Only certain dates are appropriate for launch, and for this attempt, NASA is aiming for a two-hour window that opens this morning (Australian time).

If it doesn’t launch today, NASA will try for a similar window tomorrow morning, and so on through to next Tuesday, April 7.

There are a couple of reasons early April suits NASA best.

Firstly, the launch dates are timed to match when the Earth, Moon and Sun are all aligned.

That is so the astronauts can get into the right location for what is called a trans-lunar injection — the final burn that will send them towards the position where they will end up in the Moon’s orbit.

The Orion spacecraft cannot be in darkness for more than 90 minutes at a time. (nasa.gov)

The dates are also timed to ensure the Sun isn’t blocked by Earth’s shadow during the mission.

The Orion spacecraft is powered by solar panels, so it cannot be in darkness for more than 90 minutes at a time.

Because space is so cold, without power, the temperature inside the cabin could drop to a level unsafe for humans.