Two NASA astronauts aboard Boeing’s Starliner will remain at the International Space Station for months as a faulty propulsion system addresses problems including a helium leak. Back on Earth, SpaceX’s Polaris Dawn mission has been delayed by a helium problem in ground equipment.
Boeing’s Starliner spacecraft landed uncrewed in the New Mexico desert late Friday night.
Past missions that have been affected by helium leaks include ISRO’s Chandrayaan 2 and ESA’s Ariane 5. Why do spacecraft and rockets use helium, and what’s so tricky about it?
Why helium?
Helium is inert — it doesn’t react with other substances or burn — and has an atomic number of 2, making it the second-lightest element after hydrogen.
Rockets need to achieve specific speeds and altitudes to reach and maintain orbit. Heavier rockets require more energy, which not only increases fuel consumption but also requires more powerful engines, which 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 extremely cold environments, an important feature since many rocket fuels are stored in this temperature range.
This gas is not toxic, but cannot be inhaled alone because it displaces the oxygen needed by humans for respiration.
How is it used?
Helium is used to pressurize fuel tanks, ensuring uninterrupted flow of fuel to the rocket’s engines; and is also used in cooling systems.
When the fuel and oxidizer are burned in the rocket’s engine, the helium fills the empty space in the tanks, maintaining the overall pressure inside.
Because it is non-reactive, it can safely mix with the residual contents of tanks.
Is there a chance of it leaking?
Because of helium’s small atomic size and low molecular weight, its atoms can escape through small gaps or seals in storage tanks and fuel systems.
But because the amount of helium in Earth’s atmosphere is so small, leaks can be easily detected — making the gas crucial for spotting potential malfunctions in a rocket or spacecraft’s fuel systems.
In May, hours before Boeing’s Starliner spacecraft’s initial attempt to launch its first astronaut crew, small sensors inside the spacecraft detected a small helium leak on one of Starliner’s thrusters, which NASA analyzed for several days and then deemed low-risk.
Additional leaks in space were discovered after Starliner’s launch in June, leading NASA to decide to return Starliner to Earth without a crew.
Some engineers say the frequency of helium leaks in space-related systems has highlighted an industry-wide need for innovation in valve design and more precise valve-tightening systems.
Are there any alternatives?
Some rocket launches have experimented with gases such as argon and nitrogen, which are also inert and can sometimes be cheaper. However, helium is more commonly used in industry.
Europe’s new Ariane 6 rocket ditches the helium of its Ariane 5 predecessor and uses a novel pressurization system that converts a small portion of its primary liquid oxygen and hydrogen propellants into gas, which then pressurizes those liquids for the rocket engine.
The system failed in space during the final phase of the otherwise successful maiden launch of Ariane 6 in July, adding another to a long list of pressing challenges facing the global rocket industry.
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