Our solar system has got it all: rocky worlds and giant gaseous worlds, volcanic moons, and icy frozen ones, not to mention all those asteroids, comets, and various leftover chunks from formation floating around. By now, we’ve sent robotic probes to zoom past, orbit, and land on almost every one of these bodies. So it’s rare to find an entirely new type of world to fly to, one that we’ve never been to before.
The bizarre metallic asteroid officially known as 16 Psyche is rather unique in the solar system. Composed of roughly 90 percent iron and nickel, the little world appears to be the exposed core of a former planet, which would have been annihilated in a violent impact billions of years ago. But other than these few tantalizing hints, our knowledge of Psyche remains woefully incomplete.
“We know just enough to know that we need to go there…”
“We know just enough to know that we need to go there,” says planetary scientist Lindy Elkins-Tanton of Arizona State University in Tempe.
NASA recently selected the Psyche spacecraft for one of its next Discovery-class missions. Elkins-Tanton, who leads the mission, says she was surprised and thrilled when she heard the news. It now seems that we’re in a golden age of asteroid exploration—another asteroid mission, the Lucy spacecraft, was also selected for NASA’s Discovery program, and the Dawn mission has recently explored the minor planets Vesta and Ceres. Lucy will visit a series of asteroids sharing an orbit with Jupiter to learn more about our solar system’s origin.
Asteroids are hot right now because of what they tell us about planetary formation. In the early solar system, rocky bodies are thought to have crashed into one another, accreting and merging into larger worlds. In some rare cases, one of these impacts might have completely obliterated a planetesimal, perhaps expelling its molten metallic core and leaving behind a world like Psyche. Such leftover chunks can tell scientists a great deal about these primordial processes.
One of the Psyche mission’s main goals will be to determine exactly how the asteroid Psyche formed. The idea that it’s the former core of a planetoid is dominant mainly for lack of other plausible explanations. So the Psyche team hopes to measure a remnant magnetic field that could have been present when the body was in the center of a rocky world. This is similar to the Earth’s magnetic field, which is generated by the spinning of our nickel-iron core. Once expelled from its planet’s center, Psyche’s magnetic field would have gotten frozen, leaving it as basically a giant bar magnet floating in the asteroid belt. The presence and strength of any magnetic field will tell the team a great deal about Psyche’s origin.
If it is a core, then Psyche’s surface will give researchers important insights into the core-mantle boundary inside of rocky worlds such as our own. “It’s really the only way humans will ever get to see a metal core,” says Elkins-Tanton. Places where impacts have punched below the surface will provide data about the asteroid’s internal structure, and therefore the structure of our planet’s core, as well.
Exactly what size of planet Psyche might have come from remains a mystery. Most models suggest that it could have once been in the center of a Vesta-sized world. But other simulations suggest it might have once been inside a larger Mars-sized body. In that case, the violent collision that destroyed the planet would have also split the metal core into several large droplets—of which Psyche is only one—each containing an extremely large percentage of metal. It’s possible that other Psyche-like metal worlds exist somewhere in the solar system, though astronomers have yet to find any.
Elkins-Tanton says she’s most excited to finally get images of Psyche’s surface. The best telescopes on Earth can see little more than a spot of light when looking at the asteroid, and the Psyche spacecraft will beam back the first photos of a predominantly metal world. No one is quite sure what to expect.
“There could be crazy frozen metal spires” from crater impacts, says Elkins-Tanton, as well as soaring 10- to 15-kilometer-high iron-nickel cliff faces studded with green silicate minerals.
“Let’s discover something completely new…”
Given its abundant metal reserves, Psyche might one day make a good target for human explorers. Colonizers might want to mine its metals for their spacecraft in order to push farther out into the solar system. Interestingly, Psyche also contains trace amounts of water, which was likely delivered by small asteroids and comets impacting its surface. Though locked up in hydrated silicates, this water could be extracted fairly easily and used to create either rocket fuel or fill up astronauts’ water tanks.
The Psyche mission is expected to launch next decade and reach the metallic world in 2030. Its arrival will certainly revolutionize our understanding of planetary formation and the internal structure of rocky worlds. But on some level, Elkins-Tanton says she’s hoping that Psyche ends up being surprising. “I would almost love for it to be something we’re not even anticipating,” she says. “Let’s discover something completely new about our solar system that doesn’t fit into our theories.”