It’s never mentioned in sci-fi literature, and as far as one can remember it didn’t play a part in any Star Trek episode (of either the original series or The Next Generation). But it’s a question that space scientists are increasingly having to grapple with: what exactly do you do with all the waste that accumulates on board the International Space Station, or indeed any manned space craft?

The scale of the problem is significant. One study by NASA has estimated that during a one-year mission, total food-related waste comes to a whopping 3,900 kilogrammes (kg), with disposable clothing, paper and towels accounting for nearly 2,360kg, a not-inconsiderable weight for a small craft to carry.

But this isn’t just an issue of weight – it’s also an issue of, well, space. “There really isn’t a lot of space on board the Space Station for extra things,” says Kirsten MacDonnell, who as the European Space Agency’s Project Manager of the ATV-2 (Automated Transfer Vehicle) is responsible for planning what items are brought up to the station. “They try to bring some spares to protect for failures, equipment fails over time, and you have the vital equipment that you need for the health and safety of the Space Station. And then you’re constantly having new experiments. But at the same time you have to [ask the] question: ‘Where do we put all of these things?’”

Space station waste falls into two categories. Most ‘liquid’ waste (including food waste) can be broken down via a chemical reaction, turned into a gas and then vented out into the air conditioning system and neutralised. Urine can actually be recycled back in water using the water purification system that enables 93 per cent of all the water consumed on board the Space Station to be reused.

More problematic is what is known as ‘solid’ waste. “There are a lot of empty packages – a lot of the food comes in packaging”, explains MacDonnell. “Clothing is used several times and then discarded because there is no way to wash it on board. Then a lot of cargo has to be packed with a special kind of foam to help attenuate the launch load and protect the cargo in the launch environment.

“There is also foam packaging around many items and sometimes we also use a type of bubble wrap. Then just to keep some items contained, there are pouches and zip lock bags that are used to help organisation. And once all these materials have been used, then this becomes waste in the end.”

So how is it dealt with? One thing that is a complete no-no is simply jettisoning it out into space. “Anything solid in space contributes to space debris and becomes really dangerous for any other satellites that have the same orbit or intercept the orbit in any way. We really try to avoid ejecting any solid waste from the station.” (For more on the problem of space debris, see our feature in Resource 62.)

Instead, much of the solid waste is brought down in a separate vehicle and incinerated mid-air as it enters Earth’s atmosphere. “This has a destructive re-entry into the Earth’s atmosphere and as it’s going at a certain speed it heats up and most things burn up in the atmosphere – the local power to the tanks, the cargo that is in the pressurised modules, most of it is burnt up at this point. The only things that are left that end up in the Pacific Ocean are usually the solid metallic parts, which are fairly safe.”

In 2011, with an eye to the future and the increased use of the Space Station, NASA launched its Logistics Reduction and Repurposing project (LRR), to find better long-term solutions to the problems of space waste and its disposal.

The project earmarked a number of possible solutions that are currently being worked on. One proposal that looks set to be used in the next few years is the Heat Melt Compactor, a machine that NASA has developed that heats waste to over 320 degrees Fahrenheit and compacts it into discs that are one-inch thick and eight inches in diameter. These could be held in the craft’s cargo transfer bags, but could also potentially be deployed outside the Space Station to protect it from solar flares and radiation.

Other proposals to come out of LRR include making the crew’s clothing out of polymers rather than cotton. Anti-microbial coatings combined with the polymers’ low moisture retention would mean that the garments would be able to be used for longer and would thus reduce cargo weight (plus polymer-based clothing could be put through the Heat Melt Compactor). NASA is also exploring the potential for converting space waste to energy and has looked at a number of processes including pyrolysis and gasification with a view to producing methane, which could possibly be used as rocket propellant in a closed-loop system.

But that is for the medium term. In the here and now, moves are already afoot to reduce cargo and make the best use of space onboard the Space Station. “Instead of printing everything off as they did 10-15 years ago, crew members are now using tablets and laptops”, says MacDonnell. “NASA are also conducting experiments with what they call ‘nano racks’, where you are able to perform the same experiment but just in a smaller platform. Also, instead of physically bringing down data in black boxes, they are looking at transferring the data via radio or satellite links – so, rather than a black box, you use a USB stick or something like that.”

Another solution might be a technology that is just emerging on Earth. “Earlier this year, we saw the first use of a 3D printer in space”, says Vaios Lappas, Senior Lecturer at the Surrey Space Centre and a man who has conducted research into the issue of space junk. “Long term, that is not only going to save on equipment being brought up, but means you can customise a system and make it efficient for the use that you want. It also means you could design equipment then and there on the spot! Up until now, if you wanted something to be transported to the Space Station, you’re talking about a waiting period of between three to six months.”

Of course, you might well be asking what relevance does all this have to the rest of us down here? Well, certainly the lessons learned could also have a positive impact on a lot of Earth-bound industries. “A lot of the technology used in the space station can be spun out into the marine industries”, suggests Lappas. “And these solutions could have potential for increasing recycling in areas that are volume- and mass-constrained – for example transport, shipping and the aviation industries.

“In any case, it’s important that we deal with these issues now”, he adds, “as this problem will become much more pronounced when we do longer duration flights. If we go to Mars, taking waste back down to Earth simply won’t be an option.”