HOUSTON, United States (AFP) — After a successful trip around the Moon, everything has been going smoothly on the Orion spacecraft’s journey back to Earth --- except for the $23 million toilet, which has gotten clogged.

The system designed to flush wastewater into space is malfunctioning, and US space agency NASA believes that a chemical reaction in the urine treatment system is the culprit.

The fecal disposal system, in a separate conduit, is working properly.

Astronaut Christina Koch said the so-called Universal Waste Management System was giving off “a burning heater smell.”

Rick Henfling, flight director for the Artemis II mission, stressed Tuesday that “the toilet remains operational. The challenge that we’re working through is evacuating the wastewater tank,” he said. “So we’re having to fall back to some other alternate means.”

Under Plan B, the four astronauts are using personal reusable containers called “collapsible contingency urine disposal devices.”

The toilet problem was reported just hours after liftoff from Cape Canaveral, Florida. Koch adjusted the system’s controls, restarted them, and that appeared to resolve the issue. “I’m proud to call myself a space plumber,” Koch said in her first briefing from the spacecraft, which is to splash down in the Pacific Ocean on Friday.

However, the astronauts are unable to flush the wastewater into space.

The issue has been a constant topic of discussion at press conferences held at the Johnson Space Center in Houston, Texas, the same center that in 1970 received astronaut Jack Swigert’s message: “Houston, we’ve had a problem,” after an oxygen tank explosion aborted Apollo 13’s Moon landing, starting a harrowing emergency that eventually brought those three astronauts back to Earth.