Astronauts Prepare for Mission's First Spacewalk

Two astronauts will venture outside the International Space Station Tuesday to kick off their mission?s first spacewalk while their crewmates inside begin unpacking the tons of cargo delivered by the shuttle Discovery.

Discovery astronaut Danny Olivas and station crewmember Nicole Stott plan to don their NASA-issue spacesuits and float outside at 5:49 p.m. EDT (2149 GMT) to begin a 6 1/2-hour maintenance call on the orbiting laboratory. Their main chore will be the removal of an old, enormous ammonia coolant tank.

The ammonia tank weighs about 1,800 pounds (816 kg). While it will float in microgravity when removed, it will still carry the same mass as a small compact car so Stott and Olivas will have to be careful, said Zeb Scoville, lead spacewalk officer for Discovery?s mission.

?It?s a pretty complex task in terms of mass handling,? said Scoville.

The old tank is nearly empty and will be replaced with a new one to help keep the space station?s systems cool. It will take two spacewalks to make the swap, with the second one scheduled for Thursday. Astronauts must also take care to avoid contamination from toxic ammonia while working with the tank, mission managers said.

Stott and Olivas will also retrieve a suitcase-like material exposure experiment and a European experiment from the end of the station?s Columbus laboratory.

Tuesday?s spacewalk will mark the first for Stott and the third for Olivas, who is leading all three of the spacewalks planned for Discovery?s 13-day resupply mission to the space station. The two astronauts camped out in the station?s airlock early Tuesday to purge their bodies of nitrogen in a protective measure to avoid developing the bends during their spacewalk.

?I think it?s going to be really exciting,? said Stott, who joined the station crew Sunday after arriving aboard Discovery.

Delivery day at station

While Olivas and Stott work outside the station, the 11 astronauts inside the outpost will begin unloading the nearly 8 tons of fresh supplies and science gear from a cargo pod delivered by Discovery?s crew late Monday.

One of the top items on Tuesday?s move-in list is a treadmill named after TV comedian Stephen Colbert, who won the naming rights for a new space station room in an online NASA poll earlier this year, but got the exercise gear instead.

NASA dubbed the treadmill the Combined Operational Load Bearing External Resistance Treadmill, or COLBERT, as a consolation prize after naming the new space station room Tranquility in honor of the Apollo 11 moon base. The $5 million COLBERT treadmill launched to the station in a myriad of pieces to be assembled in space.

?It?s in many, many pieces,? space station flight director Ron Spencer told reporters late Monday. ?I?m not sure when we plan on putting it together.?

The treadmill components are packed in several bags that will be stored aboard the space station until mid-September, after Discovery leaves and a new unmanned Japanese cargo ship arrives. Only then, mission managers said, will astronauts have time for the 20 hours or so it will take to build the COLBERT treadmill.

Astronauts will also move an air-scrubbing device, storage bin and new astronaut bedroom the size of a large refrigerator into the space station on Tuesday.

?The crew is going to be quite busy, even after [this] mission is over, for the next few weeks,? Spencer said of the station astronauts.

• New Video - STS-128 Spacewalk Overview
• New Video - Meet the STS-128 Shuttle Astronauts
• Video - Stephen Colbert to NASA: 'No Chubby Astronauts'

SPACE.com is providing complete coverage of Discovery's STS-128 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz in New York. Click here for shuttle mission updates and a link to NASA TV.