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Assignment: Impossible


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Visions: Rare and Precious

The views expressed are those of the author and are not necessarily those of Scientific American.


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In the series “Visions,” science fiction about the very latest research will be paired with analysis looking into the facts behind the fiction. The goal is to marry ripped-from-the-headlines science fiction with analysis into the possibilities hinted at by new discoveries.

Rare earth metals aren’t actually rare on Earth. They’re abundant on the surface, with the most common ones — cerium, lanthanum, neodymium and yttrium — more common in the Earth’s crust than lead and silver. What is true is that they are rarely concentrated enough to mine.

In space, however, it turned out to be a completely different matter. And that changed everything.

See, when the Earth was young, it was molten hot long enough for heavier elements to sink closer to its heart. This meant the only way to really get concentrations of the rare earth metals high enough to mine was to get lucky and be someplace where they were coughed up in magma from the mantle.

However, asteroids, being smaller, cooled much faster than their bigger siblings. With only a few exceptions, the interiors of these rocks didn’t separate into different layers. Instead, they remained much as they started off billions of years ago, primordial relics from the birth of the solar system. And it turned out their rare earth metals were much easier to mine.

This artist's concept illustrates the first known Earth Trojan asteroid, discovered by WISE. The asteroid is gray and its extreme orbit is shown in green. Credit: Paul Wiegert, University of Western Ontario, Canada.

The asteroids of the main belt are difficult to get to. It takes a lot of energy to fight against the siren pull of the sun’s gravity and send a craft past the orbit of Mars. But there are others much easier to get to, ones that share Earth’s path around the sun. Rocks located at points that formed triangles with sides of equal length with the sun and our planet, Earth’s Trojan asteroids, and minor planets found in even stranger orbits, such as ones that moved in horseshoe-shaped patterns.

Rare earth metals are not only rare, but useful. These 17 elements find use in computers, wind turbines, superconductors, video screens, oil refineries, hybrid and electric vehicles, catalytic converters, compact fluorescent lightbulbs, light-emitting diodes, lasers, audio speakers and microphones, cell phones, MRI machines, telecommunications, battery electrodes and even advanced weapons systems.

Rare earth metals were valuable, and that made them coveted, and that made them hoarded, and it turned out China produced 97 percent of the world’s supply of them. So when probes discovered asteroidal rare earth metals that were easy to mine, the race of every spacefaring nation to reach them began.

Aluminum baby rattle from the collection of Napoleon III.

You can never predict what happens when something rare becomes common. Aluminum was once a precious metal more valuable than gold due to how difficult it was to get. Aluminum bars were displayed alongside the crown jewels of France at the Paris Exhibition in 1855, and Napoleon III’s most honored guests dined with aluminum plates and cutlery while the rest had to make do with gold or silver ones.

But scientists eventually found a way to extract aluminum easily, and now the metal is key in strong, light alloys found in countless vital applications. When rare earth metals became common, technology advanced in ways that no one could have imagined.

Indeed, maybe we wouldn’t be around and humanity still would be.

*

On Wednesday, scientists announced the long-sought discovery of the first known sun-Earth Trojan asteroid, currently designated 2010 TK7 [animation]. Its orbit dips it above and below the plane of Earth’s orbit, making it a poor destination to visit, as it would take a lot of propellant to reach. However, there could be many asteroids at the sun-Earth L4 and L5 points hidden by daylight that might be easy targets.

It remains highly uncertain how abundant rare earth metals really are in asteroids, and Keith Long with the U.S. Geological Survey estimates a deposit would need to have about 2 percent rare earth content to really be in the ballpark of economical to visit. Still, he adds that chondritic meteorites seem enriched with rare earth metals, offering tantalizing hints that the asteroids they came from might be as well.

There’s only one way to find out…

Thanks to Martin Connors and Keith Long for helpful discussions on this story.

You can email me regarding Visions at toohardforscience@gmail.com.

Charles Q. Choi About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time, he has traveled to all seven continents. Follow on Twitter @cqchoi.

The views expressed are those of the author and are not necessarily those of Scientific American.





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