A powder gun like the one used in Harp and a gas gun like Sharp both work in the same basic way – expanding gases. The lighter the gases molecular weight, the faster it expands in air. Gunpowder has a molecular weight of 22, slightly less than the average molecular weight of the gases in air at around 28, but ignited gunpowder is incredibly hot, so that’s why it expands so quickly. The hydrogen used in gas guns, however, has a molecular weight of two so expands extraordinarily quickly in air.

“Harp would have used a powder gun to get [the projectile] to about 2 km/s, then used a rocket to get the rest of the way to the 8 km/s required for orbit,” says Higgins. “Gas guns (like Sharp) have proven to get to speeds of 6 km/s.” So the projectile is already travelling much closer to the required speed to enter orbit.

The benefit would be that less space would be needed for propellant to carry it fully into orbit, providing much more room for satellites, fuel or building materials.

Sharp demonstrated that gas gun technology could reach escape velocity – and Bull was aware of Hunter’s work – but there’s no evidence he was inspired by this technology.

Bull wasn’t ignorant to the possibility that Iraq could use his supergun technology to fire missiles, but he justified his actions by pointing out that it would be an impractical weapon, says Hall. Its size meant that it would not be possible to move the gun once it was constructed; it only pointed in one direction, was slow to fire, could be easily located and easily destroyed if anyone wished to. Everyone would know where it was, and everyone would immediately know if it had been fired from the seismic tremors it caused. The recoil force from the gun would have totalled 27,000 tonnes – equivalent to a nuclear explosion – and would have registered as a major seismic event around the world. “It was completely vulnerable to air attack,” says Hall. “You couldn’t move it. But of course when one allows one’s brain to think about what Hussein then did, it’s tempting to consider it as a military threat.”