LONDON - Mining in space is moving from science fiction to commercial reality but metals magnates on this planet need not fear a mountain of extraterrestrial supply - the aim is to fuel human voyages deeper into the galaxy.
Within three years, two firms plan prospecting missions to passing asteroids. When even a modest space rock might meet demand for metals like platinum or gold for centuries, it is little wonder storytellers have long fantasised that to harness cosmic riches could make, and break, fortunes on Earth.
But with no way to bring much ore or metal down from the heavens, new ventures that have backing from some serious - and seriously rich - business figures, as well as interest from NASA, will focus on using space minerals in interplanetary "gas stations" or to build, support and fuel colonies on Mars.
There may be gold up there, but the draw for now is water for investors willing to get the new industry off the ground.
Governments believe it has a future; NASA has a project that may put astronauts on an asteroid in under a decade and on Mars in the 2030s. And if the costs seem high, grumblers are told that one day the new skills might just save mankind from sharing the fate of the dinosaurs - if we can learn how to stop a massive asteroid smashing into Earth. "We are dreamers," declares the web site of Deep Space Industries (DSI), next to an image of a wheel-like metal station hooked up to a giant floating rock. But what the US-based start-up firm calls the first small steps in a "long play" to develop the resources of space are about to happen.
A priority is using hydrogen and oxygen, the components of water locked in compounds on asteroids, to refuel rockets.
Early in 2016, the first of DSI's exploration satellites, smaller than toasters, will hitch-hike into space on rockets carrying other payloads and start scouting for suitable rocks. The same year, another US-based venture, Planetary Resources, expects to launch prospecting craft hunting viable asteroids. "They are the low-hanging fruit of the solar system," said Eric Anderson, an American aerospace engineer and co-founder of Planetary Resources, which lists Google's Larry Page and Virgin billionaire Richard Branson among its backers. "They are just there and they are not difficult to get to and they are not difficult to get away from," he told Reuters.
Meteorites - chunks that survive and fall to earth after asteroids disintegrate in the atmosphere - yield significant amounts of precious metals like platinum, rhodium, iridium, rhenium, osmium, ruthenium, palladium, germanium and gold.
Planetary Resources estimates some platinum-rich asteroids just 500 metres across could contain more than the entire known reserves of platinum group metals. Studies based on observation and meteorites suggest space is even richer in iron ore.
Wall Street research firm Bernstein notes that a big asteroid called 16 Psyche, in the asteroid belt between Mars and Jupiter and measuring some 200 km (130 miles) across, may contain 17 million billion tonnes of nickel-iron - enough to satisfy mankind's current demand for millions of years.
But costs and technical hurdles rule out hauling resources down to Earth in the foreseeable future, experts say. The real value in asteroid mining is for further space travel - and so hydrogen and oxygen reserves are as attractive as any metal. "It's ridiculous to believe that asteroid resources will ever compete with terrestrial alternatives and Earth markets,"said Brad Blair, a mining engineer and economist.