Written by Nivedita Bhattacharjee
BENGALURU (Reuters) – Two NASA astronauts aboard Boeing Co’s Starliner will remain on the International Space Station for months due to a propulsion system glitch that has included a helium leak, while on Earth, SpaceX’s Polaris Dawn mission has been delayed due to helium problems in ground equipment.
Boeing’s unmanned Starliner spacecraft landed in the New Mexico desert late Friday.
Previous missions affected by troublesome helium leaks include the Indian Space Research Organisation’s Chandrayaan 2 mission and the European Space Agency’s Ariane 5 mission. Why do spacecraft and rockets use helium, and what’s so complicated about it?
Why helium?
Helium is an inert element – it does not react with other substances or burn – and its atomic number is 2, making it the second lightest element after hydrogen.
Rockets need to achieve specific speeds and altitudes to reach and maintain orbit. A heavier rocket requires more power, which not only increases fuel consumption, but also requires more powerful engines, which are more expensive to develop, test, and maintain.
Helium also has an extremely low boiling point (-268.9 °C or -452 °F), allowing it to remain a gas even in extremely cold environments, an important feature because many rocket fuels are stored in this temperature range.
The gas is non-toxic, but it cannot be inhaled on its own, because it replaces the oxygen that humans need to breathe.
How is it used?
Helium is used to increase the pressure in the fuel tanks, ensuring that fuel flows to the rocket engines without interruption; as well as for cooling systems.
As fuel and oxidizer are burned in rocket engines, helium fills the resulting empty space in the tanks, maintaining the overall pressure inside.
Because it is non-reactive, it can safely mix with the remaining contents in tanks.
Is it prone to leakage?
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 so little helium in Earth’s atmosphere, leaks can be easily detected — making the gas important for identifying potential malfunctions in rocket or spacecraft fuel systems.
In May, hours before Boeing’s Starliner was set to launch its first crew of astronauts, small sensors inside the spacecraft detected a small helium leak on one of the Starliner’s engines, and NASA spent several days analyzing it before deeming it low risk.
Additional leaks in space were discovered after Starliner’s launch in June, contributing to NASA’s decision to return Starliner to Earth without its crew.
Some engineers say the repeated helium leaks through space-bound systems have highlighted an industry-wide need for innovation in valve design and more precise valve-tightening mechanisms.
Are there any alternatives?
Some rocket launch companies have experimented with gases such as argon and nitrogen, which are also inert and can sometimes be cheaper. However, helium is more common in industry.
The new European rocket, Ariane 6, has ditched the helium of its predecessor, Ariane 5, in favor of a new compression system that converts a small portion of the basic liquid fuel, consisting of oxygen and hydrogen, into a gas, and then compresses these liquids for the rocket engine.
The system failed in space during the final stage of the successful debut launch of an Ariane 6 rocket in July, adding to the long list of pressure challenges facing the global rocket industry.