Space shuttle Atlantis undocked from the International Space Station today for its three-day return voyage to terra firma.

All systems are clear for the STS-129 mission's planned landing at Kennedy Space Center on Friday morning — with the small exception of a blockage in the craft's waste water dump line.

During a purge of the Atlantis' waste water tank on Wednesday, mission control noticed a reduction of flow from the waste water store nozzle, located on the left side of the shuttle. Astronauts used a camera on the spacecraft's robotic boom arm to see if an ice formation was clogging the plumbing, but didn't see any major obstructions.

"Capcom Megan McArthur told Commander Charles Hobaugh that it appears there is a slight discoloration that could be frost, but no ice. It is likely that there is a blockage in the line, which will not be a concern for a planned Friday landing," NASA said on its website.

NASA reckons it's unlikely that further waste dumps will be necessary before the landing. Our initial estimates confirm this hypothesis — barring a major Tang binge during the return journey.

Solid waste from the shuttle's toilet is compacted, dried and stored in bags where it is returned to Earth for cremation. Liquid waste from the toilet along with condensed moisture collected from the spacecraft's humidity sensor goes to the waste water tank, located beneath the crew compartment mid-deck floor.

The waste water tank's usable capacity is about 165 pounds and measures 35.5 inches long by 15.5 inches in diameter, according to NASA. A human bladder can hold about 2.2 pounds of urine at maximum capacity.

With seven crew members aboard Atlantis, we will assume each will have a full bladder twice per day, totaling 30.8 pounds of wee. On a three-day journey, (that's 92.4 pounds total), it leaves a good 72.6 pounds left over for collected shuttle humidity. ®

Bootnote

You'll just have to forgive the use of pounds in our estimates. That's the standard unit of mass used for measuring astronaut urine.

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