An international team of researchers has entered a mockup of a spacecraft to spend 105 days in isolation to simulate conditions on a flight to Mars.

The all-male crew of four Russians, a German and a Frenchman say they feel excited to help pave the way for a Mars mission.

They said before entering the craft Tuesday that they will focus on medical and physiological experiments. Their communications with the mission controllers will have 20-minute delays to imitate a real flight.

The experiment won't try to tackle the issue of shielding the crew from deadly radiation — the biggest technological challenge in building spacecraft for a future Mars mission.

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