A spacecraft built to take astronauts to Mars is being put through its paces.

The American space agency's Orion craft is built to be carried by a Delta IV heavy-lift rocket from Florida.

Scientists hope Orion will return humans to the moon by 2020 and transport the first visitors to Mars in the 2030s.

But the 140 million mile journey to Mars will involve extreme hazards threatening the lives, health and psychological well-being of the astronauts.

:: How big is the radiation risk?

Space is filled with dangerous radiation that the Earth's atmosphere shields us from. The greatest threat comes from high energy streams of subatomic particles pouring out of the sun that can damage DNA, leading to cancer.

Nasa's Curiosity Mars rover was hit by 0.66 sieverts of radiation during its 253 trip to Mars. That's the equivalent of receiving a whole body CT X-ray scan every five or six days.

Intense bursts of radiation and particles erupting from the sun, in large solar flares or coronal mass ejections, have the power to disrupt electrical equipment and deliver potentially lethal doses.

Radiation shielding coating the spacecraft can offer some protection. Cosmic radiation from outside the Solar System is harder to avoid but tends to occur at a low level.

:: Is there a danger of being hit by a meteorite?

Big and small rocks are continually flying around the Solar System, millions of which go undetected. On a long journey through space the risk of being hit is not negligible, and even a tiny meteorite could wreak unimaginable damage.

Bigger objects can be steered around if detected in advance, and Nasa has been developing armour-like materials and double-layer walls to protect against smaller objects.

The three-year long Mariner 4 mission ended in 1967 when it was peppered by a stream of sand-like micrometeorites travelling at up to 150,000 mph.

:: How easy is it to land on Mars?

The answer is, not easy at all, which is why so many Mars missions have failed. Overall the success rate of landing on Mars is only about 30%.

This is one area where it pays off to have humans on board rather than relying on computers. During the Apollo moon missions, Nasa never lost a single lander, despite some very close shaves.

Mars is much more challenging because it has an atmosphere and appreciable gravity.

The Red Planet's thin atmosphere presents special problems, With a pressure only one hundredth that of the Earth, there is too much of it to avoid burning up and not enough to provide a good braking effect.

When the Curiosity rover's entry vehicle hit the Martian atmosphere at 13,300 mph it was protected by the largest aeroshell ever built, which both prevented it burning up and helped slow it down with friction.

:: How bad is a Mars trip for your health?

Without gravity, bones become brittle and muscles start to waste away. There is also evidence that gravity affects the way the brain works.

The astronauts will have to undergo strict exercise regimes to make up for the lack of gravity. Another solution would be to provide at least some degree of artificial gravity by spinning the spacecraft as it travels.

Psychological health is considered a very imported issue for long distance space travellers.

The astronauts will need to be mentally tough to cope with being so far from home in the knowledge that so far out in space there can be no hope of rescue.

The crew will also have to be capable of working as a team and highly disciplined. Nasa is looking at using voice patterns and biomonitors to spot signs of disputes before they get out of hand.

:: Why go to Mars at all?

Many experts say only so much can be done with robots. Humans are just so much better at finding and selecting samples, and spotting the unexpected.

Others argue that it will inspire children and unite humanity in a common adventure.

Some say that confining ourselves to a planet with limited resources is simply not an option if the human race is to survive.