It’s a heady time for planetary scientists who study asteroids—the sprawling mass of debris left over from the formation of the solar system, confined largely into a washer-shaped belt between the orbits of Mars and Jupiter. On 24th September, a capsule the size of a small coffee table parachuted into the Utah desert containing a sample of an asteroid called Bennu. Collected by Osiris-Rex, a robotic mission led by Nasa, it was the triumphant culmination of a journey that began with the mission launch in 2016. Osiris-Rex arrived at Bennu, an asteroid about 500 metres across, in 2018. In 2020 it scooped up an estimated 250g of material from its rubble-like surface—the biggest asteroid sample ever collected in situ—before dispatching the capsule back to Earth.
Bennu’s rocky fabric is thought to include a lot of carbon-rich material, which is expected to have stayed in a fairly pristine state since the solar system’s formative days. The sample deposited in Utah will therefore give us a glimpse of the stuff from which the Earth and other rocky inner planets (those inside the orbit of Jupiter) were made. Bennu’s carbon-containing space rock might be like the rock that seeded the Earth with this element, vital for the appearance of life.
It's for such insights into the formation of the planets that scientists are turning to the asteroids, which might otherwise seem like a far less glamorous destination for space missions than the planets themselves. Asteroids range in size from a few whoppers like Ceres, a dwarf planet just over 900km across, down to countless objects the size of small boulders. Collectively the objects that make up the asteroid belt are thought to have a total mass of about 3–4 per cent that of our Moon.
Impressive though it is as a feat of space engineering, Osiris-Rex is not the only reason for asteroid experts to feel that their moment in the sun is here. On 12th October—if all goes to plan—another Nasa mission to an asteroid will blast off from the Kennedy Space Center in Florida. Called Psyche, it is heading for an asteroid of the same name, which looks to be one of the strangest “worlds” we will have ever encountered close up.
Asteroid Psyche is estimated to be about the size of Switzerland—280km across at its widest point. That’s not big enough for our Earth-based telescopes to be able to see it as anything more than a blur. We don’t even know for sure what shape it is, although the way it reflects light and radar suggests that it is elongated, like a typical potato. But what marks it out as unusual is that the reflected light seems also to imply that it is composed not, like many other asteroids, of rock and ice but of almost pure metal: iron, with a bit of nickel and a smattering of silicate rock or sulfur. “We’re sending a robotic probe to go find about a new kind of world, a world with a metal surface,” says planetary scientist Lindy Elkins-Tanton, a professor at Arizona State University and head of the Psyche mission.
How did these metals come to be concentrated into a big lump in the early solar system? It’s thought that Psyche is the remnant from the core of a failed planet.
The solar system formed from a disk of gas and dust surrounding the nascent Sun, about 4.6bn years ago. Stars like this—our Sun is a fairly average example—condense from vast clouds of interstellar gas as gravity gradually pulls the matter into a dense ball, surrounded by a disk of matter called the solar nebula from which planets may form. Most of the gas is hydrogen, the lightest element and by far the most abundant in the universe. Apart from a small amount of “primordial” helium and a smidgen of lithium also created in the Big Bang, all the other chemical elements, such as carbon, oxygen and metals, are forged by nuclear fusion of light atoms inside stars themselves. The brew of elements in the early solar nebula was supplied by older stars that exploded in supernovae and scattered these heavier elements across the cosmos. (The shockwaves of such stellar explosions are also thought to supply the trigger of the collapse from which new stars are born.)
While the Sun itself is made almost entirely of hydrogen and a little helium, its surrounding disk of gas and dust condensed into clumps called planetesimals. These in turn collided and merged into planets. In that messy, chaotic process, some collisions might be catastrophic enough to shatter a nascent planet to smithereens. It’s thought that the young Earth was almost destroyed this way in a collision that smashed off enough debris to form the Moon.
The asteroids are a mix of debris either left over from such collisions or which never got to coalesce in the first place. Jupiter’s gravitational influence has shepherded most of them into the current asteroid belt. Whereas the minerals of the Earth have been chemically changed over time by the heat of the planet’s interior and processes like weathering on the surface, many of the asteroids won’t have experienced such planetary-scale transformation and are likely to have been barely altered since they formed.
When a rocky planet formed, its gravity caused the heaviest, densest elements—generally metals—to sink into the centre, like syrup sinking to the bottom of a cocktail. This is why the Earth’s core is mostly iron and nickel, still molten even now from the fury of the planet’s formation. It’s the churning of this metallic core that produces the Earth’s magnetic field.
Psyche is believed to be a piece of the core of a planet that began to form this way but was then blasted apart by an immense collision. “Our hypothesis is that it's part of the core of a planetesimal that formed in the first two million years of our solar system,” Elkins-Tanton tells me. If that’s so, it would give us the first direct glimpse of a planetary core. We can’t look directly at Earth’s, because the pressures and temperatures down there are too intense.
“We are interested in how rocky planets form, and how they get to be in the structure that they are, with a metal core and a rocky exterior,” Elkins-Tanton says. “And it turns out that there’s one place in the solar system that we can go to learn about the metal cores of rocky planets: this asteroid Psyche.”
It will take about six years for the Psyche mission to reach its destination, after first flying past Mars around 2026. Once it arrives, the onboard instruments will examine Psyche’s surface from afar: taking images and analysing the elements it contains. In a first for uncrewed space missions, it will broadcast its findings back to Earth not as radio waves but using an infrared laser, which can transmit 10 to 100 times more information in a given time.
Sadly, Psyche won’t glisten like a polished lump of steel. “In my dreams, it's a polished slab of shining meteorite metal with gleaming inset gold and yellow crystals,” Elkins-Tanton laughs. “But we don't have any photos of its surface. And the truth is, it's been hanging out in space for four and a half billion years, and it's probably not shiny.”
“This is exploration of a kind of world humans have never seen”, she adds. “And probably everything I tell you today is going to be wrong when we get there.”
The six-year travel time will be a long wait—but planetary science is a waiting game, and not just after launch. Getting Nasa to agree to a mission is a long and nerve-shredding process of persuasion: there are just too many interesting places in the solar system to be able to send spacecraft to them all, and competition for the available funds and resources is intense. “You have to do week-long review processes with the external review board,” says Elkins-Tanton. “We wrote over 1,500 pages of proposals. And then you go through two gigantic gates: one where they knocked 27 proposals down to five, and then the second one where they knocked five proposals down to two and selected us for flight. It's an epic process—marathon after marathon.”
What helped to swing the decision, she says, is the narrative they constructed. “You've got to have a story. It’s got to be a story that brings you along and makes you feel a part of it. And it's especially hard to tell a compelling human narrative about an object that we've never visited, and we have no pictures of. So one of the things we had to do was create art based on the science, to inspire people and make them to want to be part of it.”
Psyche is the culmination of a remarkable career for Elkins-Tanton, who has described her scientific and personal journey in her 2022 book A Portrait of the Scientist as a Young Woman. As well as recounting her formative experiences as a geologist at the Massachusetts Institute of Technology and elsewhere (including hair-raising expeditions to Siberia in the 2000s in search of ancient volcanic basalts), the book is a sometimes harrowing and sobering account of the challenges facing female scientists. The problems Elkins-Tanton describes—sexual harassment and bullying, being constantly sidelined, overlooked and patronised by male colleagues—are still routinely reported by women in science.
Elkins-Tanton proposes ways in which scientific research can (and reasons why it must) be made less gladiatorial and confrontational, and more collaborative and genuinely inquiring. A massive project like the Psyche mission can, after all, hardly expect to get off the ground without the ability of the leadership to foster a team spirit. “The thing that I really love is the feeling of people doing things together,” she says. “The era of the hero scientist is past. Now, the really biggest things have to be done in interdisciplinary teams. And that's fortunate for me, because that's what I really love.”
She has no illusions about the uncertainties of space launches. The mission has already been postponed once from August 2022, due to what Elkins-Tanton calls a “very expensive and painful error”—as well as a “proof through failure that team culture is everything.” And the intended date this time around has just slipped from the 5th to the 12th October to allow for a last-minute check of the spacecraft’s thrusters. But there’s some leeway for such tweaks within the current 21-day launch window, and Elkins-Tanton is confident that the journey to a metal world will start in October. “And it’s going to be amazing.”
