Since the ancients named it for the God of War,

it has had a unique hold over the human imagination.

Even before humans set foot on the moon,

it was seen as the next step but why?

There are similarities between Mars and Earth.

It has a 24-hour day and four seasons.

Though its air is too thin to breathe,

and its surface too cold for unsheltered life,

it is far more astronaut-friendly than any of the other planets.

It has huge canyons and vast volcanoes.

It's riverbeds, deltas, and dried lakes speak of a past more watery than its arid present.

Yet, although it is scarce and briny,

there are still some liquid water on Mars today, and water is key to life.

The possibility that Mars once had life

or might just possibly still have it today

is one of the reasons more space probes have been sent there than to any other planet.

NASA's InSight probe,

which landed in November is the latest,

and Europe's ExoMars rover will arrive next year.

NASA talks of sending humans to Mars on a new spacecraft within the next two decades.

Elon Musk's space technology company SpaceX has plans as early as 2024.

Such missions with crews would be among the biggest technical challenges humans have ever undertaken.

The journey can only be made when the planets are properly aligned.

That only occurs roughly every 26 months,

but even then, it takes around nine months

to travel from one planet to the other.

The challenge of a life-support system that can operate for years without resupply is daunting.

The cost will be high and the hurdles immense.

But the challenge is so exciting,

it seems certain that some will try it,

and quite likely that some will die trying.

Coming up next Thursday, the hunt for oceans in space.