BENGALURU, Sept 7 (Reuters) - Two NASA astronauts aboard

Boeing's ( BA ) 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 ( BA ) 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?

WHY HELIUM?

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 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 non-toxic, but cannot be breathed on its own,

because it displaces the oxygen humans need for respiration.

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 fuel and oxidiser are burned in the rocket's engines,

helium fills the resulting empty space in the tanks, maintaining

the overall pressure inside.

Because it is non-reactive, it can safely mingle with the

tanks' residual contents.

IS IT 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 the Earth's

atmosphere, leaks can be easily detected - making the gas

important for spotting potential faults in a rocket or

spacecraft's fuel systems.

In May, hours before Boeing's ( BA ) Starliner spacecraft made an

initial attempt to launch its first astronaut crew, tiny sensors

inside the spacecraft detected a small helium leak on one of

Starliner's thrusters that NASA spent several days analysing

before deeming it low-risk.

Additional leaks were detected in space after Starliner

launched in June, contributing to NASA's decision to bring

Starliner back to Earth without its crew.

The frequency of helium leaks across space-related

systems, some engineers say, have highlighted an industry-wide

need for innovation in valve design and more precise

valve-tightening 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. Helium, however, is much more prevalent in the

industry.

Europe's new Ariane 6 rocket ditched the helium of its

predecessor Ariane 5 for a novel pressurization system that

converts a small portion of its primary liquid oxygen and

hydrogen propellants to gas, which then pressurizes those fluids

for the rocket engine.

That system failed in space during the final phase of

Ariane 6's otherwise successful debut launch in July

, adding to the global rocket industry's long list of

pressurization challenges.