Explanatory-What is helium and why is it used in rockets? By Reuters

By Nivedita Bhattacharjee

BENGALURU (Reuters) – Two NASA astronauts aboard Boeing’s (NYSE: ) Starliner will remain on the International Space Station for months due to a faulty propulsion system, whose problems include leaking helium. Back on Earth, SpaceX’s Polaris (NYSE: ) Dawn mission was delayed due to helium issues with ground equipment.

Boeing’s Starliner spacecraft landed without a crew in a New Mexico desert Friday night.

Previous missions that have been affected by nasty helium leaks include ISRO’s Chandrayaan 2 and ESA’s Ariane 5. Why do spaceships and rockets use helium and what is so complicated about it?

WHY HELIUM?

Helium is inert – it does not react with other substances or burn – and its atomic number is 2, making it the second lightest element after hydrogen.

Rockets must reach specific speeds and altitudes to achieve and maintain orbit. A heavier rocket requires more energy, not only increasing fuel consumption, but also requiring more powerful engines that are more expensive to develop, test and maintain.

Helium also has a very low boiling point (-268.9°C or -452°F), allowing it to remain a gas even in super-cold environments, an important feature because many rocket fuels are store in that temperature range.

The gas is non-toxic, but cannot be breathed on its own because it replaces the oxygen that humans need to breathe.

HOW IS IT USED?

Helium is used to pressurize fuel tanks, ensuring fuel flows to the rocket’s engines without interruption; and for cooling systems.

As the fuel and oxidizer are burned in the rocket engines, helium fills the resulting empty space in the tanks, maintaining the overall pressure inside.

As it is non-reactive, it can be safely mixed with the residual contents of tanks.

IS HE ATTENDING SHORTS?

Helium’s small atomic size and low molecular weight mean that its atoms can escape through small gaps or seals in storage tanks and fuel systems.

But because there is very little helium in Earth’s atmosphere, leaks can be easily detected — making the gas important for identifying potential flaws in the fuel systems of a rocket or spacecraft.

In May, hours before Boeing’s Starliner spacecraft made its initial attempt to launch its first crew of astronauts, tiny sensors inside the spacecraft detected a small helium leak on one of the Starliner’s thrusters, which NASA spent several days analyzing it before deeming it low risk.

Additional leaks were detected in space after the Starliner’s launch in June, contributing to NASA’s decision to bring the Starliner back to Earth without its crew.

The frequency of helium leaks in space-related systems, some engineers say, has highlighted the need for industry-wide innovation in valve design and more precise valve tightening mechanisms.

ARE THERE ALTERNATIVES?

Some rocket launches have experimented with gases like argon and nitrogen, which are also inert and can sometimes be cheaper. Helium, however, is much more prevalent in industry.

Europe’s new Ariane 6 rocket ditched the helium of its Ariane 5 predecessor for a new pressurization system that turns a small portion of the primary liquid oxygen and hydrogen propellants into gas, which then pressurizes those fluids for the rocket’s engine.

That system failed in space during the final phase of Ariane 6’s otherwise successful debut launch in July, adding to the long list of pressurization challenges facing the global rocket industry.