'Pollution evident from space'

But Discovery Commander Eileen Collins, who landed with her six crewmates early on Tuesday in California after a 14-day historic mission, said she was sobered by signs of pollution over Africa.

"One of the things I saw was in Africa, the massive burning taking place in the central part of Africa," Collins told a news conference in California. "I'm not sure why they do that."

In Africa, Madagascar, and elsewhere, she said, "You can see deforestation taking place, you can see it in the rivers and streams".

Collins said that the rivers were brown "instead of blue ... you can see the erosion" flowing out into the ocean.

Many residents of rural Africa cut down sparse tree stands for fuel. This year's Nobel Peace Prize went to Kenyan Wangari Maathai, an environmentalist who for years fought a one-woman battle to plant new trees to prevent erosion.