Kelly McSweeney

Apr 1st 2020

Recycling in Zero Gravity for Sustainable Space Travel


Scientists are developing technology to make sustainable space travel possible. As NASA prepares for long term space missions to the moon, Mars and beyond, many challenges remain. The future of space travel requires solutions for long-duration missions with limited resources. They will need access to fuel and the essential elements that sustain life. Scientists recently demonstrated a possible solution by splitting water into oxygen and hydrogen in zero gravity using a semiconductor material and light.

Challenges for Sustainable Space Travel

It’s already difficult for astronauts to live at the International Space Station (ISS) and that’s a well-established station located nearby in low earth orbit. Longer missions will be even trickier.

According to NASA, the ISS uses a system that recycles approximately 90% of the water and 42% of the oxygen in the spacecraft. The system purifies the crew’s waste — including urine and sweat — and turns it into potable water for the crew to drink. Still, cargo spacecraft regularly visit the space station to supplement essential supplies and replace some of the system’s components. How will future astronauts get the elements they need to survive on long-term space missions? What will fuel their spaceships and the electronic gadgets they’ll need onboard?

Fuel Cell Space Vehicles

The Conversation reports that an international team of researchers demonstrated that it is possible to produce hydrogen and oxygen from water using a semiconductor material and sunlight (or light from any star) in zero gravity. These findings, which are published in Nature Communications, are a step toward long-term space travel with storable, renewable energy.

While water may be a relatively heavy supply for space travel, it’s a reasonable option because it provides a two-for-one, with hydrogen for fuel and oxygen for astronauts to breathe. Plus, it can be recycled, making sustainable space travel possible. As a bonus, launching a space vehicle loaded with water is safer than the potentially explosive alternative of rocket fuel and oxygen. If this idea is going to be realistic, scientists need to develop a method for splitting water apart and putting it back together again. They’re working on how to create water from oxygen and hydrogen, plus the opposite.

The Process

In space, we don’t have the luxury of plants releasing oxygen, but we can mimic photosynthesis. According to the Conversation, water can be split into hydrogen and oxygen via electrolysis, where scientists run an electric current through a water sample that contains a soluble electrolyte.

A new version is even better for space travel because it uses lightweight equipment and sunlight (or starlight). While solar power is useful on earth, it’s even more abundant in space, where light isn’t filtered through an atmosphere. In this alternative method, photo catalysts absorb photons into a semiconductor material in the water.

The researchers explain in Nature, “The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen.”

As a bonus that will be especially useful with limited resources in space, the process can be reversed.

How to Create Water From Oxygen and Hydrogen

The hydrogen and oxygen can be recombined, according to the Conversation, “using a fuel cell returning the solar energy taken in by the photocatalysis.” This energy can be used to power electronics, and it produces water that can be reused.

Drop Tower Testing

To test their system, the researchers dropped their experiment down a 120-meter tower. As the object accelerated toward Earth, the “drop tower” simulates microgravity by creating an opposite effect to the G forces that astronauts experience during lift-off.

The Bubble Problem

While the researchers did successfully split water into hydrogen and oxygen in this microgravity environment, a big challenge popped up: bubbles. When the water is split to create gas, bubbles form. On Earth, gravity causes the bubbles to float to the surface, but in zero gravity the bubble lurks near the catalyst, blocking the next potential bubble, and therefore obstructing overall gas production.

They attempted to fix the bubble problem by creating pyramid-shaped zones on the catalyst so that the bubble could easily move from the pointed tip and float away. But even though this forced the bubbles away from the catalyst, without gravity, they remain in the liquid and create a pesky foam that blocks the catalyst and electrodes, making the whole system inefficient.

Despite these remaining challenges, the new water-splitting method proves that there’s great potential for sustainable space travel in the future. Advanced technology and creative scientific processes can make the most of limited resources, on Earth and in space.