Helium (He)

Helium is the only element that was discovered somewhere other than Earth. It was found in the Sun, twenty-seven years before anyone identified it in terrestrial rock, and the name records the assumption everyone made at the time — that a substance seen only in a star was probably a metal.

A yellow line nobody could match

On 18 August 1868 the French astronomer Pierre Janssen observed a total solar eclipse from Guntur, in India, with a spectroscope trained on the prominences at the edge of the disc. Among the lines he recorded was a bright yellow one at about 587.5 nanometres, close to the well-known pair produced by sodium but not coincident with either. Janssen also worked out during the same expedition how to observe prominences in full daylight, which meant the observation no longer depended on an eclipse.

Norman Lockyer, in London, reached the same line independently that October, and with the chemist Edward Frankland satisfied himself that no known substance produced it. They proposed a new element and named it from helios.

The priority question has an unusually tidy answer: letters from Janssen and Lockyer reached the French Academy of Sciences within days of each other in late October 1868, and the Academy eventually resolved the matter by striking a medal bearing both men's portraits. The data card above carries Janssen's name because a card can only carry one.

The suffix is the fossil. Lockyer chose -ium because that is how metals are named, and by the time anyone knew better the word was fixed. Every other noble gas ends in -on.

Twenty-seven years to find it in the ground

William Ramsay isolated it terrestrially in 1895 by treating cleveite, a uranium mineral, with acid and examining the released gas — and there was the same yellow line. Per Teodor Cleve and Nils Abraham Langlet, working in Uppsala, obtained it independently within weeks.

There had also been a near miss. The American geochemist William Hillebrand had extracted an inert gas from uraninite in 1889 and, finding it unreactive, recorded it as nitrogen and moved on. He had helium in a tube six years early and did not look at its spectrum.

Alpha particles with a lid on them

The reason a uranium mineral gives up helium is the reason there is any helium on Earth at all. An alpha particle is a helium nucleus. Every atom of helium in a commercial gas field was emitted during the radioactive decay of uranium or thorium somewhere in the crust, picked up two electrons, migrated upward through rock, and happened to be trapped beneath an impermeable layer along with natural gas.

This makes the terrestrial supply unlike that of any other industrial gas. Nitrogen and argon can be pulled out of the air indefinitely. Helium cannot: it is light enough to escape Earth's gravity altogether from the upper atmosphere, so any that is released is gone permanently, and the replacement rate is set by radioactive decay in the crust. Concentrations high enough to extract economically — typically above about 0.3% of a gas stream — depend on a specific and uncommon geological coincidence of a uranium-rich basement, a migration path and an intact seal.

Essentially all of it is recovered as a byproduct of natural gas processing, which means helium production is tied to a fuel industry that is under pressure for unrelated reasons.

The stockpile that distorted the market for twenty years

The United States began accumulating helium in 1925, when the strategic case was airships, storing it underground in the Bush Dome reservoir at the Cliffside field near Amarillo, Texas. By the late twentieth century the Federal Helium Reserve held the great majority of the world's stored helium.

The Helium Privatization Act of 1996 instructed the government to sell it off, and set the price by a formula designed to recover the programme's accumulated debt rather than to reflect scarcity. The predictable happened: a large volume of helium entered the market at an administratively low price, private exploration was discouraged, and there was no incentive for anyone to recover and recycle what they used. A National Research Council report in 2010 said so bluntly. The reserve's remaining assets were finally sold in 2024, and the market has since had to price a non-renewable resource for the first time in decades.

What it is actually for, and it is not balloons

Party balloons are a rounding error. The largest single use of helium is cooling the superconducting magnets in magnetic resonance imaging scanners, which have historically needed around a thousand litres of liquid helium each and periodically need topping up. That one application has typically accounted for something like a fifth to a third of world demand.

The rest is spread across jobs that depend on properties nothing else combines:

• Leak detection. Helium atoms are small and chemically inert, so a helium mass spectrometer can find a hole in a sealed vessel that no other tracer would pass through.
• Semiconductor and fibre-optic manufacture, where it serves as an inert, high-thermal- conductivity atmosphere.
• Purging rocket propellant tanks, because it stays gaseous at temperatures where any other candidate would condense.
• Breathing mixtures for deep diving, where substituting helium for nitrogen avoids the narcotic effect of nitrogen at pressure.
• Arc welding shields and controlled-atmosphere furnaces.

Newer scanner designs that seal a much smaller helium charge permanently into the magnet exist largely because the supply stopped being dependable.

The liquid that climbs out of the cup

Cool helium-4 below about 2.17 kelvin — the lambda point — and it becomes a superfluid. Viscosity vanishes. A film a few dozen atoms thick creeps up the inside wall of any container, over the rim and down the outside until the vessel empties itself. Heat is conducted so efficiently that the liquid stops bubbling as it boils, because no local hot spot can survive long enough to nucleate a bubble. Pyotr Kapitsa in Moscow and John Allen with Don Misener in Cambridge reported the phenomenon within the same issue of Nature in 1938, another genuine simultaneity.

Helium-3 does it too, but only below about 2.5 millikelvin, and for a fundamentally different reason: helium-3 is a fermion, so its atoms have to pair up before they can behave collectively, in a manner analogous to electrons in a superconductor. David Lee, Douglas Osheroff and Robert Richardson found the transition in 1972 and shared the Nobel prize in 1996.

Helium is also the only element that will not freeze at atmospheric pressure however cold it gets. The attraction between its atoms is so weak that quantum zero-point motion alone keeps them mobile; solidifying it requires around twenty-five atmospheres of applied pressure even at absolute zero.

The scarcer isotope, and the detectors that ran out

Helium-3 makes up somewhere around one part per million of natural helium. Practically all of the commercially available supply is not mined at all — it is the decay product of tritium, which means it accumulates in nuclear weapons stockpiles and is harvested when those are maintained.

That supply chain collided with policy after 2001, when the United States began deploying large numbers of neutron detectors at ports and borders to screen for smuggled fissile material. Helium-3 proportional counters are the standard technology for the job, demand rose by an order of magnitude, and by 2008 there was a genuine shortage that also hit the low-temperature physics community, since dilution refrigerators depend on the same isotope. Detector designers have been working on alternatives ever since. Proposals to mine helium-3 from the lunar regolith as a fusion fuel appear regularly and remain speculative on both the mining and the fusion side.