Two astronauts cut short their spacewalk and hurried back to the safety of the international space station on Wednesday after a suit problem resulted in rising carbon dioxide levels for one of the men. NASA officials stressed that spacewalker Christopher Cassidy was never in any danger and experienced no symptoms of carbon dioxide buildup. The trouble cropped up late in Wednesday’s spacewalk, the third for shuttle Endeavour’s crew. Mission Control said that the canister for removing carbon dioxide from Cassidy’s suit did not seem to be working properly.