Forty years ago, in what is perhaps humankind's greatest accomplishment, a man stepped foot on the surface of the moon. On Sunday, the crew of the International Space Station (ISS) hung an "Out of Service" sign on the toilet.

The main toilet on the station, a multi-million dollar Russian-built unit, broke down soon after the space shuttle Endeavour arrived for its mission. Astronauts from the Endeavour have been forced to use the bathroom aboard the shuttle, while the 13 residents of the ISS are using a back-up unit on the Russian side of the station.

"We don't yet know the extent of the problem," flight director Brian Smith said, but he was optimistic that the toilet problems were "not going to be an issue." As an emergency backup, NASA said it also has urine collection bags left over from the Apollo

Scientists aren't yet sure what to blame for the toilet breakdown: bad wiring, a faulty pump or Pavel, the Russian cosmonaut with a penchant for fiber-rich cereals.