Florida ? A pair of spacewalkers successfully tackled a hefty tank removal job at the international space station Tuesday as their crewmates unloaded comedian Stephen Colbert’s namesake treadmill for all “those famously fat astronauts.”

The only concern Nicole Stott and Danny Olivas had about mass — everything is weightless up there, after all — involved the huge ammonia tank they needed to disconnect. They held the freed 1,300-pound tank steady until a robot arm grabbed it.

A new fully loaded tank will be installed on the second spacewalk of the mission Thursday night. The old one will be returned to Earth aboard Discovery.

The tanks are massive by spacewalking standards: Nearly 5 feet long, 7 feet wide and 4 feet high. The old one has been up there since 2002.