CAPE CANAVERAL, Florida - A pair of spacewalkers 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 that Nicole Stott and Danny Olivas had about mass -- everything is weightless up there, after all -- involved the huge ammonia tank they needed to disconnect.

A new fully loaded tank will be installed on the second spacewalk of the mission Thursday night.

The tanks are massive by spacewalking standards: nearly 5 feet (1.5 metres) long, 7 feet (2 metres) wide and 4 feet (1.2 metres) high. The old one has been up there since 2002.

Shortly before the spacewalk began Tuesday evening, the space station officially got its new $5 million treadmill that is named for Colbert. It was one of the first items to be unloaded from the moving van that was delivered by shuttle Discovery.

Colbert, known for his Comedy Central program "The Colbert Report," was out of the country Tuesday. But he said through his New York publicist that "my treadmill will ... help trim down those famously fat astronauts." The word "my" was underlined. "Lay off the Tang, Chubby!"

The Colbert treadmill appeared to be the chubby one. As the treadmill was being pushed across the threshold, it bumped into a camera and dislodged it. An astronaut nudged the camera back into place.

Colbert originally wanted a space station room named after himself and even won an online vote earlier this year for naming rights. NASA instead went with Tranquility for the yet-to-be-launched chamber, in honour of Neil Armstrong and Buzz Aldrin's landing on the moon's Sea of Tranquility 40 years ago this summer. The treadmill became COLBERT, short for Combined Operational Load Bearing External Resistance Treadmill.

The treadmill is in more than 100 pieces. The bags containing all those parts will remain in a corner of the space station until September, when the space station crew has time to put together the running machine.

Stott, the space station's newest resident, hitched a ride up aboard Discovery. She will spend the next three months in orbit.