ByRobin George Andrews
Published October 9, 2023
• 12 min read
Hiding within the asteroid belt exists a world suffused with metal, where the surface may be covered in cliffs and craters of iron alloys. Glimpsed only as a bright speck in telescope images, no one knows exactly what the metal asteroid, named Psyche, looks like. It could simply be a lump of rock and metal—but it could also be decorated with an array of otherworldly aspects, from frozen flows of sickly yellow lava to upright tendrils of solidified iron, like undulating spires thrust toward the sky.
Scientists hope that Psyche is “something really bizarre,” says Henry Stone of NASA’s Jet Propulsion Laboratory, the project manager of a new mission to explore the asteroid.
That mission, also named Psyche, is set to launch from NASA’s Kennedy Space Center in Florida on October 12, propelled off-world by a SpaceX Falcon Heavy rocket. The spacecraft will then begin a Herculean journey through the solar system until, in 2029, it catches up to the enigmatic metal asteroid.
“We do not know what this is going to look like,” says Paul Byrne, a planetary scientist at Washington University in St. Louis. “Who knows what we’re going to see when we get there?”
The mission—led by Arizona State University and NASA’s Jet Propulsion Laboratory—will mark the first time humanity has explored a metal-rich asteroid, an endeavor to help scientists better understand the furious formation of the solar system. But at this stage, aside from its existence, practically everything about Psyche is a mystery.
“My biggest wish is that Psyche will completely surprise us,” says Lindy Elkins-Tanton, the mission’s principal investigator and a planetary scientist Arizona State University.
Rogues of the asteroid belt
There are three broad types of asteroids in the belt between Mars and Jupiter: stony types with some metal (S-types), rocky ones with clays and other carbon-bearing compounds (C-types), and those that are thought to be highly metallic (M-types). Roughly eight percent of the asteroids in the main belt are M-types, which makes them rare, intriguing and, so far, unexplored.
The approximately 170-mile-wide Psyche was discovered back in 1852 by Italian astronomer Annibale de Gasparis, who named the asteroid after the Greek goddess of the soul. A few decades ago, scientists surmised from its brightness that it was an M-type, one with plenty of iron and a bit of nickel.
“When people bounce radar beams off it, those beams come screaming back like they would off a metallic mirror, not like off a fluffy rock,” says Jim Bell, the Psyche deputy principal investigator and a planetary scientist at Arizona State University. And tiny variations in Psyche’s orbit, and the orbits of nearby asteroids, suggest that it is extremely dense—perhaps, as was originally thought, an almost entirely metallic body.
If so, that means that Psyche is strange even by M-type standards, causing scientists to ruminate on various origin stories.
One tantalizing suggestion is that the asteroid is the exposed heart of an almost-world—a failed planet’s metal core. “The best way we know to make a big blob of metal is by differentiating a parent body and forming a metal core,” says Elkins-Tanton, referring to the process that takes place as rocky planets form, wherein denser elements in a sphere trickle inward to the depths.
Nobody has ever seen a planet’s core; they can only be indirectly sensed using powerful seismic waves or by studying the wobble of a planet as it orbits the sun. “We can’t see the core of any other planetary body. But we might be able to see that at Psyche, and that’s what’s really exciting about it,” says Brandon Johnson, a planetary scientist at Purdue University.
An exposed core would mean the crust and mantle of the parent body were torn away during a mammoth impact event. “If Psyche was the body that hit another bigger body, it can have most of its mantle stripped off in essentially one go,” says Johnson.
Alternatively, Psyche may have formed far closer to the sun, where its surface material was blown away like confetti, but its more resistant core remained. “This is partly why the planet Mercury has such an enormous iron core,” says Bell. But if Psyche formed there, how did it get to the asteroid belt beyond Mars?
The question is part of an overarching puzzle that planetary scientists wish to use Psyche to solve: “Where did material form in the first place, and how did it end up in the asteroid belt?” asks Elkins-Tanton. “I suspect that there are big parts of that process that we really have no idea about at all—things that we haven’t imagined.”
Sneaking up on an asteroid
The most recent data suggests that Psyche is a mixture of both rock and metal (mostly iron), with the latter comprising 30 to 60 percent of its volume—not quite as metallic as initially suspected, but still potentially more than half metal.
“It’s not actually the densest one,” says Johnson. But the way radar pings so enthusiastically off it “means it seems to have the most metal at the surface.” Beyond being a highly metallic world, the details of Psyche remain speculation. Even its shape is unknown; scientists often describe it as a potato, because “potatoes come in many shapes,” says Elkins-Tanton.
Fortunately, the Psyche spacecraft is equipped to uncover the truth. The probe is armed with a handful of instruments with specific tasks: a pair of spectrometers designed to decode the asteroid’s elemental makeup; a magnetometer to look for an ancient magnetic field; a multipurpose imager; and a tool to study the asteroid’s gravitational field, which reveals information about its density.
After launch, the craft will undergo a 2.2-billion-mile odyssey around the solar system, slingshotting around Mars in 2026 to get a gravity boost en route to Psyche. The journey is “like going to the moon and back 10,000 times,” says Stone.
The spacecraft will approach Psyche in the summer of 2029, orbiting the asteroid with a wide berth at first. “We’re going to sneak up on it,” says Elkins-Tanton.
Myriad closer orbits are planned—partly so the instruments get a better read on the asteroid, and partly because “we’ve got to get the lighting right for the photographs,” as Psyche inconveniently “spins like a rotisserie chicken,” says Elkins-Tanton. But the spacecraft needs to start from afar because the asteroid’s odd shape and high density mean that “Psyche’s gravity field is going to be really weird,” she says.
Go in too close without carefully plotting out future orbits, and the spacecraft could crash into the surface. “We have to build, from that first orbit, a rough gravitational model of the body,” says Stone.
After the 26-month primary mission is complete, and if NASA permits it, the spacecraft “would just be allowed to orbit closer and closer and closer,” to get as much high-resolution data as possible, until it ultimately crashes into the cold metallic ground.
Otherworldly features
During those close orbits, the landscape of Psyche will be unveiled. “We’re going to see it go from that dot to who knows what,” says Bell. “We really don’t know what we’re going to see,” he says. “It could be pretty mundane”—a rubbly mess pockmarked by craters.
But it could be utterly flamboyant. A meteorite impact on Psyche may briefly liquify the iron, causing streams of it to gush upwards and outwards from the asteroid. “When it hits the vacuum of space, it cools very quickly, and it could leave this, you know, like, this crown,” says Bell. “It’s highly speculative. But it’s fun to think about.”
There may also be the scar tissue of past, and distinctly alien, volcanism. When Psyche’s liquid iron began cooling eons ago, any sulfur-rich liquids within the protoplanetary goop would have pooled toward the top, like oil refusing to mix with water, before erupting as off-yellow sulfurous lava.
“That is something that I really hope that we see, evidence of ancient volcanism on a metal body,” says Elkins-Tanton. “I think that would be amazing.”
And if a remnant magnetic field is detected, “that’s almost a slam dunk” for Psyche being the exposed core of a prototypical world, says Bell.
Rocky planets with metallic fluids churning about in their cores (like Earth today, and Mars long ago) possess immense magnetic fields. Psyche’s diminutive size means that its liquid innards solidified long ago. But a strong magnetic field signature inscribed in its geology would suggest it once had a fluid, dynamo-generating core.
Psyched for launch
Psyche was picked for flight by NASA back in 2017 along with another asteroid-focused mission named Lucy—the victors of a semi-regular competition to decide which robotic space missions the agency will green light. “We made a bet that if we were selected from flight—which was very, very unlikely—we would get tattoos,” says Elkins-Tanton. “So now seven of us have tattoos.” Elkins-Tanton’s is a planetesimal.
Now the spacecraft is ready to fly. Psyche has several launch opportunities in October, with the 12th being the earliest.
Whenever it goes, its team will endure the nail-biting sight of a mighty conflagration sending its robotic envoy toward the stars.
Bell, who has worked on several space missions, suspects this surreal moment never loses its intensity. “You take your precious cargo, and you put it on top of a bomb,” he says. And “as that rocket lifts off the pad, you want to sing, you want to dance, and you want to throw up at the same time.”
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