The dead, metallic heart of an ancient, obliterated Mars circles the sun—and in 2022, NASA will launch a mission to the asteroid belt to explore it. The Psyche spacecraft, which will visit an asteroid of the same name, recently passed its preliminary design review at NASA’s Jet Propulsion Laboratory in Pasadena, California. Next month, NASA headquarters will decide whether or not to give the Arizona State University-led project permission to begin bending metal for the spacecraft.
Because the target asteroid is such a weird, alien object, the mission is one of true exploration. Planetary scientists have no idea what they’ll find when they get there, so they have to plan for anything.
“The state of the art is profound ignorance about what we expect to find,” says Jim Bell, the deputy principal investigator of the Psyche mission. “We are trying to prepare for any eventuality, no matter what it’s like. Our instruments will make interesting measurements, observations and discoveries that will allow us to put the history of that object back together.”
The team's leading hypothesis is that the asteroid is the once-molten metallic core of some planetary body destroyed long ago. No one knows what it looks like—whether chunks of mantle still hang from it; whether it will appear distinctively metal, a hunk of iron in space; or whether it will be covered in rock, indistinguishable by the untrained eye from any other asteroid in the solar system. And the mystery of the asteroid Psyche extends beyond its appearance into the violent past of the solar system.
Planetary scientists have been studying impact events—collisions with other objects, such as a meteorite slamming into the Earth and forming a crater—for as long as they have been studying planets. Impact events are fundamental to the study of planetary formation and determining the age of a planetary surface, but impact modeling has always involved rocky bodies, and more recently, icy ones. How might a metal body handle a bruising? Nobody really knows, and models lack the requisite input data on metallic objects to produce reliable results. Planetary scientists study human-made metal alloys, as well as metal meteorites that have fallen to Earth, but examining the surface of Psyche will provide an unprecedented picture of metal impacts on a large scale.
The exploration of Psyche, a profoundly exotic object, also introduces new terms to the planetary science vocabulary. Everyone is familiar with volcanoes as found on our third rock from the sun (and on Mars as well). Cryovolcanoes—sometimes referred to as ice volcanoes—can be found on the dwarf planet Ceres, icy outer solar system moons and perhaps Pluto.
But Psyche might have had ferrovolcanoes in its past. “The idea is that Psyche might have retained much of its heat after the impact process that ripped off its mantle and crust,” Bell says. “One way that planets get rid of their heat is through volcanism. On Earth its silicate volcanism. On [Jupiter’s moon] Io, there is silicate and also sulfur-rich volcanism. On Psyche, we may be looking at a place where it’s mostly dominated by molten iron, or iron and nickel, and maybe sulfur.” Different metals melt at different temperatures, and pockets of molten liquid from Psyche’s subsurface might have erupted long ago onto the surface.
“Maybe we will see evidence of that. Maybe there won’t be any at all. But it’s fun to speculate,” Bell says.
If the team proves that Psyche is a planetary core, then everything learned at the asteroid can be extrapolated to planetary bodies across the solar system—including Earth. The spacecraft will use a magnetometer and a gamma ray and neutron spectrometer to make chemical and magnetic measurements of the asteroid in order to determine its composition, and whether it is, in fact, a planetary core. The asteroid lacks an atmosphere, and whether it has a magnetic field is yet to be determined. What is certain, however, is that it is exposed to the radiation environment of space.
“Picture a constant rain of really high energy particles hitting the surface of this asteroid,” says Patrick Peplowski, a co-investigator on the mission. “This initiates a series of reactions, one of which is the emission of gamma rays.” Different elements produce different gamma ray emissions, and if the team sees a lot of iron and nickel, then they will know Psyche is the core of a long-gone planet.
Assuming NASA gives the go-ahead for fabrication, the team will begin building a space-qualified spectrometer, along with the rest of the science instruments and the spacecraft itself. Until then, the enigmatic asteroid Psyche beckons with innumerable questions and precious few answers.
“You don’t fly a $750 million mission if you know for sure what the answers are,” Peplowski says. And when the mission is finished, two hundred million miles away, we might know more than ever before about the center of the planet beneath our feet.