Why helium leaks delayed the return of Boeing Starliner astronauts

Bengaluru:

Two NASA astronauts aboard Boeing's Starliner will remain on the International Space Station for months due to a faulty propulsion system, problems that included helium leaks. On Earth, SpaceX's Polaris Dawn mission has been postponed due to helium problems with ground equipment.

Boeing's Starliner spacecraft landed unmanned in a desert in New Mexico late Friday.

Past missions that have been affected 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 and does not burn – and has atomic number 2, making it the second lightest element after hydrogen.

Rockets must reach certain speeds and altitudes to reach orbit and stay there. A heavier rocket requires more energy, which not only increases fuel consumption but also requires more powerful engines that 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 gaseous even in very cold environments. This is an important property because many rocket fuels are stored in this temperature range.

The gas is non-toxic, but cannot be inhaled alone because it displaces the oxygen that humans need to breathe.

HOW TO USE IT?

Helium is used to pressurize fuel tanks to ensure uninterrupted fuel flow to the rocket engines, as well as for cooling systems.

When fuel and oxidizer are burned in the rocket's engines, helium fills the resulting void in the tanks, maintaining the overall pressure inside.

Since it is non-reactive, it can mix with the rest of the tank contents without any problems.

IS IT PRONE TO LEAKS?

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.

However, since there is very little helium in the Earth's atmosphere, leaks are easy to detect – so the gas is important for detecting possible faults in the fuel systems of rockets or spacecraft.

In May, just hours before Boeing's Starliner spacecraft made its first attempt to send its first crew of astronauts into space, tiny sensors inside the spacecraft detected a small helium leak on one of the Starliner's engines. NASA investigated the leak for several days before declaring it a low risk.

After the Starliner's launch in June, additional leaks were discovered in space, contributing to NASA's decision to return the 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 innovations in valve design and more precise valve closure mechanisms.

ARE THERE ALTERNATIVES?

Some rocket launches have experimented with gases such as argon and nitrogen, which are also inert and sometimes cheaper, but helium is far more common in industry.

Europe's new Ariane 6 rocket does not use the helium of its predecessor, the Ariane 5, and instead features a novel pressurization system that converts a small portion of the primary liquid oxygen and hydrogen fuels into gas and then pressurizes these liquids for the rocket engine.

This system failed during the final stages of Ariane 6's otherwise successful debut launch in space in July, adding to the long list of pressurization problems facing the global rocket industry.

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)