The idea of mining asteroids for metals has moved from the realm of science fiction to start-up companies. One problem, however, is that only a small proportion of asteroids are rich in what we need, and most of these are inconveniently located at the further edge of the main asteroid belt. However, a study of two smaller, but occasionally much closer, asteroids reveals they are of both scientific and commercial interest. One of these could be worth an estimated 11 trillion dollars if mined over 50 years, even allowing for the effect on metal prices of a new source of supply.
The asteroid 16 Psyche is well named since it maintains a grip on the mind of those with dreams of riches in space. Psyche is the largest metal-rich body in the Solar System, thought to be made mainly of iron and nickel, with little of the pesky silicon, carbon, and oxygen that dominate the Earth's crust and most bodies in space.
However, Psyche never gets closer than 2.5 astronomical units from the Sun. The cost of returning any products to Earth would be immense, even beyond the challenges of getting mining equipment out of our planet's gravity well. NASA has a mission planned to launch next year to check Psyche out, but in The Planetary Science Journal astronomers have identified two more promising targets, 1986 DA and 2016 ED85, both classified as Near Earth Asteroids (NEAs).
"Our analysis shows that both NEAs have surfaces with 85% metal such as iron and nickel and 15% silicate material, which is basically rock," said Dr Juan Sanchez of the Planetary Science Institute in a statement. “These asteroids are similar to some stony-iron meteorites such as mesoiderites found on Earth.” The conclusion is based on a combination of the brightness and color spectrum of both, and radar reflections from 1986 DA, compared with the compositions of different classes of known asteroids.
It is thought that asteroids like this are so rich in metal because they came originally from an object that grew large enough for most of its metals to sink to the core, as has happened on Earth. When some cataclysm destroyed this minor planet, the core fragments retained their largely metallic compositions.
"We believe that these two 'mini Psyches' are probably fragments from a large metallic asteroid in the main belt, but not 16 Psyche itself," said undergraduate student David Cantillo.
In a sense, humanity has been mining space rocks for at least 3,400 years, since Tutankhamen had a dagger forged from a stony-iron meteorite. However, waiting for them to come to us is a slow process.
1986 DA is thought to be around 2.4 km (1.5 miles) wide. Although small compared to Psyche, let alone the Earth, the paper calculates 1986 DA contains 28 billion tonnes of iron, and 2.5 billion tonnes of nickel, along with smaller, but more valuable quantities of cobalt and precious metals. The authors write; “We estimated that the amounts of [these] metals present in 1986 DA could exceed the reserves worldwide.” There's also quite a lot of gold and copper. If space mining ever becomes commercially viable, a single asteroid could supply us for a very long time.
Despite their name, NEAs like these spend most of their time beyond the orbit of Mars. Now and then, however, approaches occur, including one in September 2020 when 2016 ED85 became easy to observe. Although years between such encounters, they make voyages too and from these NEAs quick and cheap if timed carefully. In contrast, most metal-rich asteroids have relatively circular orbits 2.85-3.0 times as far from the Sun as the Earth's, making them much harder to reach than Mars.