What is helium and why is it used in rockets?

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

Two NASA astronauts aboard Boeing’s Starliner will stay on the International Space Station for months because of a faulty propulsion system whose problems included helium leaks. Back on Earth, SpaceX’s Polaris Dawn mission has been delayed because of helium issues on ground equipment.

Boeing’s Starliner spacecraft landed uncrewed in a New Mexico desert late on Friday.

Past missions have that have been affected by pesky helium leaks include ISRO’s Chandrayaan 2 and ESA’s Ariane 5. Why do spacecraft and rockets use helium, and what is so tricky about it?

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

Rockets need to achieve specific speeds and altitude to reach and maintain orbit. A heavier rocket requires more energy, not only increasing fuel consumption but also needing more powerful engines, which are more expensive to develop, test, and maintain.

Helium also has a very low boiling point (-268.9° C), 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 non-toxic, but cannot be breathed on its own, because it displaces the oxygen humans need for respiration.