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.

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