Aside from installing a water-recycling system and a new fridge on the International Space Station, the three astronauts on board the Endeavour will attempt some repairs which could potentially get messy. They’re trying to fix a joint on the space station that if not fixed could make the ISS near useless as a science laboratory. The mission will take place at 7:55 p.m. on Friday and will launch from Kennedy Space Center.

The mission is the start of an ongoing two-year process to repair one of the gravest problems the ISS has had during its ten years of existence. The other problem was last year’s torn solar wing.

"On the outside, we have some never before attempted repair work," said Endeavour Commander Chris Ferguson.

The issue, which was discovered by NASA over a year ago, affects one of the two joins which rotate the ISS’ American solar wings, much like steamboat paddle wheels, so they can continuously track the sun and maximize power.

It has been noticed that the power needed to spin the joint increased, and later examinations by spacewalkers confirmed damage by clusters of small metal shavings in its race ring, a wagon-wheel shaped gear.

Scientists have studied more than 100 potential causes for damage, and they believe that it is due to gold-plated bearings which roll across the ring’s surface did not sufficiently lubricate it. The grinding and vibration that subsequently caused by it almost cracked the hardened steel ring and worn out surrounding parts.

NASA will not switch to a backup ring, but will instead in 2010 install a replacement ring to prolong the joint’s life. Ten spacewalks will be required to complete the process. The rings’ diameter is about 10 feet and their weight is 229 pounds. They weren’t designed for in-orbit repairs. The repairs aren’t expected to be very messy due to precautions taken to limit spills, and the rather dry grease used to oil the gears.

The repairs won’t hold up the ISS’ completion, but if the repairs fail, the outpost won’t always have enough power to support the science experiment that the crewmembers will undertake.

"If I have to live with this long term, then it may have the effect of reducing how much research I can get done," said International Space Station program manager Mike Suffredini.

The left and right rotary joints will be repaired during four spacewalks and over 26 hours by spacewalkers. They’ll remove 11 of 12 bearing assemblies on the starboard side, the one with the damage, and will take them in for study. The last set had been replaced previously.

The astronauts will remove each bearing assembly and put it in a sack, then using two terrycloth mitts (one dry, one greasy) and a scraper they’ll clean off the caked metal shavings. They’ll then use a modified caulking gun to apply a thin grease layer on the ring surfaces that bearings roll over. Finally, they’ll install new bearings.

The fourth spacewalk will focus on the station’s port rotary joint. There have been no problems there, but NASA is greasing that up as well, to prevent future problems.

"It is a fine, delicate task in the sense that you don't want to get grease all over yourself and everywhere else," lead spacewalker Heidemarie Stefanyshyn-Piper said of her spacewalk duties. "So you've just got to work slowly and very deliberately in what you're doing."