Truly great

What, one wonders, would modern documentary makers do without computer graphics? Tonight, the technology is employed to bring to life extraordinary human achievements of the past and future.

Secrets Of The Great Wall (World, 10pm) is a rare documentary delving into China's history. To do so requires the close co-operation of those who control access to the images in China, which is not often readily forthcoming.

But the makers of this documentary enlisted the support of The Great Wall Society, the mainland's foremost academic centre for Great Wall studies, which acted as their historical and cultural consultants, and provided access to archaeological digs.

Satellite images from space, collected by China's Remote Sensing Bureau in association with Nasa, were also the key to piecing together the wall, reconstructed with computer graphics.

This programme reveals sections of the wall long lost beneath the northern deserts and grasslands, how it was made and the role it played.

Space Colonies Living Among The Stars (Pearl, 8.30pm) takes a leaf out of George Lucas' book to illustrate what it could be like to live on Mars or beyond.

The domed biospheres that it depicts are not just science fiction, but what maverick scientists and philosophers believe are not only possible, but essential to our species' development.

Aerospace engineer Robert Zubrin and philosopher Marshall T Savage, who grew up during the race to the moon, had expected to be among the first colonists to live on Mars.

While Zubrin is disappointed that Nasa's human exploration of space was dismantled after 1972, he has been busy working out practical models as to how people can travel to Mars and turn it into a green planet.

This dramatised documentary from Discovery focuses on what could be possible. Arguments against the plausibility of such a venture are ignored. The vision of life in space may be thought-provoking, but Star Wars remains a more entertaining version of the scenario.

The movie Alive (Pearl, 9.30pm) is a reminder of just how difficult it is to survive in our world's most hostile places, let alone on Mars. It recounts the true, harrowing story of how some members of a Uruguayan college rugby team managed to survive after their plane crashed on Friday, October 13, 1972, high in the Andes mountains.

For those who remained alive after the initial impact, the crash was not the problem, but the fact that there was as little food available on the mountain as on Mars was. Once the last supplies in the aircraft were finished, the only option was the bodies of those who had died.

Frank Marshall's disaster movie is adapted from Piers Paul Read's book, and works hard to answer the moral questions posed by the act of cannibalism. Eating a friend turns out to have been the bravest, most moving thing to do in the circumstances.