In its visit to Psyche, NASA hopes to glimpse the center of the Earth. NASA’s mission to the solar system's largest metallic asteroid promises to show us the iron-nickel core of a dead planet. New research, however, hints that this asteroid is much more. The article was written by Megan I. Gannon for the journal Popular Science. Guy Libourel, professor at the Université Côte d'Azur, and Mark Wieczorek, senior scientist CNRS, lagrange laboratory (CNRS-UCA-OCA) have contributed to this research.
No one is certain what the Psyche craft will spy once it takes a closer look at the seemingly metallic surface of its namesake. Rui Ricardo, Folio Art
Shortly after 1 a.m. on February 8, 1969, a bluish-white fireball streaked across the sky above the Southwestern United States and northern Mexico. A meteor getting sucked into Earth’s gravity had exploded in the atmosphere. Scorched rocks rained over a 200-square-mile area around Pueblito de Allende in Chihuahua, where locals picked up the first bits of debris. A scavenger hunt began immediately. Kids and other residents used plastic candy bags to pick up meteorites on the sides of highways, near houses, in bean fields. Scientists also descended on the cactus-dotted chaparral landscape. NASA even sent researchers; they were preparing for the upcoming Apollo 11 mission to the moon and treated the crash like a dress rehearsal for studying lunar samples. In the first few months after the fall, teams discovered an estimated 2 tons of material, and at least 37 labs in 13 countries received samples. Since long before anyone could dream of sending astronauts and robots to collect rocks in space, looking at meteorites was the best way to see our solar system’s ancient building blocks up close.
The Allende meteorite, as it came to be known, was the biggest object of its kind ever found. And, as the poster child for the oldest material in our solar system, it became perhaps the world’s most studied meteorite. Its specimens contained grains of dust that were among the first solids forged in the nebula that swirled around the sun more than 4.5 billion years ago. That dust would condense into pebbles, then rocks, then boulders the size of cities—the size of states. It would eventually form the first mini-planets, or planetesimals, which would either grow into worlds like Earth or get blasted apart in the violent cosmic playground, some of the pieces ending up scattered in a debris field now known as the main asteroid belt between Mars and Jupiter.
Some 40 years later, the Allende meteorite landed at the center of a new mystery. Ben Weiss, a planetary scientist at MIT, found that its samples appeared to have the imprint of an ancient magnetic field. For decades, scientists had assumed the two main types of meteorites—chondrites and achondrites—came from two separate classes of parent bodies. Allende belonged to the chondrites, thought to be pristine, never-melted space rocks that formed from proto-planetary dust. Achondrites—like meteorites made from the moon and Mars—are chunks broken off of planets or relatively wee planetesimals that swell until their insides melt. In that scenario, heavy metals like nickel and iron sink to the core while lighter materials float to the surface. The assumption was that the mechanism that produces a magnetic field inside an achondrite parent body’s core was unique to that class of meteorite. But, Weiss wondered, if Allende had never been part of one of these melted space rocks, how could it be magnetized?
In 2009, Lindy Elkins-Tanton, then Weiss’ colleague at MIT, proposed that Allende might be a chunk of a hybrid object, one that had melted on the inside but not the outside, a startling theory at the time. « It caused big waves in the scientific community and made everybody really upset, » she recalls. « It was one of those little tempests in a teapot that happens in academic research. » Naturally, many scientists are loath to change long-held ideas without extensive evidence, but about a decade after sparking that controversy, Elkins-Tanton is leading a mission that could resolve unanswered questions about ancient planetesimal cores floating in space—and go back in time to study Earth’s own formation.
In August, a spacecraft will launch on a 41-month journey to visit Psyche, the biggest metallic asteroid in our solar system. The behemoth is suspected to be the iron-nickel core of a growing planet whose outer layers were stripped in cosmic hit-and-runs. We’ll never get a direct look at Earth’s core—at least not until we develop the superhuman technology to drill down 3,100 miles and withstand temperatures of 9,000°F and pressure 3 million times that of the atmosphere. Psyche, however, offers a chance to stare into a planet’s heart, to learn about the early solar system and the source of magnetic fields like the one that protects Earth from cosmic radiation and perhaps allowed complex life to evolve.
A team of nearly 800 people are in crunch time ahead of the mission, also called Psyche. But as the launch window approaches, the asteroid is shaping up to be a much stranger target than NASA may have bargained for when it approved the $850 million project five years ago. At the time, Psyche was estimated to be 90 percent metal. Fresh analyses suggest that percentage is too high. So researchers are coming up with wild new hypotheses to explain its properties—hypotheses they’ll actually be able to test after the spacecraft arrives in orbit around the asteroid in 2026.
« Probably everything I say today will be found to be wrong once we’re there. That is the beauty and the excitement and the compulsion of space exploration. » - Lindy Elkins-Tanton
Is Psyche really the exposed core of a planet? Or is it simply a pile of metal-rich rubble? A strange world with remnants of metal-gurgling volcanoes? Something dazzling, like a giant rare glittery class of meteorite? « This is the part that I love about it, » says Elkins-Tanton, now vice president of Arizona State University’s Interplanetary Initiative as well as Psyche’s principal investigator. « None of those answers that we’re coming up with to explain the existing data are simple, obvious answers. They’re all low-probability events, which maybe makes sense, because it seems like there’s only one Psyche out there. »
For now, the team’s primary notion remains that Psyche is the remnant of a shattered core. « Another is that it’s something we’ve never seen before », says Jim Bell, a mission scientist also at ASU. One idea is that Psyche could be a metal-dominated world that formed very close to the sun and somehow got out to the asteroid belt, he says. « We don’t know what those objects look like because they’re gone. They’ve fallen into the sun, they’ve merged into the terrestrial planets. So even if we’re wrong, we’re gonna learn something pretty cool. »
Maybe asteroids could make us rich via space mining, or extinct like the dinosaurs, but they are perhaps most worthy of exploration because they hold the secrets of our solar system’s past. Earth’s most ancient rocks have been melted and mashed up so many times that it’s rare to find traces of its 4.5-billion-year history. If our planet has lost all memory of its infancy, then visiting an asteroid could be like peeking at its baby pictures.
The first asteroids were observed around 220 years ago. Based on a flawed model of the solar system, astronomers had concluded there should be a planet between Mars and Jupiter. To hunt it down, a society known as die Himmelspolizei, « the Celestial Police », formed in Germany to assign each member a 15-degree slice of sky to scan. Instead of locating a single world, it found several, which we now know to be asteroids. Over the next decades, stargazers would discover bodies like Ceres, Pallas, Juno, and Vesta. In March 1852, Italian astronomer Annibale de Gasparis at the Naples Observatory identified Psyche, the 16th such object, and named it after the Greek goddess of the soul.
More advanced techniques have since slightly refined our picture of Psyche. For example, spectrometers can decipher a faraway world’s composition by looking at the different wavelengths of light that minerals reflect. By the 1970s, astronomers found that a small group of asteroids were similar to iron meteorites that had fallen to Earth. By the 1980s, they recognized Psyche as the biggest of these M-class, or metallic, asteroids in the main belt, and they theorized it was the remnant of a dead planetary core.
Psyche wasn’t on Elkins-Tanton’s mind when she joined her colleague Weiss in the fall of 2009 to brainstorm why the Allende meteorite was magnetized. Within a half hour, she had drawn a diagram on his whiteboard showing a strange hybrid object that had begun melting from the inside out under the extreme heat of radioactive isotopes. « She was basically making a really simple point that maybe it doesn’t melt all the way through, which seems so obvious », Weiss says. « I’ve never done this before or since, but we got our camera and took a picture. »
At the time, astronomers were beginning to overturn the textbook wisdom that the early solar system formed in a methodical, stately fashion. Instead they favored a violent infancy in which high-energy processes rapidly formed planetesimals and planets. The theory that Weiss and Elkins-Tanton presented to packed conference rooms in 2010 and then published in the journal Earth and Planetary Science Letters in 2011 contributed to this new view. Bruce Bills and Daniel Wenkert, two researchers at NASA’s Jet Propulsion Laboratory in California, were intrigued enough by the idea to invite the MIT scientists out to Pasadena to the lab’s Innovation Foundry, an incubator for mission ideas. Could they design a space voyage that would let them actually see the insides of asteroids and find out if some could indeed be these hybrid bodies? As JPL experts looked at potential targets and calculated trajectories, the group very quickly realized one of its candidates was Psyche—not just any building block, but the one most likely to be an actual core, something scientists have never observed. Elkins-Tanton and her team began working on a proposal to visit.
Early one morning in January 2017, Elkins-Tanton’s cell phone lit up while she was spending winter break in the snowy hills of western Massachusetts. It was Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate. Service was terrible, but before the call got cut off, she heard, « I can tell I just woke you up, but you’re going to be glad I did. » This was the payoff of the grueling competitive process of pitching a mission to NASA’s Discovery program, the agency’s midsize planetary exploration arm, designed to fund cheap, efficient missions every few years.
In the course of her career, Elkins-Tanton had been in many unexpected situations as she chased compelling geologic questions. While working on her Ph.D., when she wanted to reconstruct the temperature and composition of rock inside the moon 3.5 billion years ago, she looked at soil that the Apollo astronauts had brought home. Later, when she was investigating a 250-million-year-old volcanic eruption that spurred climate changes that nearly wiped out life on Earth, she traveled by cargo helicopter and small boat to remote corners of Siberia hunting rocks. Despite all this, it never occurred to her that a paper she’d authored would lead to an actual mission to space. It also never occurred to her to get a tattoo, but a few months after that fateful call, she was sitting in a parlor getting her first ink: a cross section of a planetesimal on her hand. The artist had suggested she consider a less conspicuous location, but Elkins-Tanton wasn’t interested. « This tattoo is on my hand because this mission is about doing, building, making, going, not just about sitting still and thinking or being afraid. »
The announcement of a new NASA mission can cause a gravitational shift in the world of space research. As the target of a real craft, Psyche began to attract more observation. Coveted telescope time and lab hours were suddenly being devoted to the obscure object. But looking at Psyche—which is only about 172 miles long—is not easy from Earth. (If it were, there’d be no need to go visit.)
« You have to remember, with asteroids, when we look at them in most telescopes, you don’t see anything except a dot, » says Michael Shepard, a planetary scientist who specializes in remote sensing and asteroids at Bloomsburg University in Pennsylvania but isn’t part of the Psyche team. Researchers like him have had to get creative when they want to determine the size, surface features, and composition of a faraway and relatively small object like Psyche.
Shepard has led several projects to measure it and other M-class asteroids, the results of which began to hint that Psyche might not be as metallic as previously thought. Before its collapse in 2020, the huge reflector dish at the Arecibo Observatory in Puerto Rico was one of the few places (and by far the best) to scope out bodies’ radar reflectivity, a measure that helps determine composition. Over more than a decade, Shepard saw Psyche’s numbers drop. « That’s primarily because we only see it being bright when it’s pointed in particular directions,” he says. “The averaging effect has brought the estimate down. »
What really indicated Psyche might not be so metallic is its density. Calculating that metric requires an object’s mass and size, and with more observation, the once-inconsistent numbers for Psyche have started to converge. In a preflight assessment that Elkins-Tanton and her colleagues published in February 2020, they say the best measurements put the asteroid’s density between about 3.4 and 4.1 grams per cubic centimeter. An intact iron-nickel core should be twice that. (Water has a density of 1 gram per cubic centimeter. Most rocks are around 3. Iron-nickel is around 8.) As a result, estimates now put Psyche at just 30 to 60 percent metal.
« That paradigm of a chunk of solid iron floating through space seems like it’s no longer correct, » says Katherine de Kleer, a California Institute of Technology planetary scientist who’s not involved in the mission but has observed and studied Psyche. « So now we’re trying to understand what it is and how it formed. »
How can one explain Psyche’s missing material? Some scientists wonder if it might be all metal, but porous like a pile of rubble—but it’s unlikely an object that big lost heat quickly enough to stay holey. Because the radar reflectivity seems to be higher in certain regions, some researchers, including Brandon Johnson, a planetary scientist at Purdue University in Indiana, have theorized that iron volcanoes may have erupted through the world’s surface as it cooled from the outside in.
« I actually expected quite a bit of pushback because the idea’s kind of wild, » says Johnson, the lead author of one of the papers modeling so-called ferrovolcanism on Psyche. He was pleasantly surprised to find others running with the concept. Since no one has ever seen such a flow on Earth or elsewhere, Arianna Soldati, a volcanologist at North Carolina State University in Raleigh, tried to make one. Using a furnace at the Syracuse University Lava Project in New York, her team melted metal-rich basalt, poured the lava onto a sand-covered slope, and watched how it flowed. The patterns could help them spot the traces of similar activity on Psyche.
On the other side of the globe, equally imaginative experiments probed the apparent mixed geology of Psyche. An ancient asteroid would have been exposed to countless impacts. Guy Libourel, a cosmochemist at the Observatoire de la Côte d’Azur in France, led tests creating these collisions in miniature. At a lab in Japan, his colleagues shot little basalt beads at steel and iron surfaces at ridiculously high speeds, around a little more than 3 miles per second. (A rifle might shoot a bullet at nearly two-thirds of a mile per second.) They found the basalt melted from the heat of impact and flattened like a pancake over the surface of the target. They argue that perhaps Psyche’s metal is camouflaged by a coating of glassy rock imported via impacts. That could explain why there doesn’t seem to be so much metal on the surface—or on those of its M-class cousins, for that matter. Metal asteroids are rare, and as we get additional remote measurements of them, none seem to have densities that would indicate they’re made purely of iron-nickel cores. « We will see in 2026 what the truth is, » Libourel says.
Late this summer some members of the Psyche team will endure Florida’s heat and humidity to watch a fireball streak through the sky. A SpaceX Falcon Heavy rocket loaded with around 44,000 pounds of propellant will escape Earth’s gravity carrying Psyche, a repurposed Maxar communications satellite the size of a car. Once released from the payload faring, the spacecraft will begin a journey of 1.5 billion miles, whipping around Mars for a gravity assist and then using Maxar’s solar electric propulsion system to chug along into deep space.
« We are under the gun, » says Henry Stone, the Psyche project manager at JPL. Because the journey depends on that gravity assist, the team has a strict window for launch that opens in August and closes a few weeks later.
If all goes well, the spacecraft will arrive at its destination in January 2026 and operate for almost two years. Its cameras will capture all the asteroid’s craters, crags, and other topographic surprises of the object in high-resolution images. (The instrument is multispectral, meaning it has filters that can detect invisible signatures of minerals like oldhamite, olivine, and pyroxene that would help scientists figure out how the asteroid formed.)
More data allows mission scientists to better map Psyche and understand its gravity field, so the spacecraft will descend in a series of progressively lower orbits. All the while, its magnetometer sensors, mounted on a 6-foot boom, should find out if the body has a preserved ancient magnetic field, which would be a big clue that it was once part of a mass with a polar-spinning, partially molten, iron-nickel core. « We’ve never seen an asteroid’s magnetic field, but that thing sure seems like a good bet to look for that, » says MIT’s Weiss, who’s leading the magnetometer investigation.
The gamma-ray and neutron spectrometer, propped on another boom, will detect energy signatures created when cosmic rays blast apart atoms in the asteroid. Those measurements will help determine the elemental composition of Psyche up to a meter below the surface, charting the deposits of metals and silicates that may show whether the surface is that of a chondritic or achondritic body.
Perhaps most exciting for researchers who aren’t involved in the mission, images captured by the spacecraft will go online publicly within 30 minutes. Sharing fits with the leadership philosophies Elkins-Tanton has been honing as she manages a team of hundreds. It’s gotten her thinking about how huge science projects can be more ambitious and tackle bigger problems. How do you make sure all the participating researchers don’t scurry back to their labs with their slices of data, never to be heard from again? How do you make a project more than the sum of its parts? She’s been evangelizing for her fellow scientists to throw away the hero model that lifts up only famous and charismatic principal investigators. She doesn’t mind publishing images with glitches that need to be fixed if it means one might be full of surprises that her community can get excited about.
Other recent missions to asteroids should have prepared them for some unexpected sights. In the 2010s, when two separate sample-return missions, Japan’s Hayabusa2 and NASA’s OSIRIS-REx, approached their respective targets, Ryugu and Bennu, scientists saw that both asteroids were strewn with boulders, not covered in fine-grained regolith as expected. Researchers who have gotten sucked into Psyche’s world are excited that the space for discovery in this mission is wide open, and they’re eager to land on the questions they don’t yet know they should be asking.
« Probably everything I say today will be found to be wrong once we’re there, » Elkins-Tanton says. « That is the beauty and the excitement and the compulsion of space exploration. »
The Psyche mission is a journey to a unique metal-rich asteroid orbiting the Sun between Mars and Jupiter. The launch date is August 1st, 2022.
For more
https://www.jpl.nasa.gov/missions/psyche
https://physicstoday.scitation.org/do/10.1063/PT.6.1.20190920a/full/
Contacts
Guy Libourel - guylibourel@gmail.com
Mark Wieczorek - mark.wieczorek@oca.eu
Images copyright
Image Credit: NASA/JPL-Caltech/ASU
Image Credit: NASA/Isaac Watson
Image Credit: NASA/JPL-Caltech
