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Astronauts face numerous risks during spaceflight, such as microgravity and radiation exposure. Microgravity can reduce bone density, and radiation exposure is a carcinogen. However, these are chronic effects.

The biggest risk to astronauts is fire, as escape would be difficult on a long mission to Mars or elsewhere beyond low Earth orbit. Scientists are studying how fire behaves on spacecraft so that astronauts can be protected.

Scientists from the Center for Applied Space Technology and Microgravity (ZARM) at the University of Bremen are investigating the risks of fire on board spacecraft.

They published a new study in the Proceedings of the Combustion Institute titled “Effect of oxygen concentration, pressure, and counterflow velocity on flame propagation across thin PMMA sheets.” The lead author is Hans-Christoph Rees.

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“A fire on board a spacecraft is one of the most dangerous scenarios in space missions,” said Dr. Florian Mayer, head of the fuel technology research group at ZARM.

“There is hardly any possibility of reaching safety or escaping from a spacecraft. That is why it is crucial to understand the behavior of fires under these special conditions.”

Since 2016, ZARM has been studying how fire behaves and propagates in microgravity conditions like those on the ISS.

These conditions also include Earth-like oxygen levels, forced air circulation, and Earth-like ambient pressure. NASA has been conducting similar experiments, and we now know that fire behaves differently in microgravity than it does on Earth.

The fire will initially burn with a smaller flame and take longer to spread. This is to the fire’s advantage as it won’t be noticed as quickly.

Fire also burns hotter in microgravity, meaning that some materials that might not be flammable under normal Earth conditions can burn inside a spacecraft, creating toxic chemicals in the spacecraft’s air.

Spacecraft for Mars missions will have a different environment than the ISS. The ambient air pressure will be lower, which provides two advantages: it makes the spacecraft lighter and also allows the astronauts to prepare for external missions more quickly.

However, the lower ambient pressure introduces another critical change to the spacecraft environment. The oxygen content must be higher to meet the astronauts’ respiratory needs.

In these latest tests, the ZARM team tested a fire under these revised conditions.

PMMA stands for polymethyl methacrylate and is commonly referred to as acrylic. It is a common material used in place of glass because it is lightweight and shatter-resistant. The ISS does not use it, but it is being developed for use in future spacecraft. The Orion capsule used acrylic fused with other materials for windows, and future spacecraft will likely use something similar.

In their experiments, the researchers ignited a sheet of acrylic glass and varied three environmental factors: ambient pressure, oxygen content, and flow rate.

They used the Bremen Drop Tower to simulate microgravity.

Experiments show that lower ambient pressure reduces fire. However, higher oxygen content has a more powerful effect. The oxygen level on the ISS is 21%, just like it is on Earth.

Future spacecraft with lower ambient pressure will have oxygen levels as low as 35%. This translates into a huge increase in the risk astronauts face from fire. The results show that fire can spread three times faster than under terrestrial conditions.