The Russian-made toilet began malfunctioning last week, but the exact cause in unclear.

Until the problem is resolved, astronauts have been improvising by using plastic bags to deal with waste.

But their ordeal should end soon -- Nasa will launch the Discovery space shuttle this weekend with replacement parts on board,

"We will be taking some spare parts up," said Allard Beutel, spokesperson for Nasa.

"You can imagine you are having guests over and your one and only bathroom is broken."

"Clearly this is something you want to have working."

To make room for the replacement parts, Nasa are leaving behind items of non-essential cargo -- including several wrenches and a spare part for the space station's oxygen generator.

"Clearly, having a working toilet is a priority for us, so some of these things that we didn't need for the next six months or so could wait," said Scott Higginbotham, Discovery's payload manager.

Discovery's commander sounded an optomistic note as he arrived at the launch site.

"We hear it's in great shape," said Mark Kelly, referring to the shuttle's readiness for flight.

"As soon as we get a couple more spare parts that I'm sure some of you guys have heard about...we're going to be all ready to go."