By Nivedita Bhattacharjee
BENGALURU (Reuters) – Two NASA astronauts aboard Boeing’s Starliner will remain on the International Space Station for months because of a faulty propulsion system whose problems included helium leaks. On Earth, SpaceX’s Polaris Dawn mission has been delayed because of problems with helium in ground equipment.
Boeing’s Starliner spacecraft landed uncrewed in a New Mexico desert on Friday night.
Previous missions that have been plagued by troublesome 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 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 reach specific speeds and altitudes to reach and maintain orbit. A heavier rocket requires 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 super-cold environments, an important feature because many rocket fuels are stored in that temperature range.
The gas is not toxic, but it cannot be breathed on its own because it displaces the oxygen that humans need to breathe.
HOW TO USE IT?
Helium is used to pressurize the fuel tanks, ensuring that fuel flows to the rocket engines without interruption; and for the cooling systems.
As fuel and oxidizer are burned in the rocket engines, helium fills the resulting empty space in the tanks, maintaining the overall pressure inside.
Because it is non-reactive, it can be safely mixed with residual tank contents.
ARE YOU PRONE TO LEAKS?
Helium’s small atomic size and low molecular weight mean 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 detecting potential failures in a rocket or spacecraft’s fuel systems.
In May, hours before Boeing’s Starliner spacecraft made an initial attempt to launch its first crew of astronauts, tiny sensors inside the spacecraft detected a small helium leak in one of Starliner’s thrusters that NASA spent several days analyzing before deeming it low risk.
Additional leaks were detected in space after Starliner’s launch in June, contributing to NASA’s decision to bring Starliner back to Earth without its crew.
The frequency of helium leaks in space-related systems, some engineers say, has highlighted the industry-wide need for innovation in valve design and more precise valve adjustment mechanisms.
ARE THERE 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 much more common in the industry.
Europe’s new Ariane 6 rocket has ditched the helium of its predecessor, the Ariane 5, for a novel pressurisation system that converts a small portion of its primary propellants of liquid oxygen and hydrogen into gas, which then pressurises those fluids for the rocket’s engine.
That system failed in space during the final phase of Ariane 6’s successful debut launch in July, adding to a long list of pressurization challenges facing the global rocket industry.