
Radiation is a key concern, as the Sun can hurl out damaging, high-energy particles.
The astronauts will carry a device called a dosimeter to see how much radiation they are exposed to. They will also practise using their radiation shelter, which is under the floor of the spacecraft.
The crew need to know how to get into it quickly if a solar storm was coming their way.
Another practice session to help prepare for the unexpected will involve the astronauts climbing into their bright orange spacesuits, called the Orion Crew Survival System (OCSS).
These suits are worn to protect the crew during launch and re-entry, and also act as vital protection if there is a problem with the capsule.
The suit is like a mini-wearable spacecraft, pressurised with inbuilt life-support systems.
