Rock of wages? Work it out (Image: NASA/SPL)

The solar system is littered with millions of asteroids, but only a few can be profitably mined for valuable metals and water using current technology. That is the conclusion of a new analysis inspired by the search for life on other planets.

Recent years have seen two US companies – Planetary Resources and Deep Space Industries – established with the intent of one day mining space rocks. NASA also has asteroid ambitions, with a plan to drag one into lunar orbit for astronauts to study.

“People tend to lump it all together and say ‘Oh, there’s trillions of dollars of resources up in space’,” says Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But it is still unclear which rocks will make the best targets.


To tackle the problem, Elvis adapted a tool used to study another cosmic puzzle: the Drake equation, used in the hunt for alien life. Dreamed up in 1961 by astronomer Frank Drake, the equation provides an estimate of the number of detectable alien civilisations in the Milky Way. You just need to plug in realistic guesses for the equation’s various factors.

Elvis’s equation – shown above and detailed in an upcoming edition of Planetary and Space Science – works in a similar way. It calculates the number of mineable asteroids for a given resource by combining key factors: the asteroid’s type, its richness in resources, and the practical limitations to mining it.

First up is the asteroid’s type, which determines composition. Based on previous surveys, Elvis estimates that 4 per cent of space rocks are the right type to contain platinum and similarly valuable metals. Of these, he says, half will have a rich enough concentration of metal to be worth mining.

Got to get there

Of course, companies have to reach an asteroid to mine it. The limiting factor is the energy needed to get to an asteroid with enough mining equipment and return with the mined ore, meaning only 2.5 per cent of asteroids are accessible from Earth.

The rock must also be large enough to justify the mining expedition in the first place, so Elvis considers the fraction of asteroids larger than 100 metres in diameter, which if fully mined for platinum would be worth a little over a billion dollars at current market prices.

Putting this all together, Elvis says there are only 10 asteroids worth mining for platinum, and 18 for water for future space colonies. Engineering difficulties could make these numbers even lower. Asteroid miners should be cautious in evaluating their plans, Elvis says, as the true value could turn out to be zero.

“There’s a lot of commonality between Martin’s analyses and our own research,” says Chris Lewicki, president of Planetary Resources. He says that efforts to characterise the ore-content of asteroids may be simpler than the detailed science missions so far conducted by governments. “This may afford the opportunity for more cost-effective types of instrumentation, where the goal is simply to qualify a resource for follow-up study.”

He thinks Elvis’s estimate is based on conservative assumptions and that there could be hundreds or thousands of profitable asteroids out there. “We’ve got to actually go after the asteroids and look at them to find the right ones and really know for sure.”

The new approach addresses an important question, says Daniel Garcia Yarnoz at the University of Strathclyde in Glasgow, UK, though it is difficult to know the real numbers. “It relies greatly on assumptions on various factors, in particular the probability that the asteroid is resource-rich.”