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.
Referring to talk of city-sized settlements on Mars, he said: "The reason asteroid mining makes sense is because people might be some day where those resources are. You can't put an 80,000-person colony on Mars without using the local 'timber'.
"And if you're going to use chemical propulsion, it's going to take a lot of water to get them there."
The energy released when hydrogen and oxygen combine to make water can power rockets. The presence of both elements in compounds found on asteroids offers scope to set up space factories to make fuel for missions to Mars and beyond as well as offering "pit stops" to extend the lives of satellites.
"We're going to be looking at propellants for satellites, which is a multi-billion dollar industry to keep them alive," said Rick Tumlinson, Deep Space Industries' board chairman and a veteran promoter of commercial space development.
"We'll eventually be an oasis, a place where you can get air, and we can provide propellants. So we're a gas station," Tumlinson told a recent seminar in London.
"You can take the process leftover material, the slag, and use it for shielding, or concrete, and build large structures, and of course there is a percentage of precious metals."