When Lindy Elkins-Tanton imagines the metallic asteroid Psyche, she dreams of terrain unlike any seen before. Small craters could look like frozen splashes of water, fringed with silvery spires. Metallic lavas, squeezed out billions of years ago, might shimmer nearby. Gigantic cliff faces, cleaved from the asteroid’s metal crust as it cooled and contracted long ago, might be studded with green crystals of olivine.
Whether these vistas exist depends on whether the asteroid, a beguiling 220-kilometer-wide object discovered in 1852, really is the hunk of iron and nickel long assumed by astronomers. Now, Elkins-Tanton and her colleagues are one step closer to finding out, after a $1.2 billion NASA mission to Psyche launched today. “It’s beautifully motivating to be doing a primary kind of exploration to a body that no human has ever seen before,” says Elkins-Tanton, principal investigator of the mission, also named Psyche, and a planetary scientist at Arizona State University.
Previous missions to asteroids, which represent the leftovers from planet formation, have explored bodies with mostly rocky exteriors. Psyche will be the first to rendezvous with an “M-type” asteroid: a group of unusually reflective and dense asteroids.
For decades, scientists have wondered whether Psyche could be the denuded metal core of a larger protoplanet. According to this scenario, some 4.5 billion years ago, the original body grew massive enough that the heat from its gravitational collapse and radioactive elements partially melted the rocks, which separated into layers. The heaviest metals fell into a core. Millions of years later, a cataclysmic “hit-and-run” collision with another budding world stripped it of its outermost layers and exposed its iron-nickel heart.
The chance to get a close look at a rocky world’s metallic core was a selling point of the mission when NASA selected it in 2017, as it promised insights into the composition and history of cores in full-fledged planets. “There’s no easy way to probe what a core would be like in Earth or Venus and the rest,” says Psyche science team member Bill Bottke, a planetary scientist at the Southwest Research Institute.
Once the spacecraft arrives at Psyche in 2029, it will spend more than 2 years in orbit testing the hypothesis. Subtle shifts in the spacecraft’s radio communications with Earth will let researchers map the asteroid’s gravity field, which will reveal whether Psyche is a uniformly dense hunk of metal or glommed together from rubble. A magnetometer will search for relic fields, perhaps leftover from the churning of an ancient liquid metal core. Gamma rays and neutrons given off by the asteroid’s surface as it is bombarded by the solar wind and cosmic rays should reveal the presence of nickel, expected to make up between 4% and 12% of a metallic core’s weight. Imagers will also take pictures of Psyche’s terrain that, at their highest resolution, will reveal features as small as basketball courts.
Data from ground-based telescopes have already dealt the metal-core hypothesis some blows. “I’d call that a long shot,” says Bloomsburg University of Pennsylvania astronomer Michael Shepard. Updated estimates of Psyche’s mass and volume, some from Shepard, have found that it is only about half as dense as iron meteorites. What’s more, studies of light reflected from Psyche suggest it isn’t purely metal but probably contains some carbonaceous material, a typical asteroid ingredient, and rocky silicate minerals.
The Psyche team now estimates the asteroid is between 30% and 60% metal by volume: not a hunk of pure metal, but perhaps a somewhat porous mix of a protoplanet’s core and mantle, mangled by the hit-and-run collision and then given a carbonaceous dusting by billions of years of smaller impacts.
Another possibility is that Psyche has a metallic core hidden by an especially thin, rocky mantle. The shiny patches seen on its surface from afar could be the result of ancient bouts of “ferrovolcanism” that oozed fluid iron sulfide onto the surface. Or Psyche may not have any core or mantle at all. It might just be an unlayered conglomeration of grains assembled in a metal-rich region of the protoplanetary disk of gas and dust surrounding the young Sun.
Rare meteorites called enstatite chondrites have high metal contents and Psyche-like densities, but their parent body is thought to have formed closer to the Sun. The orbits of Psyche and its fellow M-types instead resemble the paths of asteroids that seem to have formed beyond the orbit of Jupiter. “If you’re going to make Psyche in all these crazy places, how does it get to the asteroid belt?” Bottke asks.
Getting the Psyche spacecraft to the asteroid belt has also come with challenges. It launched a year late because of slowdowns in software testing that an independent review board partially attributed to understaffing at NASA’s Jet Propulsion Laboratory, which manages Psyche. That led to budget overruns that forced NASA to divert funding from VERITAS, a planned Venus mission. “Everyone’s doing their best and, you know, sometimes the resources just don’t stretch,” Elkins-Tanton says.
Smoother sailing will hopefully come 6 years and 3.6 billion kilometers from now, once the Psyche spacecraft settles into orbit around its namesake. It promises to add to the menagerie of asteroids and comets that humans have explored. The exotic asteroid will also expand our imagination, Elkins-Tanton says: “It’s like suddenly discovering there’s such a thing as butterflies.”
