Helium (He) is a lightweight, inert noble gas with unique physical properties. While it is not used for lifting or breathing in space missions, it is an indispensable tool for safety and high-technology operations. This element plays a complex role in everything from launching the rocket to operating sensitive scientific instruments.
Pressurizing and Purging Rocket Systems
Helium’s primary function occurs long before astronauts reach orbit, serving as a critical component in operating powerful rocket engines. Launch vehicles require a precise flow of fuel and oxidizer to their combustion chambers. As propellants are rapidly consumed during ascent, a large empty space, known as the ullage volume, is created inside the tanks.
This ullage must be constantly pressurized to maintain the necessary flow rate to the engine turbopumps. Pressurization prevents the tank from collapsing inward and ensures a steady push on the remaining liquid. Helium is the ideal gas for this task because it is completely non-reactive, which is essential when dealing with volatile cryogenic propellants like liquid hydrogen and liquid oxygen.
Helium also has the lowest boiling point of any element, liquefying at only 4.2 Kelvin. This ensures it remains gaseous even when injected into extremely cold propellant tanks.
The use of helium extends beyond maintaining pressure, as it is also used extensively for purging. Purging is the process of clearing lines and tanks of hazardous residue. Helium’s inert nature allows it to safely sweep away volatile propellants without creating a risk of combustion or freezing the lines before an engine is serviced.
Helium’s Role in Leak Detection and Safety
Helium plays a significant role in verifying the structural integrity of the spacecraft and its internal systems. Spacecraft components and habitats must be completely sealed to protect crew and equipment operating under extreme pressure differentials. Due to its extremely small atomic diameter—second only to hydrogen—helium is used as a tracer gas to find microscopic flaws in welds and seals.
The method, known as Mass Spectrometer Leak Detection (MSLD), involves pressurizing a component with helium. A highly sensitive instrument then detects any minute amounts of the gas escaping. Helium molecules can penetrate holes that other gases cannot, making it the most reliable indicator of structural weakness.
This testing is performed on everything from propulsion system tubing to the seals on crew habitat modules before launch. The technique is sensitive enough to detect a leak rate equivalent to one cubic centimeter of gas escaping over 30 years. The inert property of helium ensures it will not contaminate the systems being tested or react with residual chemicals inside.
Using Helium in Scientific Payloads
Helium is also a powerful tool for scientific discovery in space, primarily through its use in cryogenics. Many highly sensitive instruments, particularly those observing in the infrared spectrum, must be cooled to temperatures near absolute zero. This extreme chilling eliminates thermal noise, which is internal heat radiation from the instrument that can overwhelm faint signals from deep space.
Liquid helium, with its ultra-low boiling point of approximately -269 degrees Celsius, is the most common coolant used to achieve these temperatures in orbit. Telescopes, such as the Herschel Space Observatory, carry large internal tanks, called cryostats, filled with liquid helium to cool detectors. The helium slowly boils off over the mission’s lifespan, keeping the instruments at a stable, ultra-cold temperature for years.
In addition to serving as a refrigerant, gaseous helium is sometimes included in scientific packages as a reference or buffer gas. Its stable, non-reactive nature makes it useful in devices like mass spectrometers, which analyze the composition of atmospheres.