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.