Earth bears scars of human destruction - astronaut

* Fifth and final spacewalk slated for Monday

* NASA trouble-shooting problem with station air-scrubber

* Shuttle to depart station on Tuesday

CAPE CANAVERAL, Fla., July 26 (Reuters) - A Canadian astronaut aboard the International Space Station said on Sunday it looks like Earth's ice caps have melted a bit since he was last in orbit 12 years ago.

Bob Thirsk, who is two months into a planned six-month stay aboard the station, said he is mostly in awe when he looks out the window, particularly at the sliver of atmosphere wrapped around the planet.

"It's a very thin veil of atmosphere around the Earth that keeps us alive," Thirsk said during an in-flight news conference. "Most of the time when I look out the window I'm in awe. But there are some effects of the human destruction of the Earth as well."

"This is probably just a perception, but I just have the feeling that the glaciers are melting, the snow capping the mountains is less than it was 12 years ago when I saw it last time," Thrisk said. "That saddens me a little bit."

If Thrisk needs a sympathetic ear, he has 12 crewmates with him, at least until Tuesday, when visiting shuttle Endeavour astronauts are scheduled to depart.

The astronauts delivered a Japanese-built experiment platform, installed new batteries for the station's solar power system and stashed spare parts to keep the station operational after shuttles are retired next year after seven more flights.

The $100 billion station, a project of 16 nations, is nearing completion after more than a decade of work.

Endeavour astronauts Chris Cassidy and Tom Marshburn are scheduled for a fifth spacewalk Monday to rewire a station gyroscope, fix insulation on its Canadian-built robot and install television cameras needed to guide a Japanese cargo vessel into its docking port. The HTV cargo hauler is slated for its debut flight in September.

"All in all I think it's an extremely successful mission in spite of a lot of really interesting curveballs that have been thrown our way," Endeavour commander Mark Polansky told reporters.

The latest glitch occurred Saturday when the station's U.S. air-scrubber shut down, prompting NASA to call in extra flight controllers to oversee the device manually. The machine strips deadly carbon dioxide, a by-product of respiration, from the station's air.

"It's not something that we want to do long term, because (of) the number of commands we have to send from the ground. But in the short term, we've got the carbon dioxide removal system back up and running and operating at close to its normal capacity," Smith said.

A backup air-scrubber is due to be launched aboard NASA's next shuttle mission, targeted for launch in August.

Endeavour is due back at the Kennedy Space Center in Florida on Friday.