The crew on the International Space Station had to make emergency repairs to its toilet—and use an emergency urinal—after a part responsible for collecting liquid waste malfunctioned. What do astronauts do with all the sewage that gets collected?
Send it back to Earth or eject it into outer space. Space toilets separate solid and liquid waste, and the solid waste is tightly bagged until it can be removed. (For detailed accounts of how you go to the bathroom in space in the first place, check out these descriptions by astronauts.) On a space shuttle, solid waste is compressed, stored, and then brought back to Earth. The space station, on the other hand, deposits the solid waste onto an unmanned vehicle (known as a "Progress module") that is eventually released toward Earth, burning up on its re-entry into the atmosphere.
Historically, space vehicles have released urine overboard. Because of the low temperatures outside, the wastewater quickly freezes into small crystals. (Apollo 9 astronaut Rusty Schweickart once described a urine dump at sunset as "the most beautiful sight in orbit.") But urine, like just about anything else humans leave in space, can turn into orbital debris. A study conducted off the Mir space station in the mid-1990s identified "flake depressions" suspected to be caused by human waste. And even tiny objects can cause damage if they are orbiting at high velocity: In one 1983 mission, a paint flake created a crack in the space shuttle Challenger's window, and wastewater was initially suspected as a possible cause of the 2003 Columbia disaster. In fact, the risks posed by frozen pee are limited: Orbital-debris experts say it is likely to sublimate from a solid form directly into gas within an orbit or two. In addition, waste that is released from the shuttle should be moving in the same direction as the spacecraft, limiting the possibility of a collision.
Although NASA technology has improved markedly since the days of urine collection and transfer assemblies and "Apollo bags" (which are still used as backup in case of a toilet malfunction), the space shuttle still has a system that dumps wastewater (PDF) during orbit. On the space station—which currently uses Russian technology—urine is instead sent back to Earth along with the solid waste, on a Progress module.
In the near future, astronauts may begin to recycle their waste products. Later this year, NASA will send up a new system that should be able to convert urine—along with humidity in the air—into clean water. Once it is up and running, the system should be able to recover about 90 percent of the water in the urine and provide much of the water supply needed for the space station. (The rest of the urine becomes a concentrated brine disposed of as usual.) And in the future, the feces created in space may not go to waste, either: NASA is funding research on a fuel cell that would convert human waste into electricity.
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Explainer thanks Nicole Cloutier-Lemasters and Gene Stansbery at NASA's Johnson Space Center and Bob Bagdigian at NASA's Marshall Space Center.