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Helium-3 from Saturn

The fuel that makes the ship possible is almost absent on Earth and dissolved in the gas around Saturn.

See it in the tour: chapter ringed world, shot 1.4.

Almost none on Earth

**The Saturn system.** Saturn at the centre, the moons on their orbits and Titan's orbit picked out in gold.

Helium-3 is a rare isotope of helium with two protons and one neutron. Earth's air holds about 1.4 helium-3 atoms for every million helium-4 atoms physics. Almost all the helium-3 people use comes from one place: tritium, an isotope of hydrogen with a half-life of 12.3 years, decays into it, and the United States weapons programme keeps tritium physics.

In 2010 that source gave about 8,000 litres of helium-3 a year, 1.1 kg physics. Demand reached about 80,000 litres in 2008, and the supply ran short physics.

What a crossing needs

The Long Passage burns about 371 kg of fusion fuel per crossing challenge. The reaction takes one deuterium nucleus for each helium-3 nucleus, so the helium-3 is 222 kg of it and the deuterium 149 kg physics. That is 207 years of the 2010 supply, for one trip physics.

One crossing's 222 kg of helium-3 is 207 times what the United States made in a year from decaying tritium. The gas giants hold about 119 times more helium-3 per helium atom than Earth's air. physics challenge

Where it is

The gas giants kept the helium-3 they were born with. The Galileo probe measured Jupiter's atmosphere in 1995 and found 166 helium-3 atoms per million helium-4 atoms, about 119 times Earth's air physics. That ratio is read as the protosolar value, the helium-3 content of the cloud the Sun formed from physics.

No probe has measured Saturn's ratio. Take Jupiter's as a stand-in. Saturn's atmosphere is 3.25% helium by volume physics. Then 5.4 of every million gas molecules are helium-3, and each kilogram of gas holds about 7.6 mg of it physics. For 222 kg, the Long Passage's load, a skimmer must pass 29,000 tonnes of gas through a separator and keep every atom challenge.

Why the settlement sells it sparingly

Separating one light isotope from about six thousand times more of its heavier twin, in gas that a vehicle passes through at orbital speed, is a laboratory process done in flight challenge. The settlement runs one experimental skimmer. It sells what the skimmer separates in experimental quantities, and the economy runs on methane, water and propellant instead fiction.

The scarcity is part of why the first settlers stayed fiction. In 2010 Earth made a kilogram a year. Saturn's atmosphere holds 7.6 mg in every kilogram of gas physics.

Key numbers

Quantity	Value	Basis
Helium-3 per helium-4, Earth's air	1.39 × 10⁻⁶	physics
Helium-3 per helium-4, Jupiter	1.66 × 10⁻⁴	physics
Helium in Saturn's atmosphere	3.25% by volume	physics
Tritium half-life	12.3 years	physics
United States supply, 2010	about 8,000 L, 1.1 kg a year	physics
Fusion fuel per crossing	about 371 kg	challenge
Helium-3 per crossing	222 kg	physics
Gas to process for 222 kg	about 29,000 t	challenge
Helium-3 sales from Saturn	experimental quantities	fiction