Metal asteroids offer something rare in the solar system — the core of a planet without all the rock that normally surrounds it.
Since it is impossible to directly examine a planetary or lunar core if the parent body remains intact, metal-rich asteroids where the upper mantle and crust layers have been lost to a cataclysmic crash offer a potential path to, in effect, peek inside the depths (and deep time) of an object.
The asteroid Psyche is such an object, and that’s why NASA approved a mission to the asteroid that is scheduled to launch next year. Orbiting the sun between Mars and Jupiter in the largest asteroid belt, Psyche appears to be the exposed nickel-iron core of an early planet, and as such reveals the early evolution of our solar system.
But Psyche is not the only metal-rich asteroid known to astronomers, and it certainly is not the closest.
Two much smaller “mini-Pysches” have been detected that are also comprised of iron, nickel, and other metals ranging from platinum to rare earth elements. And these two mini-asteroids — 1986 DA and 2016 ED85 — were recently found to have their spectral signatures are quite similar to asteroid Psyche.
And unlike Psyche, which is between 180 million and 360 million miles away, these mini-Psyches orbit less than twenty million from Earth every 20 to 30 years.
“These kind of metal-rich Near-Earth asteroids are extremely rare,” said Vishnu Reddy of the University of Arizona, and co-author of a recent paper in Planetary Science Journal. “There are some 27,000 known Near-Earth objects, and only these two are metal rich. Of the 1.2 million asteroids that have been identified, only a little over a dozen are in that metal-rich category.”
Reddy has been part of a group researching unusual near-Earth objects since 2005, and so these findings are most rewarding.
“In the years ahead we can study Psyche, a large metal-rich object that is quite far away,” Reddy said. “And now we also know of two much smaller metal-rich objects that are also much, much closer to us.”
According to lead author Juan Sanchez, who is based at the Planetary Science Institute, the group’s analysis shows that both of the Near-Earth asteroids (NEAs) have surfaces with 85% metal such as iron and nickel and 15% silicate material, which is basically rock. These asteroids are similar to some stony-iron meteorites (called mesosiderites) that have fallen to Earth.
While the two “mini-Psyches” can be scientific goldmines because they are parts of cores of what Reddy calls “baby planets,” they are not likely to have the significance of Psyche, which is a much larger portion of a core.
Psyche has a mean diameter of about 140 miles (the distance from Los Angeles to San Diego) and accounts for about 1 percent of the mass of the combined objects of the main asteroid belt. The NEA metal asteroids are much smaller, with diameters of about two miles. But unlike Psyche, the “mini-Psyches” could have real importance in the fledgling but growing field of space mining because of their relative proximity.
The amount of iron, nickel, cobalt and platinum group metals contained in the asteroid 1986 DA, for instance, is reported in the paper to exceed the accessible reserves on Earth. Copper and gold is also expected to be present in abundance, but lower than the total Earth reserves.
These estimates come from spectral analysis but also from years of studying iron meteorite that fall to Earth. The elemental makeup of those meteorites is inferred to match the make-up of the metal asteroids.
The opportunity that this potentially opens up is asteroid mining on a body with lots of valuable and useful elements. The technology to conduct such mining is in its infancy, but the possibility is tempting.
“Nature has already done the really hard work in making these metals accessible,” Reddy said. “A collision blasted away vast amounts of rock and exposed the cores. These mini-Psyches offer pieces of a small planet core for study and far off, an opportunity to collect very valuable resources.”
The team used the compositions and orbits of asteroids 1986 DA and 2016 ED85 to identify four possible asteroid families in the outer region of the main asteroid belt, which is home to the largest reservoir of small bodies in the inner part of the solar system. This also happens to be the region where most of the largest known metallic asteroids including Psyche reside.
“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 co-author David Cantillo, a UA undergrad. “It’s possible that some of the iron and stony-iron meteorites found on Earth could have come from that region in the solar system too.”
The idea of sending spacecraft to asteroids and having them pick up material and bringing it back to Earth is not entirely far-fetched. Two Japanese missions (Hayabusa to the asteroid Itokawa and Hayabusa2 to the asteroid Ryugu) have accomplished this feat, and the NASA’s OSIRIS-REx spacecraft is returning home after collecting material from the asteroid Bennu.
While the missions provide a proof of concept that a spacecraft can gather material from asteroids, the amount collected was tiny and the value is overwhelmingly scientific.
A substantial number of international companies have been started with the goal of mining asteroids, but none have gotten far and some went out of business. As Reddy described it, the technology and markets needed to make asteroid mineral mining plausible have quite a few years of maturing ahead before they could become a reality.
Far more likely in the mid-term, Reddy said, is the prospect of “mining” H2O, on the moon and later asteroids. Water is present on most celestial bodies, either as ice inside rock or as components of minerals.
If the rocks can be broken and water collected, the it can be processed and then used to support astronauts or split into two valuable components — the oxygen for humans to breathe and hydrogen as a fuel.
With plans moving forward from several national space agencies to land humans on the moon again and this time have them establish an outpost or future colony, the need for technology to “mine” water out of rocks is considered an essential part of future missions.
One of those involved in preparing for that day is Moe Momayez, associate professor of mining and geological engineering at UA. He and a colleague recently won a $500,000 grant from NASA to study new and improved ways to drill on the moon, and how to program a swarm of robots that can perform mining tasks.
He said the field of off-Earth mining is expanding fast and that he expects it to be part of lunar missions within a decade.
“Several times we applied for a grant to do this kind of work but never were selected,” he said. “Now we are selected for a grant — yes, a small one but with others maybe coming — and I think that tells you something about what is becoming important to NASA.”
Breaking rocks to get at the water, and later mining for minerals, will be especially hard on the moon (and on asteroids) because there is no gravity and no atmosphere. Everything related to mining, he said, will have to be secured in a vessel or else it can quickly disappear.
“Most Earth mining involves blasting but that won’t work on the moon because you need oxygen to blast and it’s not present,” he said. “There’s so much rethinking that needs to be done.”
Momayez — like most aspiring off-Earth miners — is focused now on getting at lunar water. The technology and economy for mining lunar or asteroid minerals and metals is well in the future, and will probably be used primarily to support other space ventures, he said. The cost of bringing the iron or nickel or even titanium (from the moon) back to Earth seems prohibitive.
One group of minerals and elements that might become cost-efficient to mine on asteroids and bring to Earth are the platinum group and rare earth elements — precisely what are expected to be present in substantial deposits on Psyche and the mini-Psyches.
“Yes, if platinum or rare-earth minerals are found in a pure form, it might be economical to haul them back to Earth.,” Momayez said. “But we know they’re not found on the moon, and so they would be coming from metal-rich asteroids.”